As mentioned earlier, the off-season cover on a vegetable garden can also contribute to soil fertility. While crop rotation evens out nutrient demands on the soil, it will still be necessary to bring nutrients into the system, simply because you will be removing nutrients in the form of crops. Elsewhere we saw that crop residues stimulate biological activity in the soil and so release its inherent fertility. Here we’ll just discuss the different types of cover crops and green manures, and their uses within an overall crop rotation scheme.
There are some problems with using green manures in the home garden, particularly one with raised beds, but they can be overcome easily with a little attention to detail. One of the first things that makes a difference with cover crops and green manures is proper selection. You should choose the cover crop based on the results you want: are you trying to unlock an impenetrable subsoil, protect the topsoil from winter erosion, or discourage a particular pest or disease that has become established in your garden? It helps to analyze which of these things is of primary importance, and sow accordingly. Second, make sure you’ve selected a species and variety of cover crop appropriate to your region’s climate, your garden’s soil type, and the season.
Here is an example of a fairly comprehensive site applicable to the Northeast USA. Similar pages exist for other areas in North America and elsewhere.
There are basically four ways that these crops can be of use: as soil protectors, soil builders, soil conditioners, and soil cleansers. One of the simplest and best uses of a green manure is as a winter cover crop to protect the soil from erosion. The standard in the northeast USA is winter rye, which is the type of rye grown for grain. It is sown immediately after harvest, but always by October 15 in, say, USDA Zone 4. Rye grows quickly in the cool fall weather and covers the soil; in the spring it regrows, but if mowed at eight to twelve inches tall it will die off and can be turned under (though not without some effort). After allowing three weeks to a month for it to decompose, the plot can be planted. This three-week waiting period in the spring was a problem for me, as the season in my garden was very short and every week matters. It is becoming common now to mix hairy vetch, a hardy legume, with the rye to provide extra nitrogen for the crop that is to follow, while decreasing the density of the rye, which makes turning it under easier. Vetch will add up to eighty pounds per acre of nitrogen to the soil over the course of its life. In experiments at the Rodale Institute in Pennsylvania, researchers have been able to sow some spring crops directly into growing hairy vetch, without tilling. They then mow the vetch and leave the cuttings in place as a mulch for the spring crop.
However, I found another winter cover crop to solve my spring timing problem in USDA Zone 5 and lower. Annual ryegrass has an advantage over winter rye in a short-season area: sown on clear ground, anytime from early August till late October, ryegrass grows quickly enough to choke out any emergent weeds and forms a very dense, fine-leaved cover. By midwinter it dies off from the cold (about 0 F, -17 C ) and mats down to form a protective mulch that decomposes quickly in the spring after turning under. This provides erosion protection and saves the three-week waiting period. Oats are another alternative that provides many of the same benefits.
Consider your own climate and soil, then look at the possibilities for winter cover crops in your own garden. Once you’ve created a rich, friable soil, it’s a waste to let it sit exposed to nutrient-leaching rains and erosion. Keep some kind of crop growing year-round, and you’ll lock in the nutrients—first in the cover crop, and then in the soil as it decomposes after turning under—that your vegetables will need when their growing time comes.
If you leave garden crops in the ground late into the fall you can actually broadcast the cover crop seed into the plot immediately after your last cultivation, once the crop is nearly mature, but before harvest. Irrigate if the weather is dry; the cover crop will grow right up among the maturing vegetables, and by harvest time will already be well established. As a “living mulch” it will help control fall weeds, too. Don’t worry about competition: the crop’s roots are well established and its growth is already slowing as it nears maturity; just pay attention to the timing.
Legumes are the great soil-builders. Their ability to work with Rhizobium soil bacteria to extract the abundant nitrogen in the atmosphere can be a primary source of fertility in gardens that have enough organic matter to support a high level of biological activity. They can also be used as a fall cover crop, if you get them in early enough to become established by the onset of freezing weather (check with your local USDA Extension office for the proper sowing time in your area). To really build up the nitrogen reserves of the soil, however, a plot should be turned over to alfalfa or other legume cover for a full season. The usual method in the northeast USA is to sow the legume (we use alsike clover) in early spring, in combination with spring oats, which grow quickly and provide protection for the young legume plants. You can harvest the oats once they mature, or simply mow them once the young clover or alfalfa is fully established, leaving the clippings on the ground as a mulch for the legume. It isn’t necessary to give up one whole section of the garden for this purpose. Just a portion of each bed could be sown to cover crops each year; eventually the whole bed will have been improved.
While alfalfa is considered the king of the legumes, it requires a neutral, well-drained soil to thrive. Clovers are a bit more forgiving, and there are many different strains, adapted to different conditions. You can even sow leftover bean or pea seed. Whatever legume you use, be sure to get rhizobial inoculant at the same time you buy the seed, and treat the seed with it—by dusting the inoculant powder on moistened seed—before planting. Even if the plot has had the same legume growing in it before, the inoculant more than pays for itself by guaranteeing optimal nitrogen fixation.
Legumes like alfalfa and clover also function as soil conditioners because they have strong, deep roots that can penetrate the subsoil, and over time break up a hardpan that is interfering with proper drainage of the topsoil. Culture is the same as for nitrogen fixation (discussed above), but leaving the crop in place for at least a whole season is even more fundamental. Any deep-rooted crop can perform the same function. Chicory has been used for this purpose, as has comfrey. You should stay away from comfrey, though, as it is hard to get rid of once established. When my grandfather’s place was sold some years ago, the new owners didn’t plan to have a garden, so I dug out his compost to help get me through the first year on my own land. I wondered what the large rough-leaved plant growing on the side of the pile was, but didn’t think too much about all the small root pieces that my shovel took along with the rich black humus in the pile. I found out when I planted the garden, though. It took me two years of cover cropping and clean cultivation to get rid of the comfrey. Now I keep one clump over by the goose yard, as it is their favorite food, but I don’t let it anywhere near the garden.
The other use of cover crops and green manures as soil conditioners is simply to stimulate the overall biological activity of the soil. This process was discussed in Chapter 4. All we need to say here is that you can get a cover crop grown, turned under, and fully decomposed in as little as six weeks, but the benefits will last for much longer than that. We often use leftover mustard or cress seed, both of which are inexpensive and grow quickly. The crop used is not as important as keeping the soil covered with the growing plants. When you enhance the natural nutrient cycling of the soil by this green manuring, you ensure available nutrients for your next vegetable crop. It is like putting money into a Christmas club account.
The last use of cover crops and green manures I want to mention is as a cleansing crop. A thickly planted crop of ryegrass (in cool weather) or buckwheat (in warm weather) will grow so dense, so quickly, that it will choke our just about any weed known. We have described how these crops can be used to eradicate established weeds, but they can be used in an ongoing weed control program as well. Other cover crops help cleanse the soil of pests and diseases by the allelopathic excretions of their roots: some marigolds have been shown by Dutch research to kill harmful soil nematodes; and rye has been shown to kill certain disease spores. All these crops and cropping systems represent ways that the gardener can finesse a desired effect from the community of his or her garden, what the English writer Edward Hyams called “the soil community.”