Fate of applied phosphate and sulfate in weathered acid soils under leaching conditions

Abstract

The ability of a soil to sorb ionic forms of phosphorus (P) and sulfur (S) is a major determinant of the movement of sulfate and phosphate ions in the profile. Earlier research is equivocal concerning the effect of concurrent applications of phosphate and sulfate on soils. Any interaction could have significant consequences for fertilizer management. An experiment was conducted with two contrasting acid soils in open PVC columns. The soils were a gleyed podzolic (Aquic Haplustalf) (GP) (30 µg P g(^-1)) soil adsorbed at a soil solution concentration of 0.2 µg P mL(^-1)), 13 µg S g(^-1)) soil adsorbed at a soil concentration of 5 µg S mL(^-1))), and a red earth (Haplohumult) (RE) (30 µg P g(^-1)) soil, 48 µg S g(^-1)) soil adsorption). A lime x P x S factorial combination was applied to simulate residual P and a topdressing of ³²P- and ³⁵S-labelled fertilizer was made as a recent application. The columns were watered to 125% field capacity, at 3 day intervals, until no labelled S or P appeared in the leachate. There was considerable movement of applied P downward in the column of the low-sorbing GP soil. At the end of nine intermittent leachings, the ³²P-labelled P recently applied to the surface had moved to the second soil layer (7.5-15 cm), thereby increasing the Colwell P content of this layer from 16 to between 40 and 50 µg P g(^-1)) soil. If the soil had been previously treated with P, the recently applied P was leached to the third (15.0-22.5 cm) and fourth (22.5-30 cm) soil layers. In contrast, applied P moved little in the medium-sorbing RE soil. Lime and S application also had no effect on the distribution of applied P in the coil columns. The amounts of recently applied P fertilizer lost by leaching from the RE were less than 2 µg P column^-1) of less than 2% of the native+fertilizer P present in the leachate. The S, applied as Na₂SO₄, was completely leached out of the GP soil by the nine intermittent leachings irrespective of lime or P treatment. The losses of ³⁵S-labelled S recently applied to the surface (>70%) occurred mostly with the second and the third leachings. In the RE soil, a previous application of S resulted in a loss of 34-57% of the applied S after 13 intermittent leachings. Most of his S was lost during the leaching episodes. The application of 98 mu g S g(^-1)) soil as a recent S application to the soil surface resulted in increased losses of about 74-121 µg S g(^-1)) soil or 37-60% of the applied S. If the soil had a previous S treatment, the S leached amounted to between 36% and 52% of the total S applied. Liming enhanced leaching losses of applied S, and P application only increased S losses in the limed treatment.