Soil Phosphorus Tests on California Subclover-Annual Grass Pastures

We evaluated the phosphorus fertility of 19 typical, California, annual grassland, pasture soils by comparing nine soil P tests: total, Bray and Kurtz methods 1 and 2, two 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 percent of the variation in subclover-annual grass forage yield and about 76 percent of the variation in forage P content. These were about 14 percent higher than with the Olsen extractants. When we consider relative precision and facility (a 60-second extraction time), the Bray and Kurtz 1 method appears to be of greater potential value for routine testing of annual pasture soils in north-coastal California.