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Improve soils with cover crops

On our farm we have used yellow blossom sweet clover for many years as a plow-down crop to build our soil. This vigorous, sweet-smelling biennial legume has provided our fields with organic matter and nitrogen since we began organic farming.

Every third year clover is under-seeded with whatever cash crop fits into our rotation for that year. The next year, the clover is allowed to grow until it just begins to bloom, and then it is disked down and left as mulch on the fallow ground for most of the summer. One really dry summer when forage was in short supply, we harvested some clover for hay. We have not done that since then because doing so seriously reduced the crop yields in the following year.

In most years, careful management of our clover crop results in more N for the next crop, a steady increase in soil organic matter, improvement in soil tilth, better moisture retention, less compaction, more healthy microbiology and happy bees. The soil-building green manure crops in our rotation also provide weed control.

We plant alfalfa and alfalfa-grass mixes on some fields. We harvest hay from these plots and sometimes allow the cattle and sheep to graze them. After several years, the alfalfa is worked down, and the field is planted to grains or oil seeds. Alfalfa’s root system leaves not only nitrogen in the soil, but its extensive perennial roots provide a bounty of organic matter, as well. Our fields are almost never bare, reducing the chances of wind and water erosion.

This system of soil building breaks many disease and insect cycles. The diversity of plants encourages beneficial insects such as bees, lady bugs, seed-eating beetles, earthworms and parasitic wasps. 

There are, of course downsides to everything. Clover and alfalfa weevils can munch down newly emerging plants. Cutworms can overwinter in the fields. Sometimes it rains at the wrong time, and disking gets delayed. One summer I had to work down clover that was more than 8 feet tall! Imagine, however, the amount of organic matter added to that field. 

New research on cover crops indicates that more benefits may be found in growing cocktail mixes of several different plant species. Different legumes fix different levels of N. The diversity of plants in a mix reduces the problems of diseases and insects. Adding crops with longer roots may aid in breaking up compacted soils. Mixing grasses with legumes decreases the danger of bloating if you are going to use the cover crops for grazing.

The mix needs to be designed to fit the soil type, the moisture availability, the length of the growing season, the crops which follow, and the needs of the field. The USDA Northern Great Plains Research Laboratory in Mandan, N.D., has developed a cover crop chart that contains all kinds of useful information on possible crops.

Soil-building practices do work. A nine-year study of farming systems at the USDA Henry A. Wallace Beltsville Agricultural Research Center in Beltsville, Md., surprised researchers. They found that while no-till practices were good at increasing soil organic matter, the highest carbon levels were found in fields where organic soil-building practices had been used. These practices, however, are not limited to use by organic farmers. Anyone can make soil building a goal of their farming system.

Jacobson writes from Wales, N.D.


COVER CROP: Mustard can be used to suppress weeds and increase soil organic matter.

This article published in the March, 2011 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.