SEARCH

SEARCH BY CITATION

Keywords:

  • agronomic measures;
  • crops;
  • fertilization;
  • nutritional quality

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

Crops, as the basic source of essential substances and nutrients, do not always contain sufficient amounts of these essential nutrients to meet dietary requirements. In this review paper, we discussed the effects of fertilization and other agronomic measures on the nutritional quality of cereal, oilseed and protein crops, tuber plants and vegetables. Research indicates that application of N, P, K and S fertilizers generally increases crop yield as well as nutritional quality. For example, fertilizer increased protein concentration in cereals and pulses, oil concentration in oilseed crops, starch concentration in tubers, and concentration of essential amino acids and vitamins in vegetables. However, excessive fertilizer application, especially N fertilizer, can result in undesirable changes such as increases in nitrate, titratable acidity and acid to sugar ratio, while decreasing the concentration of vitamin C, soluble sugar, soluble solids, and Mg and Ca in some crops. Other agronomic measures, such as tillage and crop rotation, organic farming, soil moisture management, and crop breeding and genetic engineering can also have a large effect on food crop quality, though the potential benefits of these measures for improving crop quality has not been fully exploited. Research literature on this subject suggests that more information is needed in order to achieve an increase in the concentration of essential microelements, prevent accumulation of toxic levels of elements such as Cu, Mo, Zn, Ni, Se and nitrate, and other dangerous or toxic substances and elements in crops. Copyright © 2007 Society of Chemical Industry


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

One of the most important roles of agricultural crop production is to provide almost all essential mineral and organic nutrients to humans. Basic substances that humans require include carbohydrates, lipids and proteins (amino acids), as well as 17 mineral elements and 13 vitamins. Of the lipids, linoleic acid and linolenic acid cannot be synthesized by humans and thus must be obtained from dietary sources. The ten essential amino acids—arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine—must be obtained in various amounts from ingested plant protein.1 For the 17 mineral nutrients, they are usually grouped into two categories: macronutrients, including nitrogen (N), sulfur (S), potassium (K), calcium (Ca), phosphorus (P), chlorine (Cl), sodium (Na) and magnesium (Mg), are needed in a high amount by humans (mg d−1), and micronutrients or trace minerals, such as iron (Fe), zinc (Zn), manganese (Mn), fluorine (F), copper (Cu), molybdenum (Mo), chromium (Cr), iodine (I) and selenium (Se), are needed in a relatively small amount (µg d−1). Some other elements such as arsenic (As), boron (B), nickel (Ni), silicon (Si) and vanadium (Va) are also reported as essential trace elements for human growth and health.2 Protein, vitamins, minerals and other constituents of essential human nutrients are now known to be of great significance in maintaining human health and well-being. Insufficient dietary intakes of these nutrients may impair functions of brain, immune and reproductive systems and energy metabolism, and result in learning disabilities, reduced work capacity, chronic diseases such as cancers, cardiovascular and degenerative diseases associated with aging, even more serious illnesses, and death.

Today, although the production of energy and protein appears to be adequate to feed the developed world, agriculture systems in many developing countries still do not provide enough nutrients to meet human needs.3 As a result, it is estimated that over 800 million people do not receive enough energy (calories) and protein to meet their daily requirements.4 Nearly half of the estimated 10.4 million deaths of children younger than 5 years of age that occurred in 1995 were attributed to protein and energy malnutrition.5 Concerning mineral nutrients and vitamins, it is estimated that over half of the world's population does not consume enough Fe, I and vitamin A in their food. Other deficiencies, including Ca, Zn, Se, folic acid, vitamin C, vitamin E, vitamin D, thiamin, vitamin B12 and niacin, are almost certainly impairing the health and productivity of a large number of people in the developing world, especially poor women, infants and children.2

Supply of essential nutrients for human health by crop production and measures for the improvement of nutritional quality of crop plants have spurred great interest in recent years. The objective of this report is to summarize information on the effects of fertilizer application and other agriculture production practices on nutritional quality of crops.

EFFECTS OF FERTILIZATION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

Cereal crops

Wheat

The increase of protein concentration in grain is normally associated with improved nutritional and commercial quality of wheat.6 Protein concentration in wheat grain is influenced by genotype, environmental conditions and availability of nutrients in soil (particularly N). These factors interact to influence the ability of the plant to produce protein and incorporate it into developing wheat kernel.

Nitrogen fertilization

Nitrogen is an essential component of amino acids, which are in turn the building blocks of proteins.7 Nitrogen constitutes approximately 17% of the protein molecule. Consequently, protein concentration in plants is highly dependent on N availability. The supply of N to a crop is determined by the amount of plant-available N in soil at sowing time, N released during the growing season through mineralization of soil organic matter and N applied as organic or inorganic fertilizer.

Hedlin et al. showed that the grain protein concentration of wheat grown after the incorporation of alfalfa or sweet clover green manure was higher than when wheat was grown after cereals or fallow,8 apparently due to more plant-available N in the soil at sowing time and more N mineralized from the alfalfa or sweet clover residues during the growing season. Many other studies have shown relationships between increasing soil total N and/or nitrate-N concentration and increasing wheat grain yield or protein concentration.9–11 However, when the amount of N present in the soil, in addition to the mineralized-N over the growing season, is not sufficient to produce optimum wheat grain yield and protein concentration, application of N fertilizer becomes essential to achieve full wheat grain yield and quality potentials.

Spring wheat yield and protein content were affected by N application rate (Table 1).12 This response was more evident as N deficiency increased in the native soil.13 Studies on winter wheat also showed that grain protein concentration increased as residual soil N and N fertilizer application rates increased.14–16 Apart from N rate, timing of N application also impacts protein concentration in wheat grain. Availability of N to plants early in the growing season stimulates vegetative growth and increases crop yield, but this in turn can lead to dilution of protein concentration. On the other hand, application of N at late growth stages has less influence on grain yield and more on grain protein concentration. Ottmana et al. reported that N application near anthesis pronouncedly increased grain yield and protein concentration of durum wheat (Table 2).17 Wuest and Cassman found that grain protein concentration in irrigated hard red spring wheat increased as the pre-plant N rate was increased, but larger increases in grain N concentration resulted from N addition at anthesis.18 Vaughan et al. determined that spring-applied N resulted in greater grain yield and protein concentration of winter wheat than autumn-applied N.19 In some experiments, late N applications showed no effect on grain yield, but increased grain protein, as did increasing the rate of late applied N. The effect was slightly greater when the late N applications were made at flowering as compared to at bolting.20, 21 Obviously, if the N application is too late, neither the grain yield nor protein concentration is affected.

Table 1. Effect of N rate on grain yield and protein concentration of spring wheat averaged over 5 site-years in 1985 and 1987, and grown under moderate moisture conditions12
Rate of N (kg N ha−1)Grain yield (kg ha−1)Protein concentration (g kg−1)
GlenleaHY320KatepwaGlenleaHY320Katepwa
0115119531506125.6122.4134.9
40199025811991128.8123.4132.7
80222627842356143.2131.2149.5
120241829272408148.4139.9156.8
160230529382335152.1145.3163.9
200231828722444157.4149.6163.8
Table 2. Effect of N applied near anthesis on durum wheat grain yield and protein concentration17
Rate of N (kg N ha−1)Grain yield (kg ha−1)Protein concentration (g kg−1)
Year 1995Year 1996Year 1995Year 1996
070206960115132
3473107100127141
6773806900140151

Under dry conditions, the uptake of fertilizer N from the soil may be impaired. Apart from root, leaves are another nutrient absorptive organ in plants. Uptake of N from foliar applications are less dependent on soil moisture conditions and could be effective when root uptake is impaired due to dry soil or reduced root activity at late growth stages. Timing of foliar sprays also influences the response of both grain yield and protein concentration to applied N. Foliar spraying spring wheat with urea solution before flowering frequently increased grain yield, while spraying at flowering was most effective in increasing protein concentration.22 The increase in protein due to applied N declined when foliar applications of urea were made after flowering, and foliar application of urea 10–12 days before ripening had no effect on protein concentration in wheat grain. In a study on winter wheat, yield response to foliar-applied urea decreased as application was delayed beyond flag leaf emergence, but the increase in protein concentration was more consistent than the increase in grain yield.23 Overall, soil applications of N fertilizer are more effective in improving grain protein concentration when applied before anthesis, while foliar applications appear to be more effective when applied after anthesis.

Phosphorus fertilization

Phosphorus fertilization usually has no direct influence on protein concentration in wheat. However, in some instances P has been found to influence N uptake and metabolism, for example, NP-fertilized wheat plants absorbed more N than N-fertilized plants due to better root development from emergence to tillering.24 In other studies, application of P in combination with N fertilizer was observed to decrease protein concentration compared to application of N alone, presumably due to dilution of protein by increased grain yield.13, 25 Holford et al. reported that P tended to depress protein concentration in spring wheat grain, particularly in the absence of N fertilizer.26

Potassium fertilization

Potassium is closely related to N assimilation in plants. In growth chamber studies, K nutrition has been observed to increase the rate of amino acid translocation into the grain as well as the conversion of amino acids into grain proteins.27 But under field conditions K fertilization showed no effect on protein concentration of winter wheat in Canada and Italy.10, 28 A field experiment by Dick et al. with five barley cultivars on a K-deficient soil showed that application of K tended to increase grain yield but decrease grain protein content (Table 3).29

Table 3. Effect of K fertilizer on grain yield and protein concentration of five barley cultivars averaged over 22 field experiments29
K rate (kg K2O ha−1)BonanzaCentennialConquestGaltGateway
 Grain yield (kg ha−1)
028202630243027402350
3430002940276031702790
 Protein concentration (g kg−1)
0200134142127138
34126128140123132
Sulfur fertilization

Unlike vegetative tissues, which may contain up to 50% of the total S as sulfate, mature wheat grain contains little sulfate, usually 1–5% of the total S content.30, 31 The majority of S in mature wheat grain is present in proteins as cysteine and methionine. Sulfur deficiency decreases concentration of S-containing amino acids in wheat grain, and its baking quality. For example, in pot experiments, S deficiency markedly decreased the concentration of cysteine and methionine in wheat grain and flour, with the effect being more pronounced on cysteine than on methionine.32, 33 Similarly, Wrigley et al. showed three- and two-fold decreases in the concentration of cysteine and methionine, respectively, in S-deficient wheat grain compared to the S-sufficient grain.34

