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Using Cover Crops: A History of Green Manure

Chinese Milk Vetch (Astragalus sinicus)

Perhaps in the newspaper or on TV you have heard the phrase “cover crop” from a University of Minnesota agronomist. You may have even seen a farmer who planted rye in a soybean field—a tinge of green in an otherwise dreary October. It is easy to see gardening as separate from agriculture; but in reality, the veil we put between them is thin. Chemistry and physics are universal. A nitrate molecule does not care if it is in a 4x8’ raised bed or in a 600-acre farm. If there are no roots or organic matter to absorb it come spring, the nitrate will find its way to groundwater.

Using cover crops in both areas may have stopped that nitrogen loss from occurring, and potentially could have sustained the soil. Grown primarily for improving the land rather than harvest, cover crops (aka “green manures”) have been in use by humans for longer than you might have imagined.

During the early 20th century, Dr. Franklin Hiram (F.H.) King, a renowned professor of soil physics, toured Asia to examine the agricultural practices of peasant farmers. He marveled at their industriousness and lack of reliance on mineral fertilizers used to grow crops—particularly rice—for several thousand years on the same ground. One curious habit he observed was how a cover crop of Chinese Milk Vetch (Astragalus sinicus) was sown just before harvest. Around flowering, this crop was cut and the field was flooded for rice planting. Instead of leaving the vetch residue in water, which could have lost valuable nitrogen to the air, farmers composted it off-site before application to the flooded rice.

Ancient Romans also knew the value of cover crops. A former legionary turned farmer, Lucius Columella, realized how important legumes such as lupine, black medic, and vetch were to productive estates. He wished landowners, no matter how large or small, paid close attention to their soil, and criticized “absentee” owners who leased land with little care of what was done with it—sound familiar today?

Even the founder of our country, George Washington, was an early advocate for cover crops and extended rotations. In 1786, he wrote a letter looking for not just barley seed, but clover as well to be used as a cover crop:

“…I wish to divide my seed-time, and am desirous of sowing clover and other grasses with barley in preference to other grain, I would gladly take fifty bushels…"

So with all of these famous people advocating cover crops, are you willing to try it on your garden? This winter, I will be writing an article each month to educate you in sowing cover crops. This will include species selection, timing of seeding, and answering your questions.

Works Cited

  • Columella, L. J. M., and J. M. Gesner. 1781. De re rustica: Book II. Societas Literata.
  • Montgomery, D. R. 2012. Dirt: the erosion of civilizations. Univ of California Press.
  • Mount Vernon Ladies’ Association. 2015. George Washington as Farmer. Mount Vernon, VA
  • King, F. H. 1911. Farmers of forty centuries: or, Permanent agriculture in China, Korea and Japan. Dover Publications, Inc.
  • Washington, G. 1919. The Agricultural Papers of George Washington. RG Badger.