Green Manure as a Source of Organic Matter

We all know what a brown manure is, but what is a green manure? Green manures come from plants, in fact, green manure is the plants themselves after they’ve been plowed or tilled back into the soil. That’s right, a green manure is nothing more than a bunch of plants that are grown in the garden for the express purpose of tilling them straight back into the soil usually before they produce seeds of their own (so the garden doesn’t become ‘weedy’ with plants from the covercrop).
Now, if you’ve never heard of green manure before, this may sound really wasteful or even bizarre. Why would you grow plants in your garden for no other reason than to turn around and kill them all by tilling them back into the soil? That’s a good question. And it does sound a little crazy at first, but there are colossal benefits that come from growing plants and turning them back into the soil. But before I get into the hows and whys let me briefly discuss some terminology with respect to green manures.

Plants that are used for green manures are called secondary crops because they’re not grown as a source of food like fruit and vegetable crops. The other name for secondary crops, which is the more commonly used name, is cover crops because they cover the bare soil in the garden when fruit or vegetable crops are not being grown at the moment, typically during the off season such as late fall, winter in milder climates, or early spring. A lot of gardeners use the terms cover crops and green manures interchangeably but they are different things: cover crops are the live plants grown for green manure, and green manure is the dead cover crops once they have been plowed or tilled back into the soil.

Remember from my podcast on organic matter that I mentioned that it is primarily the element carbon that creates the amazing benefits that we so desire to have in our soils. Plants, from a chemical standpoint, are primarily composed of carbon. Therefore, one of the ways to greatly increase the amount of organic matter, or carbon, in your soils is by plowing plants themselves back into your soil. This boost in organic matter from green manures is the major benefit to cover cropping, and we could leave the conversation on cover cropping at that, because that is such a gigantic benefit in and of itself, but it is actually only one of several benefits that come with growing cover crops for green manure.

One of the other benefits to cover cropping, as I’ve already briefly mentioned above, is covering your garden soil during the off-season or at any other time of the year when you’re not actively growing a garden. Creating a soil-cover smothers and crowds out weeds, and it also prevents weeds from sprouting in the first place. A lot of weed seeds are stimulated to sprout when they are exposed to sun light. A cover crop shades the soil and thus prevents a multitude of weed seeds from ever germinating at all.

Further still, there are some cover crops that have the ability to chemically inhibit or kill seeds in the soil. They actually manufacture and exude organic chemicals from their roots into the soil that are toxic to seeds. Sounds pretty wild but it’s true. This biological phenomenon is referred to as negative allelopathy. Allelopathy can be a pretty fancy tool in the hands of gardener if used wisely. You obviously don’t want allelopathic chemicals killing the seeds you plant for your garden, but knocking off weed seeds would definitely save a lot in the way of time and effort from pulling weeds, not to mention, being able to avoid using herbicides.

Allelopathic cover crops are best used when there will be a healthy space of time between when the cover crop is plowed into the soil and when you’ll start planting your garden for the season. For example, using an allelopathic cover crop in the fall and plowing it into the soil during the winter, months before your spring planting. That way the cover crop will have a chance to kill weed seeds during the off season but there will be sufficient time for the allelopathic chemicals to dissipate before you plant your good seeds in the spring.

Another plus associated with using a cover crop to cover bare garden soil is preventing erosion and nutrient leaching. Allowing soil to be bare leaves it vulnerable to the elements which may erode away valuable garden soil and the nutrients it contains. Cover crops do an outstanding job of preventing soil erosion and keeping soil nutrients in the garden where we want them and where they belong. Herein lies an important point I probably ought to explore for a minute. Sometimes people get confused and think that growing cover crops that don’t produce food are a waste of soil nutrients. This is not true simply because cover crops are plowed or tilled back into the soil as green manures thus returning all the nutrients they absorbed right back into the soil. No nutrients are lost in this process; they’re merely moved around. They are taken from the soil by plants and then returned to the soil again from those same plants once they are plowed back into the soil.

Another benefit to cover cropping is that it provides organic matter conveniently. What I mean by that is, growing a cover crop is essentially growing organic matter on site. You don’t have to go out and buy peat moss or brown manure or make compost and then transport them all to the garden. Green manures are grown as a cover crop in the garden already, right on the spot: there’s no hauling, lifting, or dragging organic matter to your garden. Most cover crop seeds are best obtained by ordering them online and having them delivered directly to your door for you. Then all you have to do is scatter the seeds evenly by hand across the soil surface of your garden, gently rake the seeds into the soil so they’re covered by a little bit of earth, water them, and watch them grow. It really is that convenient. It’s not like planting fruits or vegetables where you have to be more precise in how and where you plant in your garden. As the name implies, with cover crops all we’re doing is trying to cover or indiscriminately blanket the garden. It doesn’t need to be, and shouldn’t be, some methodical or meticulous process.

