Reaction of surface-applied superphosphate with soil. II. Movement of the phosphorus and sulphur into the soil

Abstract

Phosphate moving into moist soil from particles of surface-applied superphosphate penetrated a hemispherical zone beneath the particle. The size of this zone and the distribution of phosphate through it were governed by the phosphate sorption capacity of the soil, the size of the particle, and the soil moisture. In calcareous soils the calcium carbonate content was important in determining phosphate penetration. Phosphate from superphosphate particles penetrated rapidly into moist (100 cm tension) soil during the first 5 or 6 days after contact and continued to move through the soil at a decreasing rate throughout the whole of an 8-week period. Movement of sulphate from surface-applied superphosphate into soil was governed by the particle size of the fertilizer and the soil moisture, both of which influenced the rate of solution of the calcium sulphate. The distribution of the sulphate through the soil was influenced by the sulphate sorption capacity of the soil. The rate of penetration of sulphate from surface-applied superphosphate into calcareous soil was considerably less than when the superphosphate was applied to acid soils. Leaching influenced the movement of both phosphorus and sulphur into the soil. It caused distortion of the hemispherical distribution of phosphate beneath superphosphate particles, resulting in deeper penetration of phosphate below the particles and smaller horizontal movement away from them. It also greatly enhanced the solution of sulphate, especially from small particles. The results indicate that the optimum particle size of superphosphate will vary from soil to soil, but a 2.5 mm diameter particle is likely to satisfy a wide range of soil and environmental conditions.