Effects of NPK and biochar fertilized soil on the proximate composition and mineral evaluation of maize flour

Abstract Series of farming practice methods have been employed to increase maize production but there is no adequate information on the effect of these methods on the nutritional and mineral content of organically grown maize. This study investigated the effects of inorganic and biochar fertilized soils on the proximate composition and mineral content of maize. Maize seeds were planted on organically fertilized soil (sawdust and gliricidia biochar), chemically fertilized soil Nitrogen Phosphorus and Potassium (NPK fertilizer), and soil without any amendment as control. The proximate compositions (protein, ash, crude fat, carbohydrate, and moisture) and mineral contents (Na, Mg, K, Ca, Fe, and Zn) of the maize flour samples were determined using standard methods. The results showed that protein content ranged from 4.58% to 7.24% (protein), ash 0.82% to 1.09%, crude fat 3.84% to 4.61%, moisture 9.76% to 10.60%, and carbohydrate 76.85% to 80.31%. There was no significant (p ≤ 0.05) difference among the proximate compositions except for protein and carbohydrate. Maize planted on NPK fertilized soil had the highest crude protein content of 7.24%. Other results obtained included sodium (55.65 mg/100 g), magnesium (35.87 mg/100 g), and iron (6.78 mg/100 g). Maize from soil without amendments was significantly higher than maize from NPK fertilized and biochar fertilized soils. Also, maize from control plot had the highest calcium content value of 48.95 mg/100 g. We concluded that maize planted with NPK fertilizer had higher nutrient than those planted with biochar application. Also, the mineral content of maize planted in control plot was higher than those on the amended soil.


| INTRODUC TI ON
Maize serves as the main source of dietary energy in Nigeria apart from cassava flakes, rice, wheat, and sorghum. As a multipurpose crop, it can be used in fuel making (ethanol), as feeds for animals (poultry and livestock) and as foods (e.g., agidi, eko, ogi, tuwo, mosae.t.c.) Maize had been reported to have great nutritional value and can be used as raw material for producing many industrial products (Afzal, Nazir, Bashir, & Khan, 2009). Johnson (2000) reported that maize (Zea mays) is the second most widely produced cereal crop worldwide which is produced in the entire world except Antarctica. Maize remains an important part of human diet in many developing countries.
Biochar is a carbon-rich coproduct resulting from pyrolyzing biomass under high-temperature, low-oxygen conditions for biofuel production (Laird, 2008;Lehmann, 2007) and although it is similar to other charcoals, biochar is defined by its intentional application to the soil for environmental applications (Lehmann, 2009). It contains highly condensed aromatic structures that resist decomposition in soil and thus can effectively sequester a portion of the applied carbon for decades to centuries (Lehmann, Gaunt, & Rondon, 2006).
Biochar has been reported to increase the emergence of maize in the field based on its porous nature; it helps to retain soil moisture for a longer period while increasing the relative and absolute growth rates of maize due to its increase mineral availability by increasing the cation exchange capacity (Peng, Ye, Wang, Zhou, & Sun, 2011). Positive effect of biochar as organic fertilizer on crop yield was reported to be mainly attributed to its own nutrients and indirect fertility, thus making it to be referred to as soil fertilizer and soil conditioner, respectively (Glaser, Lehmann, & Zech, 2002). Biochar has been considered as a key input for rising and sustaining production and simultaneously reducing pollution and dependence on fertilizers (Barrow, 2012).
Recently, health conscious consumers are interested in optimizing the nutritional composition of food with minimal chemical residues on foods produced through environmentally friendly agricultural practices (Amujoyegbe, Opabode, & Olayinka, 2007).
Various methods of organic soil fertility have been reported to increase the agronomical yield of crops; however, there is a paucity of information on the food quality of the produced through soil fertility by application of organic fertilizer (such as biochar) used in crop production. The study was focused on determining the effects of NPK and biochar fertilized soil on food quality of maize.

| Maize planting
The experimental site was located in an abandoned agricultural farm overgrown with Imperata cylindrical with a few scattered pawpaw and plantain stands and trees within the University Teaching and Research Farm. The soil is an Oxisol (Aubert & Tavenier, 1972; FAO/ UNESCO, 1997). The site was weeded manually and stumped before plowing. The soil within the experimental plot was relatively uniform and was randomly sampled at 10-m interval at the predetermined depth of 0-30 cm. The experiment was laid out in randomized complete block (RCB) design with three replicates. There were three blocks separated from one another by a 3-m ride. Each block was made up of eight plots, each plot measuring 4 × 4 m, and was separated from one another (within the block) by a 2-m space. Four seeds of improved maize variety, OBA SUPER-2 Hybrid, were sown at a spacing of 75 cm between rows and 50 cm within the row. The emergent seedlings were thinned to two per stand 2 weeks after germination. The maize plants were rain-fed throughout the experimental period. At maturity, which was approximately 12 weeks, the plants were harvested. All the 12 plants contained within a subplot measuring 1 by 0.75 m located in the middle of each plot were harvested.

| Soil analysis
The site was weeded manually and stumped before plowing. The soil within the experimental plot was relatively uniform and was randomly sampled at 10-m interval at the predetermined depth of 0-30 cm. A composite soil sample made up of 10 auger points was then produced. The sample was air-dried, sieved though a 2-mm sieve, and analyzed for selected key physico-chemical properties (Table 1). The soil reaction (pH) was determined potentiometrically in soil: water ratio of 1:2 using a Kent model 720 glass electrode (pH) meter. Organic carbon was determined using the Walkley and Black (1934) chromic acid digestion procedure. Total N was determined using the method of Keeney and Bremner (1966)

| Biochar production
The feedstocks were converted separately to biochars by heating in an engineered gas-ignition pyrolyser. The average temperature within the pyrolyser was 400°C. Key chemical properties of the two biochars were determined using routine procedures (IITA (International Institute of Tropical Agriculture), 1982) while their humic substances content was exhaustively extracted with 0.1 M NaOH solvent and fractionated according to Fagbenro (1988). These properties are presented in Table 2.

| Method of fertilizer application
The two biochars were applied at the rate of 2.5 t/ha, while NPK 15:15:15 inorganic fertilizer was applied at the rate of 90 kg N ha −1 .
The organic and inorganic amendments were applied to the soil by broadcasting them evenly within each plot and then mixed manually with soil using a hoe.

| Maize flour production
Modified method of Houssou and Ayernor (2002) was used to prepare maize flour. Yellow maize kernels were sorted to remove stones, dirt, and other foreign materials. Maize kernels were milled by means of Waring Blender HGBTWO, USA, into flour as shown in Figure 1.

Maize flour samples were triplicated and labeled A-D where Sample
A is maize planted on normal soil (control), Sample B is maize planted on NPK fertilized soil, Sample C is maize planted on sawdust biochar (SB), and Sample D is maize planted on GB.

| Proximate composition and minerals
Proximate analysis

Mineral analysis
The samples were ashed at 550°C. The ash was boiled with 10 ml of 20% hydrochloric acid in a beaker and then filtered into a 100 ml standard flask. This was made up to the mark with deionized water.
Sodium (Na) and potassium were determined using the standard flame emission photometer. NaCl and KCl were used as standards (AOAC, 2005). Calcium (Ca), potassium (K), magnesium (Mg), and iron (Fe) were determined using Atomic Absorption Spectrophotometer (AASmodel SP9). All values were expressed in mg/100 g.

| RE SULTS AND D ISCUSS I ON
The result of the proximate compositions of maize flour samples is in Table 3; the percentage mean crude protein content was in the range of 4.58%-7.24%. There was significant difference among the samples. Maize planted with NPK fertilizer was significantly higher in protein than in the other maize samples. This is in agreement with the report of Matt, Rembialkowska, Luik, Peetsmann, and Pehme Results showed that the use of NPK fertilizer or biochar treatment did not affect the crude fat content of the maize sample but the maize treated with NPK fertilizer had the highest crude fat content compared to maize planted on normal soil. The percentage crude fat obtained in this report was in agreement with other researchers (Matilda, Einar, Rune, & Kjarten, 1993;Ndukwe, Edeoga, & Omosun, 2005).
There was no significant difference (p < 0.05) among the samples in terms of the moisture content with maize treated with GB having the highest moisture content of 10.60%, while maize treated with SB had the lowest moisture content value of 9.76%. This result was similar to what was reported on moisture content of maize products (9%-19%) by Trabelsi, Kraszewski, and Nelson (1998). However, it was observed that the treatment of maize sample with normal soil, NPK fertilized soil, and biochar fertilized soil, respectively, resulted in no differences in moisture content. The low moisture content in maize sample treated with SB soil serves as an indication that it will have higher storability; this will minimize fungal contamination and spoilage of the maize flour.

| Minerals
The mineral content of the maize flour samples is presented in

| CON CLUS ION
The research work has been able to show the effect of soil treatments methods on the quality of maize samples. There is significant difference in the protein content of the treated maize samples, with maize soil-treated with NPK fertilizer having the highest. There was no significant difference in ash, moisture, and crude fat contents in the maize samples. Mineral content of maize planted in control plot was higher than those on the amended soil. Maize samples fertilized with GB also had the highest quantity of essential minerals.
However, SB and GB did not affect the proximate composition and mineral content of maize flour.

ACK N OWLED G M ENTS
None.

CO N FLI C T O F I NTE R E S T
All authors have no conflict of interest to report.