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Drought and alfalfa nutrition

Michael Russelle Published on 05 June 2009

Although established alfalfa can access deep subsoil water, dry topsoils limit the availability of many nutrients.

For example, boron is adsorbed to soil clays and organic matter, and its release is governed by soil pH, the composition of the soil solution, and wetting and drying cycles. When topsoils are dry, both boron availability and uptake are limited. But dry topsoils can also limit the uptake of other nutrients, including phosphorus, potassium, sulfur and molybdenum. How should you react to a potentially dry growing season?

The answer depends a lot on your specific field conditions. If your soil tests are in the optimum-to-high range or if you have applied manure to the field in the past few years, it is unlikely that the current crop will respond to additional nutrients. However, if the soil tends to be droughty, is shallow or has low fertility, you might want to consider topdressing alfalfa after the first harvest with commercial fertilizer or manure. Base the application rate on soil tests taken early this spring and follow your state regulations for manure application.

With either source, apply as soon as possible after harvest but only on firm soil to reduce damage from wheel traffic. Neither source will provide significant nutrients to the alfalfa if the topsoil remains dry, so the decision to fertilize should be made with an eye to the weather forecast.

The water content of manure slurry can improve regrowth, but broadcast slurry can coat the leaves with solids that prevent photosynthesis and suffocate the tissue. When broadcasting slurry on alfalfa, apply no more than 1.5 tons of solids per acre.

An important caveat: Manure from a herd with diseases like Johne’s should not be used on alfalfa that will be stored as hay, because the organisms persist on the soil and plant surfaces and could infect healthy livestock.  FG

References omitted but are available upon request at

—Excerpts from Minnesota Crop eNews, April 2007

Michael Russelle - USDA-ARS Plant Science Research Unit and U.S. Dairy Forage Research Center

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