What Is Green Manure?

Despite Its Name, Green Manure Is as Vegetarian as It Gets

man tilling soil
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Green manure is defined as a crop or plant that is grown and then intentionally plowed under to improve the underlying soil. The term green manure is sometimes confused with raw manure, but the two are not the same, as raw manure is animal waste. Green manure is sometimes alternatively called a cover crop, but there is a key difference. Cover crops refer to plants grown over the soil to protect it, while green manure is a cover crop specifically meant to be tilled back into the soil.

The main benefit of using green manure for organic farmers (and other farmers) is that adding organic matter to the soil releases important plant nutrients, such as nitrogen, potassium and phosphorous, plus trace minerals that come from the decomposing organic matter after you dig the crop into the soil. All of these nutrients help to increase crop yields.  

Other benefits include improved soil condition, decreased weeds, increased water-holding capacity and decreased soil loss via erosion. The green manure treatment can dramatically improve poor soils, and it is excellent for maintaining already healthy soil.

Types of Green Manure

Common cover crops used for green manure include soybeans, clover and rye. But really, you can select from a wide variety plants. Each type of crop provides certain benefits. Most plants will improve the nitrogen levels in your soil once they are tilled in. A cover crop of alfalfa can send roots down as far as 60 feet, which will pull nutrients from the depths up to the surface to benefit your next season’s crop.

Winter tare and hairy vetch have extensive root systems that prevent erosion and long leafy vines that grow in the winter, protecting the surface of your soil—serving as a built-in mulch that won’t blow away during winter storms. Woody pod vetch is super hardy and robust and helps get rid of your weed problem by crowding them out.

Some crops, such as lupin, have relatively quick life cycles so that you can perform green manure treatments in as short as eight-week cycles. Agricultural mustard matures in as little as six weeks.

You can also make a simple green manure by collecting green leaf and twigs from plants grown in wastelands, forests and other areas, although in the case of organic farming, growing green manure crops is more common.

Tilling Time

Typically, you till your green manure crop into the soil just before it flowers, which prevents the plants from going to seed (and thus growing again after you dig them into the soil). For larger plants, you may need to break them down first with hand tools or a weed whacker. Allow two to three weeks after tilling so that the organic material can decompose, and then plant your desired crop.

Bonus points go to green manure because it's an inexpensive way to improve land, and it's an eco-friendly farming practice that helps decrease emissions that would add up if you had to transport fertilizers to the farm.

If you're looking for ways to incorporate green manure into your organic farm, see the following guides: