SULFUR TESTS ON NORTHERN CALIFORNIA SUBCLOVER-ANNUAL GRASS PASTURE SURFACE SOILS

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Abstract
The sulfur status of surface soils on 17 northern California subclover-annual grass pastures was diagnosed with five soils S tests: total S, and the sulfate soluble in LiCl, phosphate, and two dilute acid solutions (Hoeft and Bray-1 methods). Test results were evaluated for their effectiveness in predicting pasture yield and relative response to applied S. First-year yields were correlated significantly only with the dilute acid extract results (r=0.52 and 0.76; P < 0.05 and 0.01 for linear regressions with the Hoeft and Bray-1 tests, respectively), indicating that more tightly sorbed inorganic and labile organic sulfate were important components of the pasture-available S on these soils. Both regression equations describing the relationship between first-year yields and S extracted by the Hoeft and Bray-1 methods were significantly improved by inclusion of the soil organic P:total S ratio. The Bray-1 test equation was further improved by including soil sand and clay levels, and the Hoeft by including organic C levels. All terms had negative coefficients.Relative yields during the first year ware not correlated with any of the S tests on these disked soils. During the second growing season, when soils were more stable, the Bray-1 method was useful in predicting pasture response to residual S fertilizer. A critical sulfate-S value of about 10 ppm was indicated and correctly identified S response on 12 of the 17 soils (70%). The sulfur status of surface soils on 17 northern California subclover-annual grass pastures was diagnosed with five soils S tests: total S, and the sulfate soluble in LiCl, phosphate, and two dilute acid solutions (Hoeft and Bray-1 methods). Test results were evaluated for their effectiveness in predicting pasture yield and relative response to applied S. First-year yields were correlated significantly only with the dilute acid extract results (r=0.52 and 0.76; P < 0.05 and 0.01 for linear regressions with the Hoeft and Bray-1 tests, respectively), indicating that more tightly sorbed inorganic and labile organic sulfate were important components of the pasture-available S on these soils. Both regression equations describing the relationship between first-year yields and S extracted by the Hoeft and Bray-1 methods were significantly improved by inclusion of the soil organic P:total S ratio. The Bray-1 test equation was further improved by including soil sand and clay levels, and the Hoeft by including organic C levels. All terms had negative coefficients. Relative yields during the first year ware not correlated with any of the S tests on these disked soils. During the second growing season, when soils were more stable, the Bray-1 method was useful in predicting pasture response to residual S fertilizer. A critical sulfate-S value of about 10 ppm was indicated and correctly identified S response on 12 of the 17 soils (70%). © Williams & Wilkins 1987. All Rights Reserved.