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‐P 1 and RAQ‐P 14 ). Without P addition, 84% of the total P was in the NaOH‐P o and residual‐P fractions across sites. Phosphorus fertilization increased Mehlich‐1 P, resin‐P, NaOH‐P i , H 2 SO 4 ‐P, RAQ‐P 1 , and RAQ‐P 14 across sites, whereas NaOH‐P o and residual‐P were unchanged. The sum of resin‐P and NaOH‐P i 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 H 2 SO 4 ‐P varied among sites. Oxalate‐extractable Fe accounted for most of the variation 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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