A chemical study of a soil under long-continued field experiments

The application of relatively large quantities of superphosphate, rock phosphate, and basic slag over a long period of years created in the surface soil a large supply of residual phosphorus. The phosphorus applied to the soil in the form of rock phosphate had at the end of a 27-year period a much higher solubility than superphosphate or basic slag phosphate. The penetration of applied phosphorus into the subsurface soil might be said to be negligible on the soil under consideration. Eight tons of lime increased the solubility of the native phosphorus in the soil, while 3 tons of limestone applied over a relatively short period of years had but little effect on the solubility of the native phosphorus. The Neubauer values indicated that the 8 tons of lime depressed the availability of rock phosphate and increased the availability of superphosphate and slag phosphate. This soil was low in replaceable potassium and the Neubauer method showed it to be low in available potassium, although it is relatively high in total potassium. The 8 tons of lime depressed the availability of potassium as determined by the Neubauer method, and tended to give lower values for replaceable potassium as determined by the chemical method. The reaction of the untreated soil was pH 5.0. The soil to which 8 tons of lime were added had a pH ranging from 6.3 to 7.0 and where 3 tons of lime had been applied the pH ranged from 5.9 to 6.3. The various phosphates caused some variation in soil reaction. The lime applications increased the replaceable calcium values. The heavy lime treatment showed little, if any, increase in replaceable magnesium, while light lime maintained the replaceable magnesium values considerably above that of the untreated check. The total nitrogen and organic matter values were maintained at a slightly higher level on the soils with the light application of lime as compared with the heavy application. The wheat grain yields and the total phosphorus content of the grain from the unphosphated plats coincided with the phosphorus solubility values of the soils from these plats. The phosphorus content of the second year spring growth of sweet clover indicated that the heavy liming apparently aided the assimilation of phosphorus from superphosphate and basic slag and tended to depress the assimilation of phosphorus from rock phosphate.