The objectives of the investigation were to relate yield and P uptake of orchard grass grown on Davidson clay loam to P fertilization and to determine the fate and availability of applied and native P in this soil. A further aim of the investigation was to elucidate mechanisms controlling the availability of P to plants in Davidson clay loam that was limed to several pH values. These objectives were accomplished by a study of orchard grass yield data obtained during a long term field investigation and by performance of greenhouse and laboratory experiments. Annual surface application of 21 lb P/A increased orchard grass yield during the initial phase of the field experiment, from 1948 to 1957. Thereafter, from 1958 to 1962, continued application of 21 lb P/A no longer increased yield. Lack of yield response to P application was attributed to plant uptake of residual P. The conclusion was supported by data from a greenhouse study. In the study, P application did not significantly affect yield or P uptake of alfalfa grown on Davidson clay loam that had received annual application of 21 lb P/A for 15 years. Total inorganic P analysis indicated that the major portion of surface applied P remained in the 0-2 inch soil layer. However, statistical analysis of the total inorganic P data showed a significant downward movement of P into the 4-6 inch soil layer. Fractionation analysis indicated the applied and native P was present in Davidson clay loam predominantly as Fe-P. Iron-P made up greater than 70% of the total inorganic P at the 0-2, 2-4, 4-6 and 6-12 inch soil depths. High accumulation of applied P as Fe-P in Davidson clay loam was explained on the basis of the high free Fe oxide content of the soil. Application of either lime or P increased yield and P uptake of alfalfa grown on Davidson clay loam which received annual application of 0 or 10.5 lb P/A for 15 years. From these results it was concluded that liming increased the availability of native and residual P in the soil. Fractionation analysis indicated that the Fe-P fraction decreased to a greater extent than Al-P or Ca-P during growth of alfalfa on the limed soil. These data indicated that liming increased the availability of the Fe-P. Iron P decreased to a greater extent than Al-P or Ca-P during growth of alfalfa on Davidson clay loam that received annual application of O, 10.5 and 21 lb P/A for 15 years. Coefficients of simple correlation for the data, showed a higher correlation between Fe-P than Al-P or Ca-P with P uptake by alfalfa plants. From these data it was concluded that Fe-P was more important than Al-P or Ca-P in supplying P to plants grown on Davidson clay loam. Higher uptake of P from Fe-P rather than from Al-P or Ca-P was explained on the basis of the greater amount of Fe-P in Davidson clay loam. It is suggested that a large percentage of the Fe-P fraction in the soil consists of relatively soluble amorphous compounds. The presence of relatively soluble amorphous Fe-P as well as the high percentage of this compound in the soil then may account for Fe-P being more available than was anticipated. / Ph. D.