Some cover crops can also fertilize your garden with nitrogen, one of the most important nutrients to plants. This is a huge benefit because nitrogen fertilizers can be expensive, and being able to provide nutrients for your own plant without external inputs, like fertilizers, is a great step towards self-sufficiency. The cover crops that have this ability belong to a specific botanical family called the fabacea or the legume family. Even if you’ve never heard of the terms fabacea or legume, I’m sure you’re familiar with some of its members like clovers, beans, and peas, which are all legumes. The ability of legumes to provide nitrogen is actually because of a partnership between the legume plants and soil bacteria called rhizobia. Technically, it is the rhizobia that provide the nitrogen, not the legumes.

Rhizobia pull atmospheric nitrogen out of the air and provide it to leguminous plants in exchange for food in the form of carbohydrates. It’s a really fascinating biological trading system: legumes can pull carbon dioxide out of the air and make carbohydrates out of it, but they can’t pull nitrogen out of the air. Rhizobia can pull nitrogen out of the air but they can’t pull carbon dioxide out of the air to make carbohydrates.

So rhizobia and legumes barter with each other: legumes get nitrogen from the rhizobia and rhizobia get carbohydrates from the legumes. This whole bartering process between rhizobia and legumes is collectively called nitrogen fixation, and it only occurs between leguminous plants and rhizobia. Why rhizobia form this symbiotic relationship only with legumes and not other plants is not known. There has been, and continues to be, a lot of scientific research specifically devoted to that topic.

A couple of pointers on legume cover crops that you should be aware of. Because legumes barter some of their resources away to rhizobia in exchange for nitrogen, they tend to not produce as much organic matter as non-legume plants which devote all of their resources to plant growth. Herein lies a trade-off between legumes and non-legumes: legumes provide nitrogen and some organic matter, non-legumes cannot provide any nitrogen but they do produce tons of organic matter.

To get the best of both worlds a mixture of legume and non-legume cover crops is ideal. The bare minimum amount of legumes in the cover crop seed mix should be at least 30%. A mixture with 30% legumes to 70% non-legumes will provide sufficient amounts of nitrogen to maintain all the plants in the cover crop. Now, here’s a key point: as a gardener, we want to have a nitrogen surplus so there will be nitrogen left over from the cover crop in order to fertilize our primary crops, our fruits and vegetables. So a mixture of 40% legumes to 60% non-legumes or even a 50-50 mix is more desirable.

You can have an even richer legume mixture if you want, and in some cases it makes sense to grow a cover crop that is 100% legumes. For example, if you already have tons of organic matter in your soil from autumn leaves or grass clippings from the previous summer. In those cases, an all-legume cover crop would be a good idea especially for gardens that have a lot of brown compost in them because brown compost is naturally nitrogen pour.

Another benefit unique to some cover crops is their ability to purge a soil of disease. The cover crops with this ability belong to the mustard botanical family. These plants, when plowed back into the soil, produce organic chemicals that act as biofumigants which are toxic to bacteria, fungi, nematodes, and other microorganisms.

Commercially, some farmers fumigate their soils with powerful chemicals like methyl-bromide which sterilize the soil of all the microorganisms so as to prevent disease from ruining their crops. There have been a lot of interesting scientific studies that have demonstrated that plants from the mustard family have natural biofumigant properties. Indian mustard, in particular, has powerful anti-disease properties. If your garden seems to have a lot of sick or diseased plants, growing a cover crop of mustards during the off season will likely help a great deal. It should be kept in mind that the organic chemicals the mustards produce as biofumigants are non-selective; that is, they can affect all microorganisms even the good ones. For this reason, it is recommended that you only use mustard cover crops as necessary, not all the time. You can also mix and match mustards with different cover crop plants but in order get the most potent biofumigant levels in your soil for disease control, mustards are best grown as a pure stand.

There are other benefits to cover cropping such as providing forage for animals like cattle or horses, but the ones I’ve discussed above are the major benefits most useful to home gardeners. Cover cropping and the green manure that comes from cover cropping is huge. It not only provides a lot of organic matter, and the magnificent benefits associated with that alone, but it also provides a lot other biological services that can greatly boost gardening success, and best of all, without costing a fortune.

If you’re looking for seeds for cover crops, there are several seed companies that sell cover crop seeds along with all their other garden seeds. Try to find a seed company that has taken the safe seed pledge, and that does not sell Monsanto/Seminis seed.

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