Mb. Jones et al., SOIL-PHOSPHORUS REQUIREMENTS FOR MAXIMUM GROWTH OF NORTHERN CALIFORNIA SUBCLOVER-ANNUAL GRASS PASTURES, Communications in soil science and plant analysis, 26(1-2), 1995, pp. 197-207
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.