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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Phosphorus Fertilizer Effects on Soil Phosphorus Pools in Acid Upland Soils

^a Dept. of Agronomy and Horticulture, Univ. of Nebraska, P. O. Box 830915, Lincoln, NE 68583-0915
^b Univ. of Hawaii–IRRI Collaboration, Soil, and Water Sciences Division, International Rice Research Institute (IRRI) DAPO Box 7777, Metro Manila, Philippines
^c Flurstr. 240, 22549 Hamburg, Germany

* Corresponding author (t.george{at}cgiar.org)

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-P_o and residual-P fractions across sites. Phosphorus fertilization increased Mehlich-1 P, resin-P, NaOH-P_i, H₂SO₄-P, RAQ-P₁, and RAQ-P₁₄ 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 H₂SO₄-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.

Abbreviations: BC, buffer coefficient • IRRI, International Rice Research Institute • LTPE, Long-term P Experiments • PDSS, P decision support system • RAQ, resin adsorption quantity • SD, standard deviation

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