Sulfur deficiency also decreases the concentration of other amino acids, such as lysine and threonine, which are more essential to the nutritional value of wheat proteins. Byers and Bolton found that both lysine and threonine decreased as a result of S deficiency, particularly when the N supply was high.32 This was probably due to a dilution effect caused by the accumulation of aspartic acid plus asparagines. This effect of S deficiency on the concentration of lysine and threonine was not observed in a different experiment conducted by Byers et al.,30 presumably due to the smaller amount of N supplied. Wrigley et al. found little effect of S deficiency on lysine concentration, but a noticeable decrease in the concentration of threonine.34

The preceding literature suggests that adequate amounts of S must be present in soil to enhance yield and ensure adequate protein concentration in the grain. Sulfur requirements of wheat are much lower than those of high S-demanding crops such as canola or alfalfa. But deficiency of S in wheat has been observed in many countries, and has become increasingly widespread in western Europe, western Canada, western USA, southern Asia, Australia and New Zealand in recent years. Cressman and Davis reported that when S is deficient, both grain yield and quality can decline.35 Application of S fertilizer to an S-deficient soil significantly increased the concentrations of S-containing amino acids cysteine and methionine, especially at late growth stages (Table 4), but the grain N concentration was decreased by a dilution effect caused by the increased yield with S application.36

Table 4. Effect of sulfur application on barley yield, and concentration of N- and S-containing amino acids in grain36
S application time (days after sowing)Yield (g per plant)Nitrogen (g kg−1)Cysteine (g kg−1)Methionine (g kg−1)
04.317.62.411.83
204.220.12.712.06
304.618.02.521.90
373.921.52.822.15
503.721.02.672.08
553.423.43.042.36
No S application1.724.61.471.57
Microelement fertilization

Information on the effects of microelement fertilizer on wheat grain yield and nutritional quality is still lacking. Graham et al. showed that application of zinc fertilizer to Zn-deficient soil at sowing significantly increased the Zn concentration in wheat grain (Table 5).2 Also, the content of Zn and several other micronutrients, such as I, Se, Cu and Ni, was usually enhanced by application of the appropriate mineral forms.37, 38 Because of its rapid oxidation in soil and its low mobility in the phloem, soluble ferrous fertilizers are usually ineffective in increasing the iron concentration in plants, especially in grain that develops months after application. These results show that micronutrient fertilization can have positive effects on wheat yield and grain quality, but additional research is needed in order to understand more about the most effective application methods.

Table 5. Zinc concentration in grain of wheat cultivars grown on Zn-deficient soil at Birchip, Victoria, with and without Zn fertilizer added to the soil at sowing2
CultivarNo Zn fertilizer (mg kg−1 dry wt)Zn fertilizer added (mg kg−1 dry wt)
Declic9.922.3
Songlen10.827.3
Excalibur10.822.3
VL66013.729.3
Corn

Corn (Zea mays L.) kernels are composed of approximately 73% starch, 10% protein and 5% oil, with the reminder made up of fiber, vitamins and minerals.39 Among feed grains, corn is one of the most concentrated sources of energy because of its high starch and low fiber content, as well as the more digestible nutrients. Due to a high degree of unsaturated fatty acid content, corn oil is widely used for human consumption. However, the major drawback of corn is its relatively low protein concentration. In addition, corn protein is of low biological value, as it does not supply all of the essential amino acids in adequate quantities and appropriate proportions.40 Corn seed protein is deficient in lysine and tryptophan, but has high levels of the essential S-containing amino acids cysteine and methionine.

Production factors that increase corn grain yield also increase starch concentration, while reducing the protein concentration of grain.41 In addition to the dilution effect caused by increased grain yield, the negative relationship between grain protein concentration and grain yield is partly associated with the higher glucose use for the synthesis of protein over that of carbohydrates.42

Nitrogen fertilization

Research by Tsai et al. indicated that protein concentration of corn grain increased with N supply,43 and there was preferential deposition of zein (a kind of protein soluble in alcohol) over other endosperm proteins. Also, the amount of fertilizer N required to maximize grain yield is not the same as the amount needed to produce maximum grain protein concentration, and the latter is usually higher than the former.

Application of N fertilizer can also change the amino acid balance in corn grain. As the corn grain protein concentration increases with the increment of N rates, zein makes up an increasing proportion of the grain protein. Since zein contains lower amounts of the most limiting essential amino acids, lysine and tryptophan, an increased zein concentration causes a decrease in lysine and tryptophan proportion, thus lessening the biological value of the corn grain protein.44 However, this can be compensated in some cases, since N fertilization increases the size of the germ, which has a better amino acid balance than the endosperm.45

Since the oil concentration in corn grain is so low, there are few studies on fertilization and other agronomic practices influencing the corn oil concentration. Welch reported that N, P and K applications slightly increased the oil concentration in corn grain, but more important was that the increased grain yield resulted in greater oil production per unit of land area.46

Phosphorus fertilization

Corn is highly responsive to early absorption of P. A small amount of P directly applied in seed row at seeding can usually result in greater P absorption and grain yield than a greater amount of side banded P applied at a later stage. Four- to five-leaf stage is usually more critical for crop response to P than application of P at other growth stages, and the increased grain yield is more from the increase of kernel number per ear, rather than from the increased kernel weight.47 Application of P can also increase the quality and yield of sweetcorn by increasing the number of marketable ears and more complete ear-tip filling.48 Imamul Huq found, however, that the application of P could not increase the protein concentration of corn grain, and neither did the N and K (Table 6).49 Corn yield was higher, but grain protein concentrations were lower in P- and K-fertilized treatments compared to unfertilized treatments. The decrease in protein concentration was probably due to the dilution effect caused by higher yields.

Table 6. Effect of N, P and K fertilizer rates on maize grain protein concentration49
Rate of N (kg N ha−1)Protein (g ka−1)P rate (kg P ha−1)Protein (g ka−1)K rate (kg K ha−1)Protein (g ka−1)
010.930121.50121.5
5010.0425109.525108.8
15010.1175110.375110.3
25011.33225109.4225108.9
Sulfur fertilization

Sulphur is a macronutrient that is taken up by most grain crops in amounts similar to that of P, namely 10–30 kg ha−1.50, 51 However, although the effect of P on corn yield and quality has been studied extensively, little is known about the effects of S on corn quality. Since corn grain has high levels of the S-containing essential amino acids cystine and methionine, it is widely believed that S deficiency will decrease corn grain quality.

Oilseed and pulse crops

Canola

Canola is an improved form of rapeseed (Brassica napus L. and B. rapa L.), with modified fatty acid composition and reduced glucosinolate content in the meal. It is one of the major oilseed crops in the world.

Nitrogen fertilization

Canola is not only an oilseed crop, but also contains a relatively high protein concentration in the seed (>400 g kg−1 of the oil-free meal) and its meal is used as a protein supplement for animals and possibly will be for humans in the near future. Because of its high protein content, canola and other Brassica species in general require sufficient N during their growth for the synthesis of protein. Increase of N supply usually increases seed yield and protein concentration (Table 7), but decreases the oil content.52 However, protein concentration was also observed to decrease with the increase in canola seed yield caused by N application.53

Table 7. Effect of N application on canola seed protein and oil content52
Rate of N (kg N ha−1)Oil content (%)Protein content (%)
Early sowingLate sowingEarly sowingLate sowing
039.535.630.934.4
2538.534.532.835.4
7535.833.838.138.9
Phosphorus fertilization

The major part of P in canola seed is in the form of phytate, the salt of phytic acid, myoinositol hexakisphosphate.54, 55 The phytate P portion of total P in canola seed has been reported to range from 33% to 50%.56, 57 Although some positive effects of phytate intake have been reported on humans, such as reduced risk of colon cancer, kidney stone, high cholesterol and caries,58 phytates, as antinutritive compounds, have low digestibility in monogasters, reduce the absorption of calcium, iron, magnesium, zinc and other trace elements, and form complexes with basic amino acids.59, 60

Lickfett et al. found that P applications significantly increased canola seed yield as well as oil and P concentrations, while the phytate-P concentration was also elevated from 0.6 to 6.0 g kg−1 (Table 8).61 Reduction of phytate level in canola seed by lowering the P supply does not seem to be practical, as this results in decreased nutritional quality of the seed. Therefore the hope for reducing phytate-P has focused on breeding canola cultivars with low phytic acid concentration. However, it has been difficult to find genotypes with high oil and a low phytic acid concentration, since there is positive correlation between these two characteristics. Also, protein concentration cannot always be increased with the increase in canola seed yield due to P application.62

Table 8. Seed yield and quality affected by different P rates, averaged across two canola cultivars61
P rate (mg P kg−1 soil)Seed yield (g plant−1)Oil content (%)P concentration (g kg−1)Phytate-P concentration-(g kg−1)
63.543.22.40.6
199.946.93.51.9
3110.247.14.53.4
10611.947.37.26.0
Sulphur fertilization

Canola requires 3–10 times more S than barley,63 owing to a combination of high protein content with a high proportion of cysteine and methionine.64 Deficiency of S at any growth stage can cause a considerable reduction in seed yield and quality. However, symptoms of S deficiency often appear at bud-forming and flowering stages. A constant supply of available S is thus needed throughout the growing season to prevent a loss in seed yield and quality. On soils marginally deficient in S, application of high rates of N can result in faster depletion of S from soil and increase the severity of S deficiency at most growth stages.65 Deficiency of S in canola can be prevented by applying sulfate-S fertilizer at seeding.63 However, in a pot experiment with S-deficient soil, canola seed yield was markedly lower from S application after rosette stage at high N rate, while it was not influenced by S application at any time with low N rates.66 Reasons for this were that much of the S applied during rosette to flowering stages at high N rate was accumulated in leaves and could not be translocated to seed.

Field experiments carried out on S-deficient Gray Luvisol (Alfisol) soils in North America showed that application of N alone tended to reduce canola seed yield, oil content and S uptake, but increase the protein content (Table 9).67 Applying S fertilizer alone tended to increase yield, oil content and S uptake of seed, but had no effect on seed protein content. Compared to N alone, application of S together with N increased seed yield, oil content and S uptake in all cases, and increased protein content of seed in most cases. However, increasing application rate of S from 15 to 30 kg S ha−1 increased yield and S uptake of seed, but had no consistent effect on oil and protein content in seed. Comparison of fertilization times at seeding, bolting and flowering showed that the earlier the S was applied, the more effective it was in increasing yield and S uptake of canola seed. There was no noticeable difference among the methods of S application at seeding, such as surface broadcast and incorporation into the soil, side banded and seed row placed.67 In conclusion, application of sulfate-S fertilizer on S-deficient soils is required to improve N-use efficiency and thereby improve yield, seed quality and S uptake of canola, with application at seeding generally more effective than at bolting and early flowering stages.

