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Soil Care Basics: Increasing Organic Matter and Mineral Availability

Please note that what follows is only a brief one-page introduction to the complex topic of cover cropping. A much more extensive introduction is The Joys of Cover Cropping in two parts in the Garden section..

Table of Contents

IntroductionManuresCompostsFertility PatchesCover Crops

Cover Crops

Often a distinction is made between “cover crops,” planted to protect the soil, and “green manures,” planted for incorporation into the soil in order to feed soil life and the following crop. For simplicity, I will refer to crops intended for both uses as cover crops.


Cover Crops

Growing cover crops is perhaps the most valuable strategy we can adopt to feed our soil, build up its fertility, and improve its structure with each passing season. Freshly-killed covers provide readily-available nutrients for our microbe friends and hence for food crop plants; and the decaying roots of cover crops open up channels into the soil which permit penetration by oxygen and water, and ultimately add to the store of humus in the soil. Legumes (clovers, alfalfa, beans and peas) are especially valuable as cover crops, since they fix nitrogen from the atmosphere into forms readily available to subsequent crop plants. (Plants cannot use atmospheric nitrogen directly.) Mixes of different cover crops are often beneficial. For example, in mixes of grasses and clovers, the grasses add a large amount of biomass and improve soil structure because of the size and complexity of their root systems, and the legumes add nitrogen to help break down the relatively carbon-rich grass roots quickly.

Cover crops should be an essential part of our crop rotations—we should work them into our cropping plans with the same deliberation that we bring to our food crops. An easy way to do so is to maintain two separate garden spaces, plant one to food crops and one to cover crops, then alternate the two types of crop in the following year. Since most gardeners cannot devote that much space to such a strategy, effective cover cropping must be fitted into a unified garden plan, a concept that in practice gets fiendishly complex. Gardeners who like jigsaw puzzles will love the challenges.


Fall Cover Crop

Fortunately, there are cover crops for each of the four seasons, and for almost any cropping strategy. Fast-growing grain grasses (rye, oats, wheat, barley) might be appropriate in early spring in those beds not planted to the early harvest crops. A cold hardy legume like peas can be started in late winter and allowed to grow two months or longer to precede a warm-weather, heavy-feeding crop like winter squash. A warm-weather legume like