Carrie Laboski, Extension Soil Fertility and Nutrient Management Specialist
Reports from Southwestern Wisconsin suggest that sulfur deficiency may already be showing up in some alfalfa fields. Sulfur (S) deficient alfalfa will appear lighter green, will be shorter, and the plant will be more spindly with smaller leaves. Some fields have patches of lighter green and may be associated with eroded knolls; while other fields have an overall lighter green color.
Sulfur deficiency can be confirmed with a tissue test. A plant sample can be collected when the crop is in the bud to 1st flower stage. Sample the top six inches from 30 to 40 stems that represent the area being sampled. Samples should be submitted to a lab in a paper, not plastic, bag. If the S concentration in the plant tissue is less than 0.25 % S, then an application of 25 lb S/a is needed, and should be applied before there is substantial regrowth. In general, yield responses to applied S will be greater as the plant tissue S concentration decreases, though the recommended rate of S does not change. Also keep in mind that if tissue S concentrations are 0.25% or greater, there will be no significant increase in yield or profitability from S applications.
If S deficiency is occurring, then a sulfate form of S must be applied to alleviate the deficiency quickly. There is no effect of sulfate source on yield, meaning that potassium sulfate, calcium sulfate, or ammonium sulfate can all be used with success. When choosing a S source for alfalfa consider overall nutrient need and price of the fertilizer material.
Potassium sulfate may be a good choice of fertilizer for alfalfa that is S deficient, because there are probably few alfalfa stands in Wisconsin that wouldn’t benefit from the potassium supplied by this fertilizer. The price of S, in potassium sulfate is $0.77/lb of S if the value of K in this fertilizer is taken into consideration. The nitrogen (N) in ammonium sulfate has no benefit to alfalfa making it a relatively expensive source of S at $0.93/lb S. For alfalfa, potassium sulfate is a better buy than ammonium sulfate. Calcium sulfate, mined gypsum, is an even better buy because the S is $0.59/lb, assuming no value to the Ca. A S fertilizer price comparison is provided in Table 1.
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