Assessment of methods for studying the dissolution of phosphate fertilizers of differing solubility in soil

Abstract

The dissolution of three phosphate fertilizers, ground (<150µm) North Carolina phosphate rock (NCPR), NCPR 30% acidulated with phosphoric acid (NCPAPR), and monocalcium phosphate (MCP) were studied using six chemical extraction methods, 0.5M NaOH followed by 1M HCl extractable P, 0.5M BaCl₂ (buffered at pH = 8.1 with triethanolamine, BaCl₂/TEA) extractable Ca, Olsen P, Bray I P and water extractable P. Two soils were used, Tekapo fine sandy loam and Craigieburn silt loam. Extractions were made after the fertilizers were incubated with the soils at the rates of 75,150 and 750µg P g⁻¹ soil for 1, 8, 24, 51 and 111 days. Percentage dissolution of the PR-containing fertilizers was found to differ significantly between the extractants, 0.5M NaOH, 1M HCl and 0.5M BaCl₂/TEA. These differences in estimated dissolution rates between methods were attributed to differences in the recovery rates of P or Ca between methods, which depended on the type of the extractant, soil P-retention capacity or Ca-saturation, and on the fertilizer application rate. No one method was found to be clearly better than others in studying PR dissolution in soils. The 0.5M NaOH extractable P was poorly related to water extractable P (R² = 0.55 and 0.13 in Tekapo and Craigieburn soils respectively), Olsen P (R² = 0.88 and 0.78) and Bray I P (R²= 0.88 and 0.78).

The average rates of PR dissolution measured by the isotopic method (F_in) were higher than those measured by 0.5M NaOH, 1M HCl and 0.5M BaCl₂/TEA methods for the periods of 1–50 and 50–111 days of soil-fertilizer contact. The descrepancy was attributed to a plant effect on PR dissolution and to a recycling effect of fertilizer P (i.e. fertilizer P which had been transformed to slowly exchangeable forms, during incubation, fluxing back to exchangeable P pool, during the labelling trial) both accounted for by F_in values but not by extraction-derived values. The changes of water extractable P, Bray I P and Olsen P with incubation time were in agreement with those predicted by F_in and F_out values. This suggests that F_in and F_out are two important parameters indicating the rates of phosphate release to and retention from plant available P pool.