Phosphorus fertilizer effects on soil phosphorus pools in acid upland soils
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Opinions vary on fertilization strategies in part because of uncertainties in methods assessing P supply across sites. We quantified the fate and extractability of fertilizer P after two to four crops with four to five P levels applied to upland rice (Oryza sativa L.)-soybean [Glycine max (L.) Merr.] rotations in three experiments in Asia. Soil P pools were measured by Mehlich-1 extractant, a modified Hedley fractionation and by mixed-bed resin capsules after 1 and 14 d (resin adsorption quantity, RAQ-P-1 and RAQ-P-14). Without P addition, 84% of the total P was in the NaOH-P. and residual-P fractions across sites. Phosphorus fertilization increased Mehlich-1 P, resin-P, NaOH-P-i, H2SO4-P, RAQ-P-1, and RAQ-P-14 across sites, whereas NaOH-P-0. and residual-P were unchanged. The sum of resin-P and NaOH-P, increased from 10% to between 20 and 30% of the total soil P. Mehlich-1 P and resin P increased similarly across sites and fitted quadratic models: the increase in Mehlich-1 P (mg kg(-1) per kg P ha(-1)) ranged from 0.050 at low P rates to >0.125 at >400 kg P ha(-1). The increases per unit P of RAQ-P, NaOH-P-i, and H2SO4-P varied among sites. Oxalate-extractable Fe accounted for most of the variations in NaOH-P-i and RAQ-P. Changes in soil P pools in tropical upland Oxisols and Ultisols following P addition are likely better reflected by NaOH-P-i and RAQ-P than Mehlich-1 P and resin P. Improvements in soil P tests are needed to better discriminate the changes in P pools from fertilization across soils.
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