SOIL PHOSPHORUS TESTS ON CALIFORNIA SUBCLOVER-ANNUAL GRASS PASTURES

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Abstract
The P fertility of 19 typical, California, annual grassland, pasture soils by comparing 9 soil P tests: total, Bray and Kurtz methods 1 and 2, 2 Olsen extractions (1:10 and 1:20 soil: extractant ratios) and the North Carolina, Truog, Morgan and Bingham methods. Soil P test results were correlated with subclover-annual grass forage production and P content in the field and in the greenhouse. Correlation of soil P with actual yields and P uptake was generally better in the field than in the greenhouse. Phosphorus in the Bray and Kurtz 1 and the Olsen 1:10 and 1:20 extracts gave the best predictions of field forage production (respectively, r = 0.82, 0.73 and 0.67; P < 0.01). The relative values of the various tests in predicting field forage P content were quite similar with actual yields. The Bray and Kurtz 1 method was the most precise (r = 0.87), and the Olsen extracts were also highly correlated (P < 0.01). Bray and Kurtz 1 extractable soil P accounted for about 67% of the variation in subclover-annual grass forage yield and about 76% of the variation in forage P content. These were about 14% higher than with the Olsen extractants. When relative precision and facility (a 60 s extraction time) was considered, the Bray and Kurtz 1 method appeared to be of greater potential value for routine testing of annual pasture soils in north-coastal California.