Survey of Soil Phosphorus in Established Alfalfa Fields in Yuma County

Abstract

A survey was conducted to evaluate the soil phosphorus levels in a cross section of established alfalfa fields in Yuma County The levels of extractable phosphorus (P) varied from 3 to 43 ppm P. All of the soils testing in the very low (below 5 ppm P) and low categories (5 -10 ppm P) were located in the Wellton-Mohawk Valley and on the Yuma Mesa. Annual soil testing in the fall should be used to identify fields which would be expected to be responsive to P fertilizer applications. Soil testing could also help identify fields less like& to respond to P additions (P values > 15 ppm). Eighty percent of the fields tested from the Yuma and North Gila Valleys were in the high and very high ranges, 16-25 ppm and > 25 ppm P respectively. These high values may reflect residual P from applications of phosphorus fertilizers to vegetable and cotton crops grown in rotation with the current alfalfa crops. Fall soil testing in these areas could help identify nonresponsive fields in order to avoid unneeded P applications.