Table 9. Effect of S application on yield, protein concentration, oil content and S uptake of canola seed67
Timing of S applicationN ratea (kg N ha−1)S rate (kg S ha−1)Seed yield (kg ha−1)Protein concentration (g kg−1)Oil content (%)S uptake (kg S ha−1)
  • a

    Fertilizer N, where applied, was all at seeding.

Seeding0040621340.50.98
 120014023037.30.38
 03077921242.02.78
Seeding12015106422841.03.13
 12030120823141.34.17
Bolting1201582323939.62.66
 1203093724140.63.50
Flowering1201564624239.72.17
 1203076624439.72.96
Boron fertilization

Boron also plays an important role in the growth and production of canola.68 Symptoms of B deficiency in canola usually do not appear until the upper parts of the plant form pods, with seed development limited to those pods located on the lower parts of the plant. Boron-deficient canola plants appearing normal in early growth stages show red margins and/or mottling at bloom stage and reduced seed set.69 Boron deficiency also delays maturity and keeps the plant in an indeterminate stage of growth, with flowers forming up to maturity. As a heavy user of B, concentration of B in canola plant tissue should not be less than 20–30 mg B kg−1 for an optimum seed yield and quality.70

Therefore, a steady supply of B during the peak vegetative, flowering, pod production and seed development stages is essential for optimum canola seed yield and quality. Boron deficiency usually occurs on sandy soils. Boron application is most effective when it is incorporated into the soil, whereas seed row band placements may have phytotoxic effects. Foliar application is an effective way to supply B to plants, especially when root activity is restricted by dry soil, and can be used if deficiency is noted in the growing season.71 However, in field experiments on sandy soils in western Canada, where the hot-water-soluble B ranged from 0.1 to 0.8 mg kg−1, Malhi et al. reported that B fertilization did not have a consistent influence on canola seed yield and oil content (Table 10),72 and protein concentration was significantly increased only in one case by B fertilization. Bullock and Sawyer also obtained similar results.70

Table 10. Effect of B application on canola seed yield, protein concentration and oil content72
Timing of applicationMethod of applicationB rate (kg B ha−1)Seed yield (Mg ha−1)Protein concentration (g kg−1)Oil content (%)
Control 01.7022044.1
SeedingIncorporation1.01.7023044.5
 2.01.7722045.6
 4.01.7522445.6
SeedingSeed row band0.51.6921444.3
 1.01.7222045.2
 2.01.6922044.2
FloweringFoliar spray0.251.6421844.0
 0.51.6822145.5

Soybean

Soybean (Glycine max L.) is a major protein crop with a seed protein concentration of more than 360 g kg−1.73 It is also an important oilseed crop. Since legumes have the ability to fix N from the atmosphere, their N requirement from the soil is lower than that of other crops. However, studies have shown that N stresses can result in decrease in seed yield and protein concentration.74, 75

About 70% of the protein in soybean seed is present as the storage proteins, which is glycinin and conglycinin. Glycinin accounts for about 60% of the storage protein, and the remaining 40% is conglycinin, although there is some variation among soybean cultivars. Glycinin is composed of five subunits whose concentration of S-amino acids ranges from 3% to 4.5%.76 In contrast, conglycinin, which is composed of three subunits, contains less than 1% S-containing amino acids,77 and the subunit of conglycinin contains only one cysteine residue and no methionine. Therefore, the ratio of glycinin to conglycinin proteins is an indicator of protein quality; the greater the ratio, the greater the concentration of methionine and cysteine per unit protein, and the better the quality of the storage protein.

Seed filling is a critical stage for legume crops. Seed yield and quality of storage protein are influenced by both S and N nutrition during seed filling. Gaylor and Sykes reported that S deficiency caused soybean to produce greater amount of conglycinin.78 An experiment by Paek et al. showed that a reduction in N supply increased the proportion of conglycinin storage protein, resulting in a decline in the glycinin/conglycinin ratio.79 Sexton et al. found that seed yield was very sensitive to S deficiency occurring during vegetative growth, but not to S deficiency during reproductive growth stage.73 The glycinin/conglycinin ratio was strongly influenced by S deficiency occurring at reproductive growth stage, but was relatively insensitive to S availability during vegetative growth. Application of S fertilizer to previously S-deficient soybean plants near the middle of the seed-filling period caused a threefold increase in the glycinin/conglycinin ratio compared to plants that were maintained in S-deficient conditions throughout seed filling (Table 11).

Table 11. Soybean seed yield, plant mass, glycinin:conglycinin ratio (G/C) and seed protein concentration in greenhouse trials where S availability was varied during vegetative and productive growth stages, and N source was either urea or nitrate73
N sourceVegetative stageReproductive stageSeed yield (g plant−1)Plant weight (g plant−1)Seed protein (g kg−1)G/C
KNO3S-deficientS-deficient9.634.33770.31
KNO3S-deficientS-sufficient14.238.93642.14
KNO3S-sufficientS-deficient30.584.33800.70
KNO3S-sufficientS-sufficient29.883.93782.05
UreaS-deficientS-deficient11.236.64310.33
UreaS-deficientS-sufficient18.344.64261.80
UreaS-sufficientS-deficient36.593.94150.71
UreaS-sufficientS-sufficient34.693.44201.59

Tuber and root crops

Potato (Solanum tuberosum L.) and sweet potato (Ipomoea batatas L.) are major tuber and root crops, respectively, with enlarged underground stems and roots as edible parts. These plants parts contain high amounts of starch, and can also supply many other macroelements, microelements, amino acids and vitamins. Potato is a major tuber crop, ranking fourth in the world in production volume after rice, wheat and maize; thus it contributes significantly to human nutrition in a great number of countries. In terms of energy, potato production potential is surpassed by only a few other crops.

Nitrogen, phosphorus and potassium fertilization

The potential of the potato crop for protein production is astonishingly high, even compared with protein-rich crops such as pea, bean and canola. In North America and Europe, the tuber yield of potato can reach 90 t ha−1, which amounts to 600–800 kg protein ha−1.80 In addition, potato protein has been shown to be of high nutritive value compared to other sources of plant protein such as wheat, rice, corn, bean and soybean.81 Because of its high yield potential, potatoes require large amounts of plant nutrients, especially N, P and K. Potassium has been proven to play an important role in the transport of starch and sugar from above-ground parts of the plant and their accumulation in tubers.

In nutrient-deficient soils, application of fertilizers can significantly increase tuber yield. Eppendorfer and Eggum reported that the increase in tuber yield was particularly high with application of N, P and K.82 Tuber starch contents responded to fertilization in a pattern similar to tuber yield and the addition of K resulted in the largest increase in starch content. Application of N fertilizer usually led to increases in tuber N concentration, but in some cases low N fertilizer rates decreased tuber N content due to the dilution effect caused by higher dry matter production. Increased application of P and K also resulted in a decreased N concentration for a similar reason (Table 12).82

Table 12. Effect of application of N, P, K and S on the dry matter yield, starch content and N concentration of potato tuber82
Nutrient rate (per tuber)Dry matter (g per tuber)Starch content (%)N concentration (g kg−1)
N1 (0.5 g N)116770.88
N2 (2.0 g N)208760.95
N3 (6.0 g N)319811.43
N4 (9.0 g N)376771.51
P1 (0.0 g P)66742.33
P2 (0.25 g P)199702.00
P3 (0.75 g P)311731.82
K1 (0.0 g K)35623.73
K2 (0.75 g K)84643.57
K3 (3.0 g K)204712.41
Sulfur fertilization

With S concentrations ranging from 1.2 to 2.8 g kg−1 in the tuber and haulm, potato is not considered a high S-demanding crop, but considerable amounts of S can be removed from the soil over the long term when yields are high. In S-deficient soil, application of S fertilizer can significantly increase tuber yield and starch content of potato, while leading to a decrease in tuber N concentration due to an increase in dry matter production. Furthermore, Eppendorfer and Eggum found that S deficiency also significantly influenced the amino acid composition of potato (Table 13).82 Specifically, the concentration of S-containing amino acids methionine and cystine decreased by 30% and 60%, respectively, in S-deficient soil, while the concentration of glutamine, asparagine and arginine was increased. More research is necessary to determine how S supply influences these non-S-containing amino acids.

Table 13. Effect of S rates on amino acid composition of proteins in potato tubers82
Nutrient rate (g S per tuber)Amino acids (g (16 g N)−1)
CysMetLysThrTrpIleLeuValPheTyrArgAspGlu
  1. Cys, cysteine; Met, methionine; Lys, lysine; Thr, threonine; Trp, tryptophan; Ile, Isoleucine; Leu, leucine; Val, valine; Phe, phenylalanine; Tyr, tyrosine; Arg, arginine; Asp, aspartate; Glu, glutamate.

0.00.61.13.92.20.82.62.94.03.62.76.419.520.6
0.150.91.24.52.91.03.24.44.63.42.84.320.515.1
0.51.41.55.03.41.23.65.35.03.82.93.818.514.1
Calcium fertilization

Necrotic lesions, such as brown center, internal brown or rust spot, as well as internal heat necrosis and hollow heart, frequently occur in potato, resulting in a serious decline in tuber quality and processing value. These defects may be associated with soil temperature and moisture, genetic characteristics, disproportionate and inconsistent tuber and plant growth rates, and certain cultural practices. However, most necrotic lesions, such as internal brown or rust spot, are related to localized Ca deficiencies in the tuber. In greenhouse pot experiments, Collier et al. demonstrated that Ca application could increase tuber Ca concentration and reduce internal brown spot.83 Kleinhenz et al. showed that a 37% reduction in the incidence of internal tuber defects was associated with split application of Ca compared to a single application.84 Calcium concentration and internal quality of individual tubers are closely related. The incidence of internal defects was 16.4% in tubers with Ca concentrations ⩽100 mg kg−1 dry weight in non-periderm tissue compared to 10.6% in tubers with Ca > 100 mg kg−1 dry weight in non-periderm tissue.

