INFLUENCE OF PHOSPHORUS SOURCES AND RATES ON SOIL pH, EXTRACTABLE PHOSPHORUS, AND DTPA-EXTRACTABLE MICRONUTRIENTS1

Phosphorus fertilization has been reported to influence micronutrient availability. The influence of applied P on extractable micronutrients depends mainly on the chemical characteristics of the soil and the P source. Soils in the western Great Plains typically have high soil pH and are saturated with Ca, which has a marked effect on micronutrient availability. Therefore, the objectives of this study were to determine the effects of different P sources and rates on (a) soil pH, (b) Bray and Kurtz no. 1 P, and (c) DTPA-extractable Fe, Mn, Zn, and Cu.Two soils (McLain sicl—fine, mixed, thermic, Pachic Argiustoll and Quinlan cl—loamy, mixed, thermic, shallow Typic Ustocrept) that differed in micronutrient content and chemical characteristics were collected from western Oklahoma for laboratory study. Soils were passed through a 2-mm screen and placed in plastic Petri dishes, and five P levels (0, 20, 40, 60, and 80 kg ha) were applied using monocalcium phosphate (MCP), monoammonium phosphate (MAP), and ammonium polyphosphate (APP); the soils were then mixed uniformly. Soils were moistened to approximately 0.33 MPa and incubated for 2 mo at room temperature.Application of P decreased soil pH in both soils, and MAP and APP had a greater effect than MCP, which was attributed to the nitrification of the added ammonium. Bray and Kurtz no. 1 P increased with P application in both soils. Monocalcium phosphate and MAP decreased DTPA-Fe, -Mn, and -Cu in McLain soil. However, high levels of P applied as APP increased DTPA-Fe, -Mn, and -Cu. Phosphorus application, regardless of source, had no effect on DTPA-Zn in McLain soil. Monocalcium phosphate and MAP decreased DTPA-Mn in the Quinlan soil; however, high levels of P applied as APP increased DTPA-Fe. Phosphorus application, regardless of source, had no effect on DTPA-Zn and -Cu in Quinlan soil.