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Evaluation of Various Extractants for Available Soil Sulfur¹

ABSTRACT

The S status of 49 Wisconsin soils was evaluated in field studies using alfalfa (Medicago sativa) as the test crop. The effectiveness of six soil S extractants for predicting yield and S uptake response to applied S were evaluated by means of correlation analyses. The best extractant, considering all dependent variables and the S range, mean and standard deviation for each procedure, was 2N HOAc containing 500 ppm P as Ca(H₂PO₄)₂ · H₂O [Ca(H₂PO₄)₂-HOAc]. The correlation between extractable S and response to applied S was not improved by inclusion of percent organic matter, percent sand, or subsoil SO₄-S in the regression equation. However, inclusion of soil pH with extractable S did significantly improve the prediction of S response.

The best equation for prediction of yield response to applied S in the field was: Yield response (metric tons/ha) = 50.81 – 7.22 pH – 8.19 S + 1.171 (pH x S) + 0.167 (S)² – 0.00341 (pH x S)² where S is the ppm of S extracted with Ca-(H₂PO₄)₂-HOAc. Significant response to applied S would be expected on soils having a calculated yield response of 0.40 metric tons/ha or greater when extractable S is between 6 and 10 ppm S. Soils testing less than 6 ppm would be expected to respond and those testing greater than 10 ppm S would not be expected to respond to application of S, regardless of the calculated response value. Use of the regression equation and the limits of 6 and 10 ppm S resulted in correct prediction of S response at 77.6% of the field sites.

¹ Contribution from the Dep. of Soil Sci., Univ. of Wisconsin, Madison, Wisconsin 53706. Research supported by the College of Agr. and Life Sci. (Project No. 1620), Univ. of Wisconsin, Madison, Farmers Union Central Exchange and The Sulphur Institute. Presented before Div. S-4, Soil Science Society of America, Aug. 19, 1971, New York, N.Y.

² Formerly Project Assistant now Assistant Professor, Agr. Res. and Ext. Center, South Dakota State Univ., Rapid City, South Dakota, 57701, Professor and Associate Professor, Dep. of Soil Sci., Univ. of Wisconsin, Madison, Wisconsin 53706, respectively.

Received for publication October 4, 1972. Accepted for publication December 12, 1972.