Green Manure

What?
Green manure and cover cropping are relatively synonymous; both describe plants we purposefully plan on knocking down and digging into the soil without harvesting from them. In fact, the harvest is the plant itself, feeding the soil from which it grew.

Where I'm hired to maintain an organic vegetable garden, I planted part of the beds with a summer mix of buckwheat and cowpeas. In other seasons and for other purposes, I'd plant something different.

Cover crop just after emerging from the soil.

The plants in bloom, just before cutting down.

Why?
Cover cropping can solve a myriad of garden problems. Some crops can protect soil from erosion, some can fight against root knot nematodes, others build tilth and inhibit the growth of weeds, but the most common use for cover cropping is to increase soil fertility by adding organic matter and often nitrogen. In my client's garden, I wanted to keep soil that wasn't growing food during the summer as healthy as possible, housing microbes and beneficial critters. I also wanted to continue to add nitrogen, so the buckwheat and cowpea combo was perfect. The plants grow well in the heat; the buckwheat builds organic mass very quickly; working in symbiosis with mycorrhizae, the cowpeas collect nitrogen from the atmosphere.

The felled crop.

I planted the mix last month, dosed with inoculant to give the cowpeas a jump start. Yesterday, the plants were mostly blooming—buckwheat is a great beneficial insect attractant, by the way—and ready to be cut down. Since it was only about 30 square feet I had planted, it was easy for me to use shears to cut the plants, after which I shoveled them into the soil. Turning the plants bottoms up, I could see the root nodules on the cowpea plants, housing nitrogen. That nitrogen will feed other plants, along with the nitrogen and other minerals the buckwheat will release as it decomposes. Additionally, the decomposed material will eventually serve as a sponge, helping the soil to maintain even moisture. It will be a happy home for all kinds of good insects and micro-organisms. Since the weather is currently quite dry, I watered the area I had turned under to help kickstart decomposition. In three weeks, remnants of the green manure will have turned into rich soil matter, and this area will be ready for planting fall crops.

Nitrogen nodules on cowpea roots.

More Resources:
Sustainable Agriculture Research and Education's Write-up of Cover Crops
Sustainable Agrigulture Research and Education Program's Explanation of Cover Crops
Rodale's Organic Life on Cover Crops