A COMPARISON OF SEQUENTIAL EXTRACTION PROCEDURES FOR MEASURING PHOSPHATE ROCK RESIDUES IN SOILS

Laboratory studies have been undertaken to evaluate sequential extract ion procedures to measure phosphate rock (PR) residues in a range of t emperate (New Zealand) and tropical (Indonesian) soils. The amounts of soil and PR-P were measured by using a sequential procedure which inv olved pre-extraction with either 1 M NaCl or 0.5 M NaCl/TEA (pH 7), fo llowed by 1 M NaOH and mineral acids. The acids included varying conce ntrations of HCl or H2SO4 (0.5-4 M) and a tri-acid mixture of HCl, HNO 3 and HClO4 (5:5:7). Amounts of PR residues in the soil were estimated from increases in amounts of the acid-extractable P (DELTAacid-P) or Ca (DELTAacid-Ca) in the PR-treated over untreated soil. Amounts of PR -P residues (measured by DELTAHCl-P) in New Zealand soils varying in p H- and Ca levels were completely recovered (-5%) when the soils were p re-extracted with 0.5 M NaCl/TEA, rather than 1 M NaCl (or no pre-extr action). Pre-extraction of acid soil/PR mixtures with 1 M NaCl caused dissolution of PR during the extraction. Although the DELTAHCl-Ca also estimated the amount of PR residues well, particularly after a 0.5 M NaCl/TEA pre-extraction, this method was less sensitive than the DELTA HCl-P method, particularly in the soils with high native Ca saturation . In contrast to the high recovery obtained from the New Zealand soils , the recovery of the PR-P residues in the Indonesian soils in the HCI extracts following the 0.5 M NaCl/TEA and 1 M NaOH extractions were l ow ranging from 62 to 86%. Although increasing the concentration of HC l (upto 4 M) did not cause any improvement in the recovery of PR-P, ex traction with H2SO4 achieved complete recovery of PR-P. For all soils, a sequential extraction involving 0.5 M NaCl/TEA-1 M NaOH-0.5 M H2SO4 is recommended for measuring PR residues in pairs of PR-treated and u ntreated field or laboratory incubated soils.