Assessing sulfur fertility levels for alfalfa
The most accurate measurement of plant nutrient uptake is a tissue test.
Michigan agriculture is very diverse and most farmers will have several crops per year they need to manage for their operation. One of the many hats they are required to wear is for soil fertility. Knowing the different fertility requirements for each crop can be a daunting task because these requirements can vary dramatically from crop to crop. For alfalfa growers, the most basic plant nutrients needed are phosphorus and potassium. However, another alfalfa nutrient of increasing concern is sulfur (S). Farmers may ask, “Are there sufficient levels of S in my soils? Will the alfalfa crop respond to S fertilization? How much S should be applied?”
Alfalfa is a heavy user of S, and a crop of alfalfa hay yielding 4 tons per acre can remove up to 20 pounds S per acre. As a result of the Clean Air Act and the reduction in sulfur dioxide emissions from power plants, S levels in Michigan soils have been decreasing for many years (see figures).
Knowing how much S is available for plant uptake from a soil sample is difficult because the reliability of soil testing for S is poor. The most reliable way to determine if alfalfa plants are deficient in S is to have a plant tissue test performed. Plants with S deficiency will show symptoms that include stunting of plants, yellowing of youngest leaves and veins, and reductions in yield. Generally, the coarser the texture of the soil, the less S it will hold and the greater the potential will be for S deficiency.
To take samples for an alfalfa tissue test, Michigan State University Extension recommends collecting the top 6 inches from 35 alfalfa shoots at the bud to first flower stage and submitting them to the MSU Soil and Plant Nutrient Laboratory for analysis. The sufficiency level of tissue S for alfalfa is above 0.25 percent to insure the crop has the nutrients needed for high yields.
Alfalfa growers applying S fertilizers without knowing whether the plant needs additional nutrients can be incurring an unnecessary expense. Consider taking a tissue sample just prior to first cutting to assess the level of S uptake before applying fertilizer that may or may not be warranted.