Increases in plant tissue Ca concentration are more likely to result from altered flux of Ca in xylem sap than from increasing Ca supply to root. However, this hypothesis does not apply to Ca in potato tubers. Kratzke and Palta showed that tuber Ca levels were affected by the Ca supply in the region containing developing tubers, but not main roots.85 This indicated that Ca concentration in tubers was associated primarily with the amount of Ca taken up by roots connected to tubers, or with the Ca concentration of the soil solution surrounding tubers. Therefore, placement and timing of Ca will influence the efficiency of field-applied Ca intended to increase Ca concentration in tubers. Simmons et al. demonstrated that pre-plant strip application of gypsum in the field with side-dressed calcium nitrate (Ca(NO3)2) fertilization increased tuber periderm Ca concentrations and improved tuber grade and size on low-Ca sandy soils.86

Potato is a chloride-sensitive crop, and toxicity symptoms are readily noticeable when the chloride content in the growing medium exceeds 0.2%. Potassium chloride (KCl) can increase the soluble sugar concentration and decrease starch content in tubers more than potassium sulfate (K2SO4). Therefore, when KCl and CaCl2 fertilizers are applied, special attention should be paid to the effects of Cl on tuber quality.

Vegetables

Leafy vegetables

Leafy vegetables such as cabbage, spinach, lettuce, and celery provide humans with many kinds of vitamins and mineral elements. Since these crops are usually harvested at vegetative growth stages, and the edible parts—young stems and leaves—can accumulate relatively large amounts of nitrate, leafy vegetables have been found to be the major source of nitrate uptake by humans. Nitrate that accumulates in leafy vegetables originate from residual soil N, as well as from the application of organic and inorganic N fertilizers.87 For this reason, the key measure for reducing nitrate accumulation in vegetables lies in proper N fertilizer management.

Nitrogen fertilization

Ammonium-N or urea fertilizer can be transformed very quickly into nitrate-N when applied to aerobic soil, therefore increasing soil nitrate-N. This could lead to an increase in the nitrate-N concentration in vegetables. Other N sources containing nitrate and/or ammonium-N are also widely used in vegetable production in different countries. Field experiments have shown that nitrate concentrations in leafy vegetables were positively correlated to N rates, and N fertilizer added to the soil was the major cause of nitrate accumulation in vegetables (Table 14).88

Table 14. Nitrate-N concentration in the edible part of different leafy vegetables at seedling stages88
N rate kg N ha−1Nitrate-N concentration (mg kg−1 FW)
Chinese cabbageGreen cabbageRapeSpinach
07798051146368
909388551335494
18010118891413549
27098310821265537

Effects of different N sources on vegetable growth and nitrate accumulation have attracted great attention in recent years. In a water culture experiment, nitrate concentration in spinach was decreased by 79–98%, while fresh weight decreased by 22–26% when fertilized with ammonium or urea compared to treatments fertilized with nitrate.89 Under field conditions soil is aerobic, and ammonium and urea N are rapidly transformed to nitrate-N; therefore the effect of fertilizer type on nitrate concentration in vegetables is usually not so pronounced. Wang and Li showed that ammonium chloride, ammonium nitrate, sodium nitrate and urea had no significant effects on nitrate N levels in cabbage and spinach regardless of harvest time, although nitrate N fertilizers tended to increase nitrate accumulation compared to the ammonium N fertilizers.90

Nitrate accumulation in vegetables is caused by the imbalance between nitrate absorption and reduction by plants. When nitrate-N is adequate in soil, plants usually absorb much more nitrate than they can reduce. Therefore, reducing soil nitrate and nitrate absorption by plants are of great importance for the purpose of reducing nitrate accumulation in leafy vegetables. Application of coated N fertilizer has been reported to control the release rate of N from fertilizer and significantly decrease nitrate in soil. Ombodi et al. showed that band applications of polyolefin-coated fertilizer (POCF), such as polyolefin-coated urea, polyolefin-coated urea with dicyandiamide in the coating, polyolefin-coated diammonium phosphate and polyolefin-coated ammonium sulfate, decreased nitrate concentrations in spinach by 90%, while markedly increased vitamin C concentrations compared to uncoated ammonium sulfate treatments.91

Nitrate levels in vegetable leaves are lowered through the natural, ongoing process of nitrate reduction, but this process requires time. Some researchers have suggested that nitrate concentration in leafy vegetables can be lowered to safe levels when harvested 7–8 days after N fertilization. However, this has not been proven by field experiments.88, 90 It is well known that plant growth as well as nitrate concentration of vegetables depends on many internal and external factors. For this reason, no single factor can be used to determine the waiting period between fertilization and harvest. Instead, a large number of factors including fertilizer rates, water supply, vegetable species, growth stage, cultivation methods, ambient temperature, and illumination intensity should all be taken into consideration.

Phosphorus fertilization

Phosphorus promotes root growth, enhances nutrient and water use efficiency, and increases yield. Absorption and reduction of nitrate is an energy-consuming process, and the energy is supplied by adenosine triphosphate (ATP). Consequently, the metabolism of nitrate is related to P supply. In pot experiments, Gao et al. found that low available P in soil led to an increase in nitrate levels in cabbage and spinach, and that a high soil N:P ratio was one of the key causes for nitrate accumulation in vegetables.92 However, Wang and Li found that crop response to P fertilizer depended on soil available P as well as on crop species.93 In soil that is deficient in available P, crops generally respond well to P application. In contrast, the addition of P fertilizer to a soil containing 109 mg kg−1 (Olsen-P) decreased nitrate concentration by 20.1% in green cabbage, but increased nitrate by 17.3% in cabbage and 18.9% canola. Phosphorus fertilization had no significant effect on nitrate in spinach.

Potassium fertilization

Potassium can accelerate transport of nitrate from roots to aboveground parts. Zhou et al. showed that compared to control, nitrate concentration in cabbage was decreased by 14.6% (from 482 to 412 mg kg−1) with application of 75 kg K ha−1.94 Effects of K on growth and nitrate accumulation in leafy vegetables vary with crop species and cultivar. Hydroponic experiments showed that the addition of K decreased nitrate concentrations in cabbage by 26.0% compared to the control treatment. In contrast, the addition of K increased nitrate concentrations by 8.2% in spinach.92 Furthermore, application of K to treatments containing high N levels inhibited spinach growth, but had no significant effect on cabbage.

Fruit vegetables

The edible parts of fruit vegetables such as tomato, cucumber, green and hot peppers and eggplant usually contain little or no nitrate. However, the application of N, P and K fertilizer can still influence their yield as well as commercial and nutritional quality.

Since little nitrate is transported through the phloem of plants, application of N does not usually increase the risk of nitrate accumulation in fruit. However, Ruiz and Romero showed that with increase of N rates more amino acids were transported to cucumber fruits through the phloem, and the highest levels of fruit amino acids were found in plants treated with higher N rates.95 Protein and other organic N compounds showed a trend similar to that of amino acids. However, very high N rates as KNO3 inhibited plant growth and decreased fruit yield, amino acid content, and protein and organic N concentration (Table 15). Over-application of N decreased vitamin C, soluble sugar, Mg and Ca concentrations, as well as soluble solids in tomato fruit, but increased titratable acidity and the acid:sugar ratio, leading to a decrease in commercial and nutritional quality. Furthermore, Moreno et al. found that Fe and Mn concentrations in cucumber were higher in N, P and K fertilized treatments compared to the unfertilized control.96

Table 15. Effect of N rates on accumulation of amino acid, protein, organic N in cucumber and the fruit yield95
Rate of N (g N m−2)Amino acid (mg g−1 FW)Protein (mg g−1 FW)Organic N (mg g−1 DW)Fruit yield (kg per plant FW)
51.453.2622.60.82
102.234.4227.21.85
202.484.9728.71.65
401.483.4424.11.23

EFFECTS OF ADDITIONAL AGRONOMIC MEASURES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

Tillage and crop rotation

Tillage systems can significantly influence the yield and nutritional quality of food crops through their effect on soil moisture, nutrient availability, temperature and aeration. In recent years, minimum- and zero-tillage farming practices have become very popular in dryland/rainfed areas, especially in North America and some European countries, because of their advantages in reducing soil erosion, conserving soil moisture, improving soil organic matter and tilth, and reducing labor, fuel and machinery costs. Conventional tillage is still the major soil management measure practiced in Asia, South America and Africa. Some reports indicate that the shift towards minimum tillage tends to decrease nutrient density/concentration in crops, especially for N.97 For example, it has been reported that, grain yield and protein content of wheat were lower under zero tillage than under conventional tillage in the Prairie Provinces of Canada.98

Long-term experiments showed that the negative effects of zero or minimum tillage on crops were mainly caused by an increase in N immobilization and that this effect can be overcome through the use of higher N rates.99, 100 Seed yield and N uptake of barley were lower under zero tillage compared to conventional tillage at low N rates, but zero-tillage treatments had equal or higher grain yield and N uptake at high N rates. Similarly, the yield and N uptake of barley were lower in straw-retained treatments compared to straw-removed treatments at low N rates, but the straw-retained treatments produced equal or higher yields and N uptake values at high N rates.100 This phenomenon may be caused by reduced N mineralization rates in soil under zero tillage or increased immobilization of applied N by microorganisms in the straw-retained treatments. Mitigation of these negative effects by increasing N rate therefore resulted in higher or equal yield and N uptake relative to conventional tillage or straw-removed treatments. The problem of N immobilization in zero or minimum tillage may be compensated over time as N mineralization rates increase due to increased organic matter in the soil.97

Crop rotation is an important tool for increasing yield and quality of food crops. For example, rotation of cereals with grain legumes can effectively decrease the need for fertilizer N due to N fixation by the legumes. Gan et al. showed that grain yield of durum wheat (Triticum turgidum L.) increased by 7% and grain protein concentration increased by 11% when grown after pulse crops such as chickpea (Cicer arietinum L.), lentil (Lens culinaris Medik.), and dry pea (Pisum sativum L.), rather than after spring wheat.101 After oilseeds such as mustard (Brassica juncea L.) or canola (B. napus L.), durum wheat grain yield increased by 5% and protein concentration increased by 6%. This indicates that rotation with pulse or oilseed crops is an effective way to increase the nutritional quality of wheat.

Organic farming

Farmers and consumers are increasingly concerned about the association between plant food and human health, mainly because of the contamination of food by pesticides, herbicides, nitrate and toxic elements, which are blamed on the use of chemicals and fertilizers. Consequently, demand for organically grown products has risen steadily and the number of growers adopting organic farming practices has also increased. No chemicals are permitted in organic farming systems. Instead emphasis is placed on the use of farmyard manures, crop residues or composts made of animal manure and plant residues to meet plant nutrient needs, and the use of diverse crop rotations for weed, pest and disease control.102 Foods produced by organic methods are usually believed to have a better taste and a better balance of vitamins and minerals than conventionally grown crops. However, evidence from many greenhouse and field experiments with different crops do not always support this point of view.

