Predicting Sulfur Needs with Soil Tests.

Predicting Sulfur Needs with Soil Tests The need for a reliable sulfur soil test has increased in recent years because of the worldwide increase in the incidence of sulfur deficiency, and to identify areas of excess S which may result in plant molybdenum and animal selenium deficiencies. Sulfur soil testing faces similar difficulties to that for nitrogen and phosphorus. The similar mobility of sulfate and nitrate in most profiles means that, as for N, whole profile sampling may be necessary in some instances while choosing an extractant that removes a combination of adsorbed and soil solution  sulfate and a component of organic S that will mineralize to sulfate during the life of the crop or pasture presents a challenge. Early attempts to measure plant available soil S used a variety of extractants which were not related to specific identifiable soil S pools that are known to supply sulfate to plants. Extractants which have been used that remove inorganic sulfate include H2O, CaCl2, LiCl, MgCl2, NH4Cl, mono-calcium phosphate (MCP), NH4OAc + acetic acid and Bray-1. Those which remove inorganic sulfate plus some part of the organic S include hot water, NaOH, NaHCO3 and NH4OAc. Studies have shown that the ester sulfate pool is an important supplier of S to the sulfate pool and therefore must be taken into account when selecting an appropriate soil S test. Studies in Australia have compared a range of tests and have found that extraction of the soil with 0.25M KCl at 40oC for 3 hours, and measuring the entire S in the extractant, produced results which were better correlated with plant S response than MCP with or without charcoal, H2O, NaHCO3 or total S. Not only was the correlation with plant response better with KCl-40 than the other extractants but the variation throughout the year was less. Experiments using 35S labelled soil have been undertaken to determine the pools of S removed by the various extractants. These showed that MCP removed almost the entire adsorbed S and little of the ester sulfate whereas KCl-40 removed less adsorbed S and more ester sulfate. Whilst the KCl-40 soil S test has been shown to be the most useful, care must be taken in interpretation to account for S beyond the sampling zone which may be accessed by plants.