Effect of available phosphorus in southern soils upon crop yields

Greenhouse studies were made by growing two crops of sagrain on 12 different soils, representing four samples of Susquehanna, Ruston, and Orangeburg fine sandy loams. The available P(2)O(5) in each soil, which had received different phosphate treatments, was determined by the Truog method. Susquehanna fine sandy loam, a poor soil in the field, gave as high yields in the greenhouse as Ruston and Orangeburg fine sandy loams, two excellent agricultural soils. All of the soils studied in the greenhouse contained less than 6 ppm available P(2)O(5), and 11 of the 12 responded to the first phosphate applications, but none responded to heavier applications. Sagrain yielded as well on soils with 10 ppm available P(2)O(5) as on those with 40 ppm. Maximum yields were obtained on many soils with only 8 ppm (16 pounds per acre) available P(2)O(5). Field studies were made by growing cotton on 10 different soils for five years. Soil samples were obtained from the 4-8-4 and 4-0-4 treated plats and available P(2)O(5) was determined by the Truog method. The 10 soils varied greatly in their response to phosphorus. Those containing less than 6 ppm available P(2)O(5) gave excellent response; those containing from 6 to 15 ppm gave some response; but those containing more than 15 ppm gave very little or no response to phosphorus, although available P(2)O(5) was greatly increased by the phosphate application. Cotton and sagrain failed to respond to phosphate applications on soils containing 15 ppm or more available P(2)O(5), which indicates that under southern conditions crops do not require large quantities of phosphorus. Most southern soils do not contain 15 ppm available P(2)O(5) and require phosphorus, but applications should not be made without determining the available P(2)O(5) present. It is believed that when nitrogen and potassium are limited and phosphorus fixation is at a minimum, phosphate recommendations for cotton and sorghum may be made on the following basis: Soils containing less than 6 ppm available P(2)O(5) require liberal applications, those containing from 6 to 15 ppm require light applications, but those containing more than 15 ppm require very little or no phosphorus.