Experiments in greenhouses showed that tomatoes grown on organic substrates contained significantly more Ca and vitamin C, but less Fe than did tomatoes grown in hydroponic media that contained synthetic nutrients. Phosphorus and K concentrations did not differ between tomatoes grown on organic and hydroponic substrates.103 Investigations with other vegetables showed that yield and vitamin content of carrot and cabbage were not affected by organic or inorganic fertilizer treatments,104 but concentration of N, S, Mn, Cu and B in carrot roots and N, Mn and Zn in cabbage were significantly greater in organic compared with inorganic treatments. In another 3-year field experiment where pesticides, lime and NPK fertilizer were added to the conventional treatment plots, while lime, composted manure and no insect control were applied to the organic treatment plots, Warman and Havard found that yield and vitamin C content of the potato tubers was not affected by treatments.105 In the same experiment, the yield of two corn cultivars was higher in the conventional treatment compared to the organic treatment, but there was no difference between treatments in the yield of another cultivar and the vitamin C or E contents of the kernels in any year. Organic treatment markedly increased K in potato tuber and sweet corn kernels, Zn in potato tubers and Mn in sweet corn kernels. But other elements were either not affected or were slightly decreased by the organic treatment (Table 16).105 Wheat grain protein contents were lower in organic treatments compared to conventional treatments.106

Table 16. Concentration of macro- and micro-elements in potato tuber and sweet corn kernel105
Nutrient elementPotatoSweet corn
ConventionalOrganicConventionalOrganic
Macroelements (g kg−1)
N10.310.223.423.5
P1.41.73.73.8
K13.414.712.613.1
Na0.00.00.00.1
Ca0.20.10.10.1
Mg0.80.81.41.5
S1.11.21.51.5
Microelements (mg kg−1)
B5.15.44.93.9
Fe21.121.817.515.8
Mn5.24.28.29.2
Cu4.34.04.23.5
Zn9.211.226.425.4

Soil moisture management

Soil moisture greatly influences crop yield and quality by directly affecting physiological and biochemical processes in the plants, and indirectly by changing nutrient availability in soil. Field experiments showed that wheat grain yield and protein content were significantly affected by soil moisture.107 The greatest effect of soil moisture was primarily on crop yield, since carbohydrate production seems to be more sensitive to dry conditions than protein production. In dryland farming systems, moisture supply is usually the most frequent and uncontrollable factor limiting grain yield. Increases in available soil moisture generally result in higher grain yield potential and yield response to applied N. Increases in protein concentrations only occur at very high N rates. Under severe moisture stress, the application of fertilizer N can increase wheat grain protein contents. When moisture stress is moderate, both grain yield and protein concentration may increase with N application. The relationships between soil moisture and grain yield or protein concentration have been reported in many studies. Terman et al. reported that, with adequate irrigation, the main effect of applied N on hard red winter wheat was increased yield, while with severe water deficit the chief effect of applied N was increased protein concentration.108 Campbell et al. reported that wheat grain protein concentration increased with N application up to 61.5 kg ha−1 under dry conditions, while under wet conditions it tended to increase with N application up to 164 kg ha−1.109 Larger additions of N were required to bring the grain protein concentration of irrigated wheat to the same level as that of non-irrigated wheat.

Effects of soil moisture on wheat yield and protein content also vary with growth stages. Field experiments with spring wheat in low-N soil showed that only high moisture stress between tillering and boot stages severely reduced dry matter yield, but increased protein concentration. In contrast, medium or late moisture stress did not influence protein concentration.110 Pot experiments showed that water deficit at grain-filling stage significantly decreased grain yield and nutrient accumulation in winter wheat. Supplemental irrigation at booting stage significantly increased the biomass and nutrient uptake of whole plant, but grain yield and nutrient uptake were not as high as that with supplemental irrigation at grain filling. However, the grain protein concentration was not increased by supplemental irrigation at any growth stage.111

Kniep and Mason found that N application increased corn grain yield and protein concentration, but decreased the lysine percentage in protein.112 Irrigation increased corn grain yield and reduced protein concentration, but increased the lysine percentage in protein, and therefore improved its nutritional value.

Domestic and industrial waste

Domestic life and industrial activity produce numerous organic and inorganic wastes. Increasing amounts of these wastes are recycled and applied to agricultural land worldwide. However, these wastes may also contain metal and metalloid, and organic and inorganic pollutants and other toxic or potentially harmful substances, e.g., Cr, As, Hg, Pb, F, Cu, Mn, Ni, Cd, Co, Se, dioxin, furan and pathogenic microorganisms. Most of these pollutants are either non-biodegradable or highly resistant to biodegradation. Therefore, they can accumulate in agricultural soil, be absorbed by crops and accumulate in the edible parts of plants,113 and then enter the food chain and become health hazards to humans and animals. Although some human essential nutrients may be elevated by the application of the wastes, safety of crop food is of more significance than its nutritional value.114

Sharma et al. found that Cr levels in spinach was as high as 5.4 mg Cr kg−1 DW in plants grown in silt clay loam soil and 11.7 mg kg−1 DW in sandy soil, and the plant root accumulated more Cr than shoots.115 Cadmium is more readily taken up by plants, and it is not phytotoxic at low concentrations. Uptake of Cd and its accumulation in plant are influenced by many biological factors such as crop species and cultivar, plant tissues (leaf > grain, fruit and edible root), leaf age and metal interaction. Experiments with different rice (Oryza sativa L.) cultivars showed that Cd accumulation in leaves was significantly and positively correlated with Fe, Zn and Cu, but negatively correlated with Mn.116 In pot experiments, Moral et al. indicated that Cd concentration in tomatoes increased significantly as soil Cd concentration increased, and sewage sludge application could intensify the accumulation of Cd in fruit.117 Titratable acidity, soluble protein concentration and other nutritional and commercial qualities of tomatoes were not significantly affected by sewage sludge, and the yield was even increased (Table 17).117

Table 17. Yield and quality of tomato fruit as affected by addition of Cd and sewage sludge to soil117
Treatment (mg Cd kg−1 soil)Yield (g plant−1)Titratable acidity (mg citric acid mL−1)Soluble protein (mg protein mL−1)Cd (mg Cd kg−1 DW)
With sewage sludge treatments
010354.950.02230
310895.080.02290.78
309604.860.02633.80
1009724.760.03206.33
With equivalent NPK fertilizer treatments
07115.120.03690
36395.020.03900.57
306065.390.03861.67
1008015.060.02813.67

In addition to domestic and industrial wastes, fertilizers also play an important role in Cd accumulation in plants. Some P fertilizers contain Cd and their use has been shown to increase Cd concentrations in rape, oat, potato and flax.118–120. Nitrogen fertilizers do not normally contain significant levels of Cd, but a number of investigations have shown increase in Cd concentration in plants due to N application.121–123 The increase in Cd accumulation in crops with N fertilizer application is primarily attributed to acidification, changed ionic strength and changed composition of soil solution, which may increase Cd solubility and phytoavailability.

Crop cultivar selection and breeding

Cultivar selection and breeding for resistance to diseases, increased fruit set and size, specific plant growth forms, improved grain filling and so on have played an important role in increasing agricultural productivity during the past 50 years. However, equally important but often overlooked is the nutrient composition and concentration in edible parts of crops, with regard to the food nutritional quality. Research on nutrient concentration in various plants has shown significant genotypic variation for minerals, vitamins and other phytochemicals. For example, pod Ca concentration varied nearly twofold among snap bean genotypes,124 β-carotene concentrations varied fourfold among broccoli cultivars,125 and folate concentrations varied fourfold among red beet cultivars.126 Over 40 years of breeding research also showed differences of up to 20% in the concentration of Fe and Zn in the grain of wheat cultivars.127, 128 These findings highlight the fact that significant variation in nutrient content already exists within the germplasm of these species. Thus classical breeding approaches can be used to provide nutritionally improved cultivars.

However, the negative correlation between protein content and yield in staple food crops2 caused many plant breeders to seriously question the possibility of improving nutritional quality in food crops though breeding efforts. Fortunately, unlike the relationship between protein concentration and crop yield, the concentration of some mineral macronutrients and micronutrients has been found to be positively related to high yield. For example, Ortiz-Monasterio and Graham et al. showed that the total Fe concentrations in wheat grain were not reduced by increases in yield, and the same seems to be true for Phaseolus beans.127, 128 Crop selection and breeding have great potential for nutritional enhancement of food plants.

Genetic engineering

Genetic engineering involves either the introduction of foreign DNA into a crop species or artificial modification of the plant's existing DNA. Usually DNA is introduced to or modified in a single cell, and then a whole plant is regenerated from that cell through tissue culture techniques. In recent years, genetic engineering technology has been widely used to improve crop yield and nutritional quality by increasing plant resistance to moisture stress, insects and diseases. An exciting example comes from Japan, where rice has been genetically transformed to have higher iron content in grain endosperm.129 In this work, the gene that controls the synthesis of phytoferritin, a large Fe-containing protein, in soybean, was inserted into the rice genome.

Although there is considerable debate about stability of the transformations and various concerns about the effect of genetically modified or gene-transplanted crops on ecological balance and food safety, genetic engineering technology for breeding new crop plants holds great promise for dramatic improvements in nutritional quality of food plants in the future.

SUMMARY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

Plants are the basic source of carbohydrates, lipids, proteins/amino acids, mineral elements and vitamins needed for human health. However, high concentration of Cu, Mo, Zn, Ni, Se and nitrate, when in high concentrations in plants, can be toxic to both animals and humans. Other mineral elements in soil, such as Cr, F, As, Cd, Hg and Pb, also pose health risks by entering the food chain through plant accumulation and translocation to edible harvested parts. Not all plants contain enough nutrients to meet human dietary requirements. From the viewpoint of human health, it is of great significance to increase the concentration of essential nutrients in the edible parts of crops while at the same time lowering, or at least maintaining in a safe range, the concentration of toxic or undesirable elements.

