Phosphorus Fertilizer Effects on Soil Phosphorus Pools in Acid Upland Soils

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-P1 and RAQ-P14). Without P addition, 84% of the total P was in the NaOH-Po and residual-P fractions across sites. Phosphorus fertilization increased Mehlich-1 P, resin-P, NaOH-Pi, H2SO4-P, RAQ-P1, and RAQ-P14 across sites, whereas NaOH-Po and residual-P were unchanged. The sum of resin-P and NaOH-Pi 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-Pi, and H2SO4-P varied among sites. Oxalate-extractable Fe accounted for most of the variation in NaOH-Pi and RAQ-P. Changes in soil P pools in tropical upland Oxisols and Ultisols following P addition are likely better reflected by NaOH-Pi 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.