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This article in AJ

  1. Vol. 66 No. 4, p. 578-581

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Indexes of Sulfur Deficiency in Alfalfa. I. Extractable Soil SO4-S1

  1. D. T. Westermann2



Sulfur deficiencies occur on many of the undeveloped agricultural soils found in the mountain valleys of the western United States; however, the majority of the S soil-test correlation studies have been conducted on leached and weathered soils. Identification of S deficiency on alfalfa (Medicago sativa L.) growing in three mountain valleys permitted the collection of S soil-test correlation data for these undeveloped soils. This paper reports the relationships found between extractable soil SO4-S levels and the response of alfalfa to S fertilization.

Soil samples were taken from 13 experimental sites before active plant growth commenced in the spring in 30-cm depth increments to 92 cm. Soil SO4-S was extracted by water, 0.1 M LiCl, and 0.032 M KH2PO4 and measured by the methylene-blue reduction method. Significant correlations were obtained between the SCVS extracted by all three extractants from the 0- to 30-cm soil layers and relative forage yields and total S uptake by the first harvest of alfalfa at early bloom. Critical soil SO4-S levels of 3.0, 3.0, and 4.0 ppm in this soil layer were found when extracted by water, LiCl, and KH2PO4 respectively. Inclusion of KH2PO4-extractable SO4-S to 92 cm did not improve the curvilinear regression relationships, nor did inclusion of percent soil organic matter. Increasing the soil:extractant ratio from 1:2 to 1:3 did not significantly change the amount of SO4-S extracted by the KH2PO4 solution. The 0.1 M LiCl solution is recommended for extracting SO4-S from soils with relatively small amounts of absorbed SO4-S.

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