SOIL-PHOSPHORUS REQUIREMENTS FOR MAXIMUM GROWTH OF NORTHERN CALIFORNIA SUBCLOVER-ANNUAL GRASS PASTURES

Annual grassland soils of California are generally quite deficient in phosphorus (P), but the amount of fertilizer P required to obtain opti mum pasture production is quite variable. This paper reports on the re lationships between five soil-P tests and the amount of applied P requ ired to obtain near maximum growth of subclovergrass pasture on 12 sit es representing four soil series in the north coastal range of Califor nia. Dry matter (DM) yield response curves resulting from application of nine levels of P were fitted to the Mitscherlich equation: y a{1-b( [EXP(-cx)]}, where: y = DM when x = kg/ha of P were applied, a = maxim um DM when P was not limiting, and b and c are constants that were adj usted to give the best fitting curves buy an iterative algorithm. This equation was used to calculate the amount of P required to bring yiel ds to 90% of the maximum DM represented by ''a'' in the equation. Then the regression equations relating P requirements to soil-P extracted by the Bray-1, Olsen, and modified Olsen methods, and P sorbed by the soils in 17 hours or six days were computed. Phosphorus sorbed in 17 h ours was most closely related to P requirement, followed by Bray-1 P, modified Olsen, and Olsen, respectively. Virtually all the P applied w as sorbed by all the soils in six days using the Fox-Kamprath method a nd discrimination among soils on that basis was impossible.