Protein concentration is one of the primary factors that determines nutritional and commercial quality of wheat and other grains, and it is greatly influenced by available N in the soil. Grain protein concentration usually increases when plant-available N is above the point where N is no longer the limiting factor for grain yield. Apart from remobilization and transport of N from the vegetative parts, N in grain also originates from post-anthesis uptake from soil. Application of N during late growth stages can be effective in increasing protein concentration. Foliar applications are especially effective because they are not dependent on soil moisture conditions. Phosphorus stimulates root development and increases N uptake; in this it may contribute to increases in grain yield and protein concentration. However, protein concentrations in grain may decline due to the dilution effect, if P fertilization causes a large increase in grain yield. Potassium is closely related to N assimilation, while application of K normally has little direct effect on grain protein concentration under field conditions. Sulfur deficiency decreases the concentration of S-containing amino acids, thus affecting protein quality. There is still not enough information on the practical management of micronutrient fertilizers for the purpose of increasing micronutrient concentration in grains. Although application of N, P and K can decrease corn oil concentration, the increased grain yield due to fertilization results in greater oil production per unit of land area.

Canola is not only an oilseed crop, but also contains high concentration of protein. An increase in N supply usually increases seed yield and protein concentration, but decreases oil content. Thus there is an inverse relationship between oil and protein content in the seed. Application of P on P-deficient soil can increase seed yield as well as oil and P concentrations, but the concentration of phytate-P can also be noticeably elevated. Sulfur is relatively immobile in plants, and a constant supply of available S is therefore needed throughout the growing season. On S- and N-deficient soils, application of S fertilizer alone shows no effect on seed protein content, but application of S together with N increases seed protein content in addition to increasing seed yield, oil content and S uptake. A steady supply of B during the peak vegetative, flowering, pod production and seed development stages is essential for optimum yield and quality of oilseeds on B-deficient soils.

The N requirement of legumes is less than that for other crops. However, N stresses can result in decreased seed yield and protein concentration. Seed yield is very sensitive to S deficiency occurring during vegetative growth, while the glycinin/conglycinin ratio is strongly influenced by S deficiency occurring during the reproductive growth stage for legume crops.

For tuber crops, K plays an important role in the transportation of starch and sugar from above-ground plant parts to tubers. Application of K fertilizer to K-deficient soils results in an increase in tuber yield. In S-deficient soils, application of S causes significant increase in tuber yield and starch content. Most necrotic lesions in tubers are related to Ca deficiency, and the amount of available Ca in the soil solution surrounding tubers was found to be associated with the concentration of Ca in tubers. Therefore, placement and timing of Ca application can influence the efficiency of field-applied Ca for increasing Ca concentration in tubers on Ca-deficient soils.

The accumulation of nitrate in leafy vegetables has attracted a lot of research attention. Nitrate in vegetables originates from residual N in the soil as well as organic or inorganic fertilizers applied by farmers. Field experiments show that there was no significant difference among N fertilizers on vegetable yield, but nitrate-N fertilizers tend to increase nitrate concentration in vegetables much more than ammonium-N fertilizers. Application of coated N fertilizers decreases the supply of nitrate-N in soil to plants. Edible parts of fruit vegetables usually contain little or no nitrate. Over-application of N usually decreases the concentration of vitamin C, soluble sugar, soluble solids, and Mg and Ca, while increasing the titratable acidity and acid:sugar ratio in tomato. Application of N, P and K dosage favors Fe and Mn accumulation in cucumber. More information is still needed regarding effects of fertilizer application on micronutrients in vegetables.

Besides fertilization, other agronomic practices can also significantly influence crop yield and nutritional quality. Minimum tillage tends to decrease cereal grain protein concentration, most likely due to decreased N mineralization and increased N immobilization during the growing season. Rotation with pulse or oilseed crops is an effective way to increase the nutritional quality of wheat. The general perception is that organically grown food crops taste better and have a better balance of nutrients compared to conventionally grown crops. However, evidence from many greenhouse and field experiments does not always support this point of view. In dry-land farming systems, increased available soil moisture generally raises grain yield potential and yield response to fertilizer. Effect of soil moisture on wheat yield and protein content also varies with growth stage. Moisture stress during late growth stages may restrict redistribution/translocation of N from vegetative parts to grain. The application of domestic and industrial wastes elevates the concentrations of some essential nutrients in food crops. However, food safety is of concern. The mechanisms affecting the bioavailability, absorption and accumulation of inorganic and organic contaminants in plants are largely unknown.

The concentration of minerals, vitamins and other nutrients varies significantly among crop cultivars and classical breeding approaches can be used to develop nutritionally improved cultivars. Although there is considerable debate on the stability of transformations and concerns about gene-modified or gene-transformed crops, genetic engineering technology is being widely used in crop cultivar improvement and new cultivar breeding to increase the ability of crop plants to resist moisture stress, insect and disease, and improve yield and nutritional quality of food plants.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

Fertilization is one of the most practical and effective ways to control and improve yield and nutritional quality of crops for human consumption. A considerable amount of research has been done on the effects of N, P and K fertilizer management on crop yield as well as nutritional quality, such as protein concentration in cereals and pulses, oil concentration in pulses and oilseed crops, starch concentration in tubers, and concentration of minerals and vitamins in vegetables. Other agronomic measures, such as tillage, crop rotation, organic farming, soil moisture management, crop cultivar selection and breeding, and genetic engineering contribute greatly to improving yield and nutritional quality of crops, but they still have not been fully exploited to meet the needs of the growing world population. More research is urgently needed to increase the concentration of essential microelements in food crops, prevent over-accumulation of Cu, Mo, Zn, Ni, Se and nitrate, and decrease the concentration of dangerous or toxic substances and elements such as Cr, F, As, Cd, Hg and Pb in crops used for food by rational use of macro- and micronutrient fertilizers and proper management of domestic and industrial wastes.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES

The authors would like to give their sincere thanks to the National Natural Science Foundation of China for the key project (30230230) and general projects (30370843, 40201028 and 30070429), the Ministry of Agriculture of China for the ‘948’ research program (2003-Z53), the Ministry of Education of China for the program of ‘NCET’ and the project of ‘SRF for ROCS’, and Dr KS Gill, Cecil Vera and Dr Jeff Gale for the internal review of this manuscript.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EFFECTS OF FERTILIZATION
  5. EFFECTS OF ADDITIONAL AGRONOMIC MEASURES
  6. SUMMARY
  7. CONCLUSIONS
  8. Acknowledgements
  9. REFERENCES
  • 1
    Linder MC, Nutritional Biochemistry and Metabolism: With Clinical Applications (2nd edn). Elsevier, New York (1991).
  • 2
    Graham RD, Welch RM and Bouis HE, Addressing micronutrient malnutrition through enhancing the nutritional quality of staple foods: principles, perspectives and knowledge gaps. Adv Agron 70: 77142 (2001).
  • 3
    Welch RM, Combs GF Jr and Duxbury JM, Toward a ‘green’ revolution. Issues Sci Technol 14: 5058 (1997).
  • 4
    Uvin P, The state of world hunger. Nutr Rev 52: 151161 (1994).
  • 5
    World Health Organization, Trace Elements in Human Nutrition and Health. World Health Organization, Geneva (1996).
  • 6
    Tipples KH, Duhetz S and Irvine GN, Effects of high rates of nitrogen on Neepawa wheat grown under irrigation. II. Milling and baking quality. Can J Plant Sci 57: 337350 (1977).
  • 7
    Olson RA and Kurtz LT, Crop nitrogen requirements, utilization, and fertilization, in Nitrogen in Agricultural Soils, Agronomy Series (No 22), ed. by StevensonFJ. American Society of Agronomy, Madison WI, pp. 567604 (1982).
  • 8
    Hedlin RA, Smith RE and LeClaire FP, Effect of crop residue and fertilizer treatments on the yield and protein content of wheat. Can J Soil Sci 37: 3440 (1957).
  • 9
    Alkier AC, Racz GJ and Soper RJ, Effects of foliar- and soil-applied nitrogen and soil nitrate-nitrogen level on the protein content of Neepawa wheat. Can J Soil Sci 52: 301309 (1972).
  • 10
    Campbell CA, McLeod JG, Selles F, Zentner RP and Vera C, Phosphorus and nitrogen rate and placement for winter wheat grown on chemical fallow in a brown soil. Can J Plant Sci 76: 237243 (1996).
  • 11
    Grant CA and Flaten DN, Fertilizing for protein content in wheat, in Wheat Protein: Production and Marketing, ed. by FowlerDB, GeddesWE, JohnstonAM and PrestonKR. University Extension Press, University of Saskatchewan, Saskatoon, Canada, pp. 151168 (1998).
  • 12
    Gauer LE, Grant CA, Gehl DT and Bailey LD, Effects of nitrogen fertilization on grain protein content, nitrogen uptake, and nitrogen use efficiency of six spring wheat (Triticum aestivum L.) cultivars, in relation to estimated moisture supply. Can J Plant Sci 72: 235241 (1992).
  • 13
    Dubetz S, Effects of high rates of nitrogen on Neepawa wheat grown under irrigation. I. Yield and protein content. Can J Plant Sci 57: 331336 (1977).
  • 14
    Fiez TE, Miller BC and Pan WL, Winter wheat yield and grain protein across varied landscape positions. Agron J 86: 10261032 (1994).
  • 15
    Goos RJ, Westfall DG, Ludwick AE and Goris JE, Grain protein content as an indictor of N sufficiency for winter wheat. Agron J 74: 130133 (1982).
  • 16
    Sowers KE, Miller BC and Pan WL, Optimizing yield and grain protein in soft white winter wheat with split nitrogen applications. Agron J 86: 10201025 (1994).
  • 17
    Ottmana MJ, Doergeb TA and Martin EC, Durum grain quality as affected by nitrogen fertilization near anthesis and irrigation during grain fill. Agron J 92: 10351041 (2000).
  • 18
    Wuest SB and Cassman KG, Fertilizer-nitrogen use efficiency of irrigated wheal. I. Uptake efficiency of preplant versus late-season application. Agron J 84: 682688 (1992).
  • 19
    Vaughan B, Westfall DG and Barbarick KA, Nitrogen rate and timing effects on winter wheat grain yield, grain protein, and economics. J Prod Agric 3: 324328 (1990).
  • 20
    Hogg TJ, Late nitrogen application to improve grain protein of irrigated durum wheat. Report, Saskatchewan Irrigation Development Center, Outlook, Saskatchewan, Canada (1995).
  • 21
    Hogg TJ and Brown S, Late nitrogen application to improve grain protein of irrigated Sceptre Durum. Report, Saskatchewan Irrigation Development Center, Outlook, Saskatchewan, Canada (1996).
  • 22
    Finney KF, Meyer JW, Smith FW and Fryer HC, Effect of foliar spraying of Pawnee wheat with urea solutions on yield, protein content, and protein quality. Agron J 49: 341347 (1957).
  • 23
    Gooding MJ and Davies WP, Foliar urea fertilization of cereals: a review. Fertil Res 32: 209222 (1992).
  • 24
    Boatwright GO and Haas HJ, Development and composition of spring wheat as influenced by nitrogen and phosphorus fertilization. Agron J 53: 3336 (1961).
  • 25
    Russell GC, Smith AD and Pittman UJ, The effect of nitrogen and phosphorus fertilizers on the yield and protein content of spring wheat grown on stubble fields in southern Alberta. Can J Plant Sci 38: 139144 (1958).
  • 26
    Holford ICR, Doyle AD and Leckie CC, Nitrogen response characteristics of wheat protein in relation to yield responses and their interactions with phosphorus. Aust J Agric Res 43: 969986 (1992).
  • 27
    Mengel K, Secer M and Koch K, Potassium effect on protein formation and amino acid turnover in developing wheat grain. Agron J 73: 7478 (1981).
  • 28
    Barraclough PB and Haynes J, The effect of foliar supplements of potassium nitrate and urea on the yield of winter wheat. Fertil Res 44: 217223 (1996).
  • 29
    Dick AC, Malhi SS, O'sullivan PA and Walker DR, Influence of interaction of barley cultivars with soil and fertilizer K and soil pH on grain yield and quality. Plant Soil 86: 265271 (1986).
  • 30
    Byers M, McGrath SP and Webster RA, A survey of the sulphur content of wheat grown in Britain. J Sci Food Agric 38: 151160 (1987).
  • 31
    Zhao FJ, Salmon SE, Withers PJA, Evans EJ, Monoghan JM, Shewry PR, et al, Responses of breadmaking quality to sulphur in three wheat varieties. J Sci Food Agric 79: 18651874 (1999).
  • 32
    Byers M and Bolton J, Effects of nitrogen and sulphur fertilizers on the yields, N and S content, and amino acid composition of the grain of spring wheat. J Sci Food Agric 30: 251263 (1979).
  • 33
    Byers M, Franklin J and Smith SJ, The nitrogen and sulphur nutrition of wheat and its effect on the composition and baking quality of the grain. Aspects Appl Biol Cereal Qual 15: 337344 (1987).
  • 34
    Wrigley CV, Du Gros DL, Archer MJ, Downie PG and Roxburgh CM, The sulfur content of wheat endosperm proteins and its relevance to grain quality. Aust J Plant Physiol 7: 755766 (1980).
  • 35
    Cressman HK and Davis JF, Sources of sulphur for crop plants in Michigan and effect of sulphur fertilization on plant growth and composition. Agron J 54: 341344 (1962).
  • 36
    Eriksen J and Mortensen JV, Effects of timing of sulphur application on yield, S-uptake and quality of barley. Plant Soil 242: 283289 (2002).
  • 37
    Allaway WH, Soil–plant–animal and human interrelationships in trace element nutrition, in Trace Elements in Human and Animal Nutrition, ed. by MertzW. Academic Press, Orlando, FL, pp. 465488 (1986).
  • 38
    House WA and Welch RM, Bioavailability of and interactions between zinc and selenium in rats fed wheat grain intrinsically labeled with 65Zn and 75Se. J Nutr 119: 916921 (1989).
  • 39
    Eckhoff SA and Paulsen MR, Maize, in Cereal Grain Quality, ed. by HenryRJ and KettlewellPS. Chapman & Hall, London, pp. 77112 (1996).
  • 40
    Pollak LM, The history and success of the Public–Private Project on germplasm enhancement of maize. Adv Agron 78: 4689 (2003).
  • 41
    Mason SC and D'Croz-Mason NE, Agronomic practices influence maize grain quality. J Crop Prod 5: 7591 (2002).
  • 42
    Penning de Vries FWT, Brunsting AHM and Van Laar HH, Products, requirements and efficiency of biosynthesis: a quantitative approach. J Technol Biol 45: 339377 (1974).
  • 43
    Tsai CY, Warren HL, Huber DM and Bressan RA, Interaction between the kernel N sink, grain yield and protein nutritional quality of maize. J Sci Food Agric 34: 255263 (1983).
  • 44
    Tsai CY, Dweikat I, Huber DM and Warren HL, Interrelationship of nitrogen nutrition with maize (Zea Mays) grain yield, nitrogen use efficiency and grain quality. J Sci Food Agric 58: 18 (1992).
  • 45
    Bhatia CR and Rabson R, Relationship of grain yield and nutritional quality, in Nutritional Quality of Cereal Grains: Genetics and Agronomic Management, ed. by OlsonRA and FreyKJ. American Society of Agronomy, Madison, WI, pp. 1144 (1987).
  • 46
    Welch LF, Effect of N, P and K on the percent and yield of oil in corn. Agron J 61: 890891 (1969).
  • 47
    Lauzon JD and Miller MH, Comparative response of corn and soybean to seed-placed phosphorus over a range of soil test phosphorus. Commun Soil Sci Plant Anal 28: 205215 (1997).
  • 48
    Sanchez CA, Burdine HW and Martin FG, Yield and quality responses of three sweet corn hybrids as affected by fertilizer phosphorus. J Fertil Issues 6: 1724 (1989).
  • 49
    Imamul Huq SM, Yield and protein quality of maize grain as affected by fertilizer application. Bangladesh J Agric 12: 169180 (1987).
  • 50
    Walker KC and Booth EJ, Sulphur research on oilseed rape in Scotland. Sulphur Agric 16: 1519 (1992).
  • 51
    Scherer HW, Sulphur in crop production: invited paper. Eur J Agron 12: 127141 (2000).
  • 52
    Fismes J, Vong PC, Guckert A and Frossard E, Influence of sulfur on apparent N-use efficiency, yield and quality of oilseed rape (Brassica napus L.) grown on a calcareous soil. Eur J Agron 12: 127141 (2000).
  • 53
    Asare E and Scarisbrick DH, Rate of nitrogen and sulphur fertilizers on yield, yield components and seed quality of oilseed rape (Brassica napus L.). Field Crops Res 44: 4146 (1995).
  • 54
    Bell JM and Shires A, Composition and digestibility by pigs of hull fractions from rapeseed cultivars with yellow or brown seed coats. Can J Anim Sci 62: 557565 (1982).
  • 55
    Marschner H, Mineral Nutrition of Higher Plants (2nd edn). Academic Press, London (1995).
  • 56
    Karvanek M, Pokorny J, Kozlowska H and Rutkowsky A, Über Rapsschrote. 5. Mitteilung: Mineralstoffe. Nahrung 8: 675680 (1964).
  • 57
    Nwokolo EN and Bragg DB, Influence of phytic acid and crude fibre on the availability of minerals from four protein supplements in growing chicks. Can J Anim Sci 57: 475477 (1977).
  • 58
    Greiner R and Jany KD, Ist Phytat ein unerwünschter Inhaltsstoff in Getreideprodukten? Getreide Mehl und Brot 50: 368372 (1996).
  • 59
    Atwal AS, Eskin NAM, MacDonald BE and Vaisy-Genser M, The effects of phytate on nitrogen utilization and zinc metabolism in young rats. Nutr Rep Int 21: 257267 (1980).
  • 60
    Marquard R, Zuchtziele bei Raps im Hinblick auf die Qualität von Rapsschrot. Fat Sci Technol. 95: 557561 (1993).
  • 61
    Lickfett T, Matthäus B, Velasco L and Möllers C, Seed yield, oil and phytate concentration in the seeds of two oilseed rape cultivars as affected by different phosphorus supply. Eur J Agron 11: 293299 (1999).
  • 62
    Jain NK, Vyas AK and Singh AK, Yield and quality of Indian mustard (Brassica juncea) as influenced by phosphorus and sulphur fertilization. Ind J Agric Sci 66: 539540 (1996).
  • 63
    Grant CA and Bailey LD, Fertility management in canola production. Can J Plant Sci 73: 651670 (1993).
  • 64
    Bole JB and Pittman UJ, Availability of subsoil sulphate to barley and rapeseed. Can J Soil Sci 64: 301312 (1984).
  • 65
    Janzen HH and Bettany JR, Sulfur nutrition of rape-seed. I. Influence of fertilizer nitrogen and sulfur rates. Soil Sci Soc Am J 48: 100107 (1984).
  • 66
    Janzen HH and Bettany JR, Sulfur nutrition of rape-seed. II. Effects of time of sulfur application. Soil Sci Soc Am J 48: 107112 (1984).
  • 67
    Malhi SS and Gill KS, Effectiveness of sulphate fertilization at different growth stages for yield, seed quality and S uptake of canola. Can J Plant Sci 82: 665674 (2002).
  • 68
    Wooding FJ, Interior Alaska crops respond to boron application. Agroborealis 17: 4749 (1985).
  • 69
    Nyborg M and Hoyt PB, Boron deficiency in turnip rape grown on gray wooded soils. Can J Soil Sci 50: 8788 (1970).
  • 70
    Bullock DG and Sawyer JE, Nitrogen, potassium and boron fertilization of canola. J Proc Agric 4: 550555 (1991).
  • 71
    Mortvedt JJ, Boron diet essential for crops. Farm Chem February: 2 (1994).
  • 72
    Malhi SS, Raza M, Schoenau JJ, Mermut AR, Kutcher R, Johnston AM, et al, Feasibility of boron fertilizer for yield, seed quality and B uptake of canola in northeastern Saskatchewan. Can J Soil Sci 83: 99108 (2003).
  • 73
    Sexton PJ, Paek NC and Shibles RM, Effects of nitrogen source and timing of sulfur deficiency on seed yield and expression of 11S and 7S seed storage proteins of soybean. Field Crops Res 59: 18 (1998).
  • 74
    Holl FB and Vose JR, Carbohydrate and protein accumulation in the developing field pea seed. Can J Plant Sci 60: 11091114 (1980).
  • 75
    Lhuillier-Soundéléa A, Munier-Jolain NG and Ney B, Dependence of seed nitrogen concentration on plant nitrogen availability during the seed filling in pea. Eur J Agron 11: 157166 (1999).
  • 76
    Fukushima D, Recent progress of soybean protein foods: chemistry, technology, and nutrition. Food Rev 7: 323351 (1991).
  • 77
    Sebastiani FL, Farrell LB, Schuler MA and Beachy RN, Complete sequence of a cDNA of a subunit of β-conglycinin. Plant Mol Biol 15: 197201 (1990).
  • 78
    Gaylor KR and Sykes GE, Effects of nutritional stress on the storage proteins of soybeans. Plant Physiol 78: 582585 (1985).
  • 79
    Paek NC, Imsande J, Shoemaker RC and Shibles R, Nutritional control of soybean seed storage protein. Crop Sci 37: 498503 (1997).
  • 80
    Kermira DAS, Dobbelt udbytte af industrikartofler. Gødningen 84: 67 (1992).
  • 81
    Kofranyi E, Die biologische wertigkeit gemischter proteine. Die Nahrung 11: 863873 (1967).
  • 82
    Eppendorfer WH and Eggum BO, Dietary fibre, starch, amino acids and nutritive value of potatoes as affected by sulfur, nitrogen, phosphorus, potassium, calcium and water stress. Acta Agric Scand B Soil Plant Sci 44: 107115 (1994).
  • 83
    Collier GF, Wurr DCE and Huntington VC, The effect of calcium nutrition on the incidence of internal rust spot in the potato. J Agric Sci Camb 91: 241243 (1978).
  • 84
    Kleinhenz MD, Palta JP, Gunter CC and Kelling KA, Impact of source and timing of calcium and nitrogen applications on ‘Atlantic’ potato tuber calcium concentrations and internal quality. J Am Soc Hortic Sci 124: 498506 (1999).
  • 85
    Kratzke MG and Palta JP, Calcium accumulation in potato tuber: role of the basal roots. Hortic Sci 121: 10221024 (1986).
  • 86
    Simmons KE, Kelling KA, Wolkowski RP and Kelman A, Effect of calcium source and application method on potato tubers and cation composition. Agron J 80: 1321 (1988).
  • 87
    Schenk MK, Nitrogen use in vegetable crops in temperature climates. Hortic Rev 22: 185223 (1998).
  • 88
    Wang Zhaohui, Tian Xiaohong and Li Shengxiu, The cause of nitrate accumulation in leafy vegetables. Acta Ecol Sin 21: 11361141 (2001).
  • 89
    Zhang CL, Gao ZM, Zhao YD and Tang WM, The effects of different nitrogen forms and their concentration combinations on the growth and quality of spinach. J Nanjing Agric Univ 13: 7074 (1990).
  • 90
    Wang Z-H and Li S-X, Effects of N forms and rates on vegetable growth and nitrate accumulation. Pedosphere 13: 309316 (2003).
  • 91
    Ombodi A, Miyoshi S and Saigusa M, Effects of band applications of polyolefin-coated fertilizers on the nitrate and oxalate content in spinach. Tohoku J Agric Res 49: 101109 (1999).
  • 92
    Gao ZM, Zhang YD, Zhang DY, Shi RH and Zhang MF, Effects of N, P and K on the accumulation of nitrate and the activity of nitrate reductase and superoxidase in leafy vegetables. Acta Hortic Sin 16: 293298 (1989).
  • 93
    Wang Zhaohui and Li Shengxiu, Effects of N and P fertilization on plant growth and nitrate accumulation in vegetables. J Plant Nutr 27: 539556 (2004).
  • 94
    Zhou Yimin, Ren Shunrong and Wang Zhengxiang, The effect of application of N fertilizer on the accumulation of nitrate in vegetables. Acta Agric Boreali-Sin 4: 110115 (1989).
  • 95
    Ruiz JM and Romero L, Cucumber yield and nitrogen metabolism in response to nitrogen supply. Sci Hortic 82: 309316 (1999).
  • 96
    Moreno DA, Víllora G and Romero L, Variations in fruit micronutrient contents associated with fertilization of cucumber with macronutrients. Sci Hortic 97: 121127 (2003).
  • 97
    Nyborg M, Solberg ED, Izaurralde RC, Malhi SS and Molina-Ayala M, Influence of long-term tillage, straw and N fertilizer on barley yield, plant-N uptake and soil-N balance. Soil Tillage Res 36: 165174 (1995).
  • 98
    Malhi SS, Grant GA, Johnston AM and Gill KS, Nitrogen fertilization management for no-till cereal production in Canadian Great Plain: a review. Soil Tillage Res 60: 101122 (2001).
  • 99
    Malhi SS, Nyborg M and Solberg ED, Influence of source, method of placement and simulated rainfall on the recovery of 15N-labelled fertilizers under zero tillage. Can J Soil Sci 76: 93100 (1996).
  • 100
    Malhi SS and Nyborg M, Effect of tillage and straw on yield and N uptake of barley grown under different N fertility regimes. Soil Tillage Res 17: 115124 (1990).
  • 101
    Gan YT, Miller PR, McConkey BG, Zentner RP, Stevenson FC and McDonald CL, Influence of diverse cropping sequences on durum wheat yield and protein in the semiarid northern Great Plains. Agron J 95: 245252 (2003).
  • 102
    Stockdale EA, Lampkin NH, Hovi M, Keatinge R, Lennartsson EKM, Macdonald DW, et al, Agronomic and environmental implications of organic farming systems. Adv Agron 70: 261327 (2001).
  • 103
    Premuzic Z, Bargiela M, Garcia A and Iorio A, Calcium, iron, potassium, phosphorus and vitamin C content of organic and hydroponic tomatoes. HortScience 33: 255257 (1998).
  • 104
    Warman PR and Havard KA, Yield, vitamin and mineral contents of organically and conventionally grown carrots and cabbage. Agric Ecosys Environ 61: 155162 (1997).
  • 105
    Warman PR and Havard KA, Yield, vitamin and mineral contents of organically and conventionally grown potatoes and sweet corn. Agric Ecosys Environ 68: 207216 (1998).
  • 106
    Starling W and Richards MC, Quality of commercial samples of organically grown wheat. Aspects Appl Biol 36: 205209 (1993).
  • 107
    Campbell CA, Selles FA, Zentner RP, McConkey BG, McKenzie RC and Brandt SA, Factors influencing grain N concentration of hard red spring wheat in the semi-arid prairies. Can J Plant Sci 77: 5362 (1997).
  • 108
    Terman GL, Ramig RE, Dreier AF and Olson RA, Yield–protein relationships in wheat grain as affected by nitrogen and water. Agron J 61: 755759 (1969).
  • 109
    Campbell CA, Davidson HR and Warder FG, Effects of fertilizer N and soil moisture on yield, yield components, protein content and N accumulation in the aboveground parts of spring wheat. Can J Soil Sci 57: 311327 (1977).
  • 110
    Campbell CA, Davidson HR and Winkleman GE, Effect of nitrogen, temperature, growth stage, and duration of moisture stress on yield components and protein content of Manitou spring wheat. Can J Plant Sci 61: 549563 (1981).
  • 111
    Wang Zhaohui and Li Shengxiu, Effects of water deficit and supplemental irrigation at different growing stage on uptake and distribution of nitrogen, phosphorus and potassium in winter wheat. Plant Nutr Fertil Sci 8: 265270 (2002).
  • 112
    Kniep KR and Mason SC, Lysine and protein content of normal and opaque-2 maize grain as influenced by irrigation and nitrogen. Crop Sci 31: 177181 (1991).
  • 113
    Ducrot C and Meffre C, Risque sanitaire toxicologique lié à l'épandage agricole des boues des stations d'épuration: synthèse bibliographique. Rev Med Vet 147: 439444 (1996).
  • 114
    Stenger A, Experimental valuation of food safety: application to sewage sludge. Food Policy 25: 211218 (2000).
  • 115
    Sharma AD, Brar MS and Malhi SS, Critical toxic ranges of chromium in spinach (Spinacia oleracia L.) plants and in soil. J Plant Nutr 28: 15551568 (2005).
  • 116
    Liu JG, Liang JS, Li KQ, Zhang ZJ, Yu BY, Lu XL, et al, Correlations between cadmium and mineral nutrients in absorption and accumulation in various genotypes of rice under cadmium stress. Chemosphere 52: 14671473 (2003).
  • 117
    Moral R, Pedreno JN, Gomez I, Palacios G and Mataix J, Tomato fruit yield and quality are affected by organic and inorganic fertilization and cadmium pollution. J Plant Nutr 19: 14931498 (1996).
  • 118
    Singh BR, Cadmium and fluoride uptake by oats and rape from phosphate fertilizers in two different soils. Norw J Agric Sci 4: 239249 (1990).
  • 119
    Sparrow LA, Chapman KSR, Parsley D, Hardman PR and Cullen B, Response of potatoes (Solanum tuberosum cv Russet Burbank) to band-placed and broadcast high cadmium phosphorus fertiliser on heavily cropped krasnozems in north-western Tasmania. Aust J Exp Agric 32: 113119 (1992).
  • 120
    Grant CA and Bailey LD, Effects of phosphorus and zinc fertiliser management on cadmium accumulation in flaxseed. J Sci Food Agric 73: 307314 (1997).
  • 121
    Willaert G and Verloo M, Effect of various nitrogen fertilizers on the chemical and biological activity of major and trace elements in a cadmium contaminated soil. Pedologie 43: 8391 (1992).
  • 122
    Grant CA, Bailey LD and Therrien MC, Effect of N, P, and KCl fertilizers on grain yield and Cd concentration of malting barley. Fertil Res 45: 153161 (1996).
  • 123
    Grant CA and Bailey LD, Nitrogen, phosphorus and zinc management effects on grain yield and cadmium content in two cultivars of durum wheat. Can J Plant Sci 78: 6370 (1998).
  • 124
    Quintana JM, Harrison HC, Nienhuis J, Palta JP and Grusak MA, Variation in calcium concentration among sixty S1 families and four cultivars of snap bean (Phaseolus vulgaris L). J Am Soc Hortic Sci 121: 789793 (1996).
  • 125
    Schonhof I and Krumbein A, Gehalt an wertgebenden Inhaltsstoffen verschiedener Brokkolitypen (Brassica oleracea var italica Plenck). Gartenbauwissenschaft 61: 281288 (1996).
  • 126
    Wang M and Goldman IL, Phenotypic variation in free folic acid content among F1 hybrids and open-pollinated cultivars of red beet. J Am Soc Hortic Sci 121: 10401042 (1996).
  • 127
    Ortiz-Monasterio I, CGIAR Micronutrients Project, Update (No. 3). International Food policy Research Institute, Washington DC (1998).
  • 128
    Graham RD, Senadhira D, Beebe SE, Iglesias C and Ortiz-Monasterio I, Breeding for micronutrient density in edible portions of staple food crops: conventional approaches. Field Crops Res 60: 5780 (1999).
  • 129
    Goto F, Yoshihara T, Shigemoto N, Toki S and Takaiwa F, Iron fortification of rice seeds by the soybean ferritin gene. Nat Biotechnol 17: 282286 (1999).