The availability of soil phosphate. I.—Examination of the soil solution with special reference to the fixation of phosphate

The effects of fertilizer treatments and of season on the constituents of the soil solution, with special reference to phosphate, have been examined, using soils tested as part of a long-term fertilizer-experiment. The concentration of inorganic phosphate in the soil solution in phosphate-treated plots was higher than that of the control throughout the year, the increases ranging from 21-5 to 66.6%. The effect of superphosphate did not increase during cropping; the concentrations of inorganic phosphate in the soil solution were low in summer and varied very little through the year. Ammonium sulphate and lime reduced the inorganic-phosphate concentration. In plots receiving dung the concentrations of inorganic phosphate were unusually high, the increases ranging from 120 to 635% in the 0–8-in. soil layer. In the presence of lime the increases of phosphate due to dung were lower. There was no indication of the downward movement of phosphate through the soil. The concentration of organic phosphorus in the soil solution decreased during the cropping season. Ammonium sulphate, superphosphate and potassium sulphate tended to increase the organic phosphorus in soil solutions but lime tended to reduce its accumulation in July. Dung did not increase the organic phosphate greatly, especially in the presence of lime. Simple and partial correlations between inorganic phosphate and other constituents in the soil solution were studied. Inorganic phosphate examined in April was negatively correlated with both pH and calcium. The relationship with pH was most clearly observed in plots without phosphate treatment and that with calcium in the superphosphate-treated plots. The relationship was unchanged when the correlation coefficients were calculated between phosphate and calcium with pH or nitrogen constant, and between phosphate and pH with calcium or nitrogen constant. There was no significant correlation in the July and September samples. Colloidal absorption of phosphate appears to dominate the fixation of phosphate in the soil although there is evidence of chemical precipitation by calcium.