Indexes of Sulfur Deficiency in Alfalfa . I . Extractable Soil SO - S ' 4

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 SO 4•S levels and the response of alfalfa to S fertilization. Soil satrapies 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 Lid], and 0.032 M KI-12110, and measured by the methylene-blue reduction method. Significant correlations were obtained between the SOS-S extracted by all three extractants from the 0to 30-cm soil layers and relative forage yields and total S uptake by the first harvest of alfalfa at early bloom. Critical soil 50 4.S levels of 3.0, 3k, and 4.0 ppm in this soil layer were found when extracted by water, Lie], and KI-1 2110„ respectively. Inclusion of KI-1,1204-extract able 50,-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 SO.,-S extracted by the KH 2110, solution. The 0.1 M LiC1 solution is recommended for extracting SCI,-S from soils with relatively small amounts of absorbed SO,-S. Additional index words: S Uptake, Lucerne, Medicago saliva L. M OST of the sulfur in soils is in the organic form, but neither total S nor organic S has proved to be a satisfactory index of soil S availability to growing plants (16). More promising methods measure some form of "available S" such as the SO 4-S extracted 1 Contribution from the Western Region, ARS-USDA; Idaho AgricnItunit Experiment Station cooperating„ 2 Soi! Suicut Snake River Conservation Research Center, Kin111, -,.h . , ill 8334i. by water and dilute salt or phosphate solutions or the SO4-S extracted plus a fraction of the organic S. Water-extracted SO4-S has generally not been well correlated to S uptake by plants except on relatively unweathered soils (3). The phosphate solution extracts adsorbed (1) as well as soluble 50 4-S and have been used successfully by many workers (2, 3, 4, 9). Attempts to account for the organic S contribution to the plant's S requirement have also been made by measuring the S released by heating (6, 14, 15), or the total S extracted by salt solutions (3). A Washington study (7) showed that the S uptake by wheat (Triticum aestivum. L.) from 53 soils in the greenhouse was correlated to the SO4-S extracted by 0.1 M LiC1 or 5 mM MgC1 2. The 504-S extracted by water or 0.1 M CaC1 2 from the top 15 cm of soil was also related to the S available to legumes under field conditions in Canada (12). That study also showed that the ability to predict S deficiencies was increased by measuring 504-S to a 30-cm soil depth, sampling the soils in the, spring before active growth commenced, and by air-drying the samples. A similar relationship between soil 504-S and plant response has been reported under Australian field conditions, except that adsorbed soil 50 4-S was measured (8). Whereas some degree of correlation has been found for almost every index of soil S availability, the majority of the studies were not done under field conditions. In addition, most studies have been conducted primarily on leached and weathered soils, whereas many agricultural soils of the mountainous areas in the western United States are relatively undevelo These soils may be subjected to seasonal leaching from snow-melt but can become droughty during the growing season unless irrigated. Sulfur deficiency was identified on alfalfa (Medicago sativa L.) growing in the mountain valleys of southern Idaho and exploratory studies indicated that soil SO 4-S would be a good