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Extraction with 0.01 m CaCl2 underestimates the concentration of phosphorus in the soil solution



The aim of this paper was to compare the concentration of P in soil extracts prepared with water and a ‘soil solution proxy’ (‘SSP’, that is, a salt solution similar in ionic composition and strength to the actual soil solution) with that in 0.01 m CaCl2 extracts, which is usually taken as a measure of soil P intensity. Seventy widely ranging agricultural soils from the Mediterranean part of Spain were used. Soil/solution ratio was 1:10 and extraction time 3 days. For 0.01 m CaCl2, a short extraction time of 30 min was also used as the reference method. CaCl2-P(3 days) and CaCl2-P(30 min) were not significantly different for the 40 noncalcareous soils group, but CaCl2-P(3 days) was significantly larger than CaCl2-P(30 min) for the 30 calcareous soils group. The Water-P/CaCl2-P(30 min) ratio was not significantly related to any soil property, its mean being 6.3 for the noncalcareous and 5.8 for the calcareous soils group. The mean SSP-P/CaCl2-P(30 min) ratio was 2.6 for the noncalcareous and 3.1 for the calcareous soils group, and decreased slightly with increasing ionic strength of the soil solution in the noncalcareous soils group. These results are consistent with the promoting influence of the Ca ion and ionic strength on P adsorption by permanent-charge soils. The fact that extraction with 0.01 m CaCl2 generally results in underestimation of the actual concentration of P in the soil solution should be considered when CaCl2-P is used as a soil P test.