Cannabis Chronicles

Green Manures

A Green Manure is a crop grown for the purpose of supplying the soil with nutrients and organic matter. It is called a “cover crop” when the green manure is grown for the added purpose of reducing soil erosion. Green manures are usually legumes or grasses, and they are grown with the simple intent that they will be turned back under the soil. Cover crops and green manures are certainly cost effective for large-scale farmers, but many backyard gardeners have no idea how simple and effective they are to use. And, as we mentioned earlier, they do offer a “manure” option for growers who choose vegan organics.

Green manures improve soil in a variety of ways. Green manures add significant amount of organic matter into the soil. Like animal manures, the decomposing of green manures works to enhance biological activity in the soil. Green manures can also diminish the frequency of common weeds, and when used in a crop rotation, they can help to reduce disease and pests. When turned under, the rotting vegetation supports beneficial bacterial populations. As those decomposers do their work, nutrients stored by the cover crop are returned to the soil.

Alfalfa roots regularly grow to depths of five feet or more, soybeans and clover can reach almost as deep. Since their roots go deeper than folk would commonly cultivate with a rototiller or plow, a green manure crop can bring subsoil minerals up to where even shallow rooted plants can reach them. Green manures also help to improve overall soil structure, because those deep reaching roots leave behind minute channels deep into the soil. When these deep roots decay, they provide organic matter that promotes long-term soil building.

Except for buckwheat (a member of the rhubarb family) and rapeseed (related to the cabbages), all commonly used green manures are either legumes or grasses. Rye and oats are two good examples of grass family members that are commonly used as green manures. When we think of legumes, beans and peas are the “classics” which come to mind, but the legume family also includes relatives such as clover and alfalfa. Members of the legume family can be particularly valuable as green manures, due to their ability to “fix” nitrogen from the atmosphere.

In the legume family, a very specific type of bacteria works in league with plant roots. These microorganisms, called nitrogen fixing bacteria, form nodules on the plant roots where they work in a form of partnership with their host. Functioning in concert with the plant roots, nitrogen fixing bacteria transform atmospheric nitrogen (which plants otherwise can’t use), into ammonia, which plant roots can easily absorb.

If one of these plants is uprooted, the small nodules become visible as white or pinkish bumps the size of a large pinhead. The more nodules visible the better, since more nodules equals more nitrogen fixed. To assure that enough of these bacteria are present, commercially sold legume seeds are often treated with a bacterial innoculant. Make sure to get the appropriate innoculant for your specific legume crop if it’s necessary to inoculate your own soil or legume seed stock.

Each kind of legume requires a specific species of bacteria for effective nitrogen fixation, and each innoculant works for only a few species. It’s usually possible to buy an innoculant mix designed for all peas, snap or dry beans, as well as lima beans. Soybeans will require their own specific innoculant. A totally different innoculant will be needed to serve the needs of the vetches (as well as fava beans.) Still another nitrogen fixing bacteria will work with all the true clovers, but sweet clovers will require yet another innoculant.

With careful stewardship, a legume cover crop can enrich the soil with enough nitrogen to supply most of the following years crop nitrogen needs. Commonly used legumes for cover crops include: alfalfa; fava, mung and soy beans; a whole variety of clovers; cowpeas and field peas; common or hairy vetch; the lupines; and finally our favorite name among the legume cover crops - Birdsfoot trefoil.

Although the grasses and other non-legumes do not have the ability to fix nitrogen from the atmosphere, they still provide all the other benefits of green manures. Other non-legume crops grown for green manure include; barley, bromegrass, buckwheat, millet, oats, rapeseed, winter rye, ryegrass, grain sorghum, and wheat.

Seed for cover crop and green manures doesn’t need to come from fancy little packets at the garden center. Purchase grass and legume seeds by the pound, if you can, to save money. Farm and agricultural supply centers, what we call “feed & seed” stores, usually offer the most economical source. If your garden area is small, a single pound of seed may go a long way. With the smaller seeds, a pound could be expected to last through a couple of plantings. The larger seeds of legumes, like beans and peas, don’t store as well, so it’s advised to purchase them fresh annually.

The use of green manures and cover crops is relatively simple, the primary necessity being the time to grow the plants. Some preplanning is always helpful to make sure the correct crop is selected to best meet the grower’s needs. So, for example, if enriching soil nitrogen levels is a goal, then it’s best to choose a cover crop from the legume family due to their ability to fix nitrogen.

Some green manure plantings tolerate poor soil quality better than others, so some cover crops may be chosen because they tolerate particularly acidic (or alkaline) conditions. If a grower needs to break up hardpan soil and improve drainage, some cover crops grow very strong and deep roots. Such conditions call for green manures like alfalfa and birdsfoot trefoil that can thrust their roots through anything but the most dreadfully compressed soils.

As stated earlier, deep-rooted plants can also bring up essential nutrients from the subsoil. And, some do even more; they actually accumulate nutrients, concentrating them. Growing these green manures can produce a measurable (although not huge) increase in soil nutrients. Some legumes, especially red clover, can help to increase phosphorus levels. Buckwheat also increases phosphorus, as well as helping to supplement calcium. Vetches are also accumulator plants, working to increase levels of both calcium and sulfur.

Buckwheat and Rye are examples of crops often grown as green manures that also function to control weeds. Winter Rye is actually a natural herbicide; it produces chemicals that are toxic to many weed seedlings. Buckwheat works by outgrowing its weedy competitors. The large leaves of buckwheat effectively shade out many common annual weeds.

It’s also necessary to consider the seasonal needs of your garden when planning a green manure planting. Some green manures are early season crops, while others do better when planted during the heat of summer. Winter rye and winter wheat are usually planted in the late summer or fall and then turned under in the following spring.

Another key to getting the most from a green manure planting is to turn them under at the proper time. Winter cover crops of rye and wheat, for instance, should be turned under as soon as the spring soil is dry enough to work. It’s best when turning under a winter wheat to allow at least two weeks for the green manure to “work” in the soil before beginning any spring planting.

In order to assure good germination rates, it’s necessary to wait even longer for winter rye manures to be ready for replanting. A three to four week wait is suggested after turning under a winter rye crop before sowing seeds of another crop. This is due to the same herbicidal quality that makes winter rye effective in the control of weeds. In general with most grass cover crops, the best timing is to turn them under before they form mature seed.

Turning under legumes at any time will enhance the organic matter in soil and promote an active population of beneficial soil bacteria. But, to get the full benefit of a legume plantings ability to fix nitrogen, they should be allowed to grow a full season. Perennials like alfalfa, red clover, and birdsfoot trefoil can produce additional soil enriching nitrogen if allowed to grow for a second season. If allowed those two years of growth, they can be mowed multiple times, providing a high quality source of compost or material for mulching. An alfalfa cover planting can serve as a gardener’s own sure source of fresh materials for the manufacture of alfalfa teas.