Losses of phosphate from a light-textured soil in Alabama and its relation to some aspects of soil conservation

A study was made of the residual phosphate situation in a nearly level, light-textured soil of Alabama that had been used for a 26-year period in an experiment involving a 3-year rotation of cotton, corn, and oats with various legumes and with different phosphate fertilizer treatments. The soil has been considered to be uneroded or not seriously eroded, and it has been assumed that the unused phosphates were accumulating in the soil as fixed phosphates. The data presented reveal that some of these assumptions are not correct. The principal facts are summarized as follows: Where superphosphate was used for the 26-year period, 32% of the phosphate was used by the plants in making harvested products, 8% was still present as a residue, and 60% had been carried away with the clay fractions lost by erosion. Where rock phosphate was used, only 9% of the total phosphate added was used by the plants and 82% was lost by erosion, the amount remaining as a residue being 9%. The-surface soil contained only 6% of clay, yet, in this fraction was held over 50% of the total phosphate in the soil. The amount of phosphate that had moved downward in the profile was too insignificant to measure accurately; apparently, a small amount had moved down with the eluted clay. The total amount of phosphate taken out of the subsoil below the 8-inch depth in 26 years by the crops on a plat receiving only a nitrogen fertilizer was 618 pounds P2O5 per acre. Of this amount 258 pounds were removed from the field in harvested crops, 53 pounds accumulated in the surface 8 inches, and 313 pounds, or 60% lost by erosion. A nitrogen fertilizer had maintained a 35-bushel corn and oat yield per acre for a 12-year period without the use of phosphate fertilizers and had at the same time caused a slight increase in the total phosphate content of the surface 8 inches of soil by drawing phosphate from the subsoil horizons. A plat with no fertilizers produced only 24 and 12 bushels per acre yields for corn and oats, respectively, and had depleted the surface soil by 43 pounds of P2O5 per acre. The difference in the yields and phosphate contents of the soil is accounted for by the increased root growth in the subsoil of the nitrated soil and the greater upward movement of the phosphate from the subsoil in the more vigorous plants. The data are discussed in relation to the importance of saving the clay fraction in soil conservation practices.