Reaction of surface-applied superphosphate with soil. I. The fertilizer solution and its initial reaction with soil

Abstract

The phosphorus and calcium contents of the fertilizer solution from surfaceapplied superphosphate in contact with moist soil were shown to be similar to those of the metastable triple-point solution (MTPS) produced when an excess of monocalcium phosphate is shaken with water. The average phosphorus, calcium, and sulphur concentrations of saturated aqueous extracts of superphosphate at 20°C were 4.0M, 1.5M , and 0.023M respectively. The pH of these extracts ranged from 1.60 to 1.92 (0.15-0.47 units higher than that of MTPS). The sulphur content of the fertilizer solution was generally one-fiftieth to one-hundredth of the phosphorus content. In the absence of leaching both diffusion and capillarity were involved in the movement of phosphate from superphosphate particles into moist soil. The fertilizer solutes entered the soil mainly by diffusion but on soils of low moisture content the increase in pore size of the fertilizer particles, which resulted from the dissolution of monocalcium phosphate, led to a reversal of the suction gradient between soil and particle which caused movement of some of the fertilizer solution into the soil by capillarity. Leaching of superphosphate applied to dry soils by rainwater is likely to yield fertilizer solutions which have higher pH, lower phosphorus and calcium contents, and higher sulphur contents than the saturated solutions arising from superphosphate particles in contact with moist soil. When synthetic fertilizer solution reacted with acid soils the rate of removal of phosphate and calcium from solution depended upon the phosphate sorption capacity of the soil. In calcareous soil the calcium carbonate played a major part in phosphate precipitation. Appreciable amounts of iron and particularly aluminium were dissolved from each of the soils studied by the synthetic fertilizer solution to give solutions which, on standing, readily formed precipitates of phosphate. Both sulphate and organic sulphur compounds were displaced from the acid soils by the fertilizer solution but, in the calcareous soil, sulphate was coprecipitated with the phosphate.