Phosphorus pools in sulfuric acid sulfate soils: influence of water content, pH increase and P addition

Abstract

Acid soils often have low P availability limiting plant growth, which is addressed by application of inorganic fertilizers and neutralizing agents. However, little is known about the effect of addition of inorganic P and neutralizing agents on P pools in acid sulfate soils under submerged or moist conditions. Sulfuric acid sulfate soil (pH < 4) was amended with two neutralizing agents (NaOH or Ca(OH)2) to achieve soil pH 4 or 5.5, without or with addition of inorganic P equivalent to 20 kg ha−1. Soils were incubated at 25 °C in either submerged or moist conditions (100% of maximum water holding capacity). After 2 weeks, soil P pools (labile P, moderately labile P, non-labile P and residual P) and Fe and Al oxides were determined. Adjustment of pH had little effect on the measured parameters. Labile, moderately labile and non-labile P pools were higher with P addition than without P addition. With P addition, labile and non-labile P pools were up to twofold higher in submerged incubation than in moist incubation. Labile P, non-labile P and residual P represented 70%, 15% and 15% in submerged incubation and 40%, 40% and 30% in moist incubation, respectively. In submerged incubation, Fe oxides were higher in soils amended with neutralizing agents than in the original soil which can be explained by the higher pH. A high proportion of added P was available after 2 weeks of application particularly in submerged incubation. The pH increase had little effect on P availability.