What is a Green Manure Crop, and why is it so important to the Garden?

If you’ve harvested peas and turned under the plants while they were still green and tender, you have put green manure in your soil. Any green plant spaded or tilled back into the soil can be called green manure. Some green manure crops are grown just to be plowed back into the soil while they are still green and rich in organic matter. Alfalfa, buckwheat, and annual ryegrass are a few examples. There are many others.

You might hear green manure crops being called “cover crops” or “catch crops.” These names indicate two of the jobs of a green manure crop:

1. To cover bare soil at the end of the season. This protects it from erosion over the winter.

2. To catch fertilizer and moisture that have leached deep into the soil. Deep roots of a green manure crop retrieve nutrients that would otherwise be lost.

Nine Great Green Manure Benefits

• Green manure is easier to use as a fertilizer and soil conditioner than barnyard manure. Heck, I’d rather stand knee-deep in green manure than ankle-deep in the “cow kind” any day.
• A green manure crop might start out from as little as 5 or 10 pounds of buckwheat seed. Six weeks after planting you may have 2 or 3 tons of plant matter to work into the soil.
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• Animal manure as a fertilizer and soil conditioner is expensive if you don’t have your own animals. It’s usually full of weed seeds and hard on the back to lug around, spread, and till in. Besides, it takes a lot of animal manure to add a sizeable amount of fertilizer to the soil.
• Green manure crops are especially helpful to lifeless sandy soils. After a few crops, sandy soil will hold nutrients and water much better. Instead of washing right through the soil, the nutrients are trapped by the organic matter very close to the surface of the soil where plant roots can get to them. This trapping action saves on fertilizer—especially nitrogen—and on watering.
• Green manure crops provide a tremendous amount of organic matter for earthworms and bacteria in the soil. These soil creatures break down the organic matter into elements to be used by the next crop. Some acids react with soil minerals to produce extra nutrients. A green manure crop is a feast for your soil.
• Roto-tilled into heavy, clay soils, green manure crops improve and condition the soil. The organic matter wedges its way between tight clay soil particles and that allows air, water, and roots to penetrate better. Buckwheat, one of my favorite green manure crops, is exceptionally good at this. Clay soil with a lot of buckwheat decomposing in it won’t bake down and crust over as much, which makes it easier to till.
• When you cover the soil with a green manure crop, you control the loss of topsoil by water erosion and winds. If you have a large garden space, you may not want (or have the time) to plant it all to vegetables. Plant any unused sections to green manure crops and protect the soil there. It takes little time to plant a crop and till it in at the proper time.
• I strongly recommend keeping as much vegetable-growing area as possible covered during the winter.
• Green manure crops can provide free fertilizer. Some of my favorite green manure crops, such as peas and beans, are legumes. Legumes are nitrogen factories. They take nitrogen from the air and fix it to their roots in little nodules. Tilling these crops into the soil replenishes much of the nitrogen that is removed by growing other vegetables. Legumes can provide two or three times more nitrogen than grasses and other green manure crops.
• Fast-growing green manure crops such as buckwheat smother weeds. Buckwheat can cover the soil like a tarp in just a few days after sprouting. The leaves are so close together that sunlight can’t nourish any weeds. Two or three of these crops in succession will beat almost any weed problem, including the most persistent perennials.
• Green manure crops act as an insulating blanket over the soil. They keep the ground cooler in summer and warmer in winter. A green manure blanket covering the soil in fall and winter is especially important because earthworms are busiest and most numerous at this time. If you can keep them working close to the surface, they’ll produce a rich store of nutrients for next year’s crops located right where young plants can reach it. But if you leave the soil bare it will freeze early near the surface. This will force the earthworms down deep where it’s still warm enough for them to work. The fertilizer produced down there will not be of much use to your early crops.
• The best end-of-season blanket crop is annual ryegrass. It grows fast in the cool weather of fall, and won’t be killed back until a hard freeze hits. Then it will lay like a thick mat through the winter.
• Green manure roots are “go-getters.” They reach deep into the subsoil (where vegetable roots hardly ever reach) and recapture valuable nutrients. These plant foods pass through the roots, up into the plants, and wind up back in the topsoil when the crop is turned under.
• What happens to the extensive deep roots when you till the plant in just 6 or 8 inches deep in the soil? The deeper roots will decay and contribute organic matter to the cause, and plenty of it. Another bonus: they leave paths for worms and vegetable roots to follow.

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