The distribution, availability, and nature of the phosphates in certain Kentucky soils

This investigation was undertaken for the purpose of determining the nature and availability of both the native and applied phosphates in the soils of six Kentucky soil experimental fields. The influence of lime on the availability of the phosphates was also studied. The distribution or localization of the native and applied phosphates in the fractions of soil separates was determined for a representative soil for the purpose of ascertaining in what fraction or fractions of the soil separates the fixation of the applied phosphate occurs. For the relatively available phosphorus, carbonated water and N/500 sulfuric acid buffered to a pH of 3.0 were used as solvents. Very small amounts of relatively available phosphorus were extracted from the soils of the check plats. These amounts were roughly proportional to the total phosphorus which indicates that the native phosphates are essentially of one kind in these soils. Prolonged extraction of the soils gave a solubility of the native phosphates approximately equivalent to that of dufrenite. The use of lime did not seem to influence the availability of the native phosphates. The availability of superphosphate varies with the capacity of soils to fix phosphates in relatively insoluble and unavailable forms. On this basis, the soils studied may be divided into two groups. The first group of low capacity in this respect includes the DeKalb silt loam at Berea, the Decatur silt loam at Russellville, and the Memphis silt loam at Mayfield. The second group of high capacity in this respect includes the Tilsit silt loam at Greenville, the silt loam at Campbellsville derived from Waverly shale, and the silt loam at Fariston derived from Eastern Coal Basin shale. The use of lime reduces the rate of fixation into relatively insoluble phosphates in all of the soils, but equal applications of lime are much more effective in this respect on soils of the first group than of the second group. The use of lime reduces the rate of solution of rock phosphate in all of the soils. This reduction is greater in soils of the first group of low capacity to fix phosphorus in relatively insoluble forms than in soils of the second group of high capacity in this respect. Lime exercises a positive influence on crop response to rock phosphate on soils of the first group and a negative or neutral influence on soils of the second group. In the soils of the second group, where a high capacity exists for fixing phosphates in relatively insoluble forms, the rate of solution of rock phosphate is also high, but a deficiency in readily available phosphorus exists, due to its rapid fixation as relatively insoluble and unavailable phosphates. The use of lime reduces the availability of the manganese in all of the soils studied. No correlation exists, however, between the reduction in available manganese and the positive or negative influence of lime on crop response to rock phosphate. The great reduction in the availability of manganese in the case of the Fariston soil, where 6 tons of lime were applied, suggests that manganese deficiency may be responsible in part for the negative influence of lime on the phosphates where very large applications of lime have been made. Fixation of phosphates in soils occurs largely in the fine clay fraction, but appreciable amounts are also fixed in the coarse clay and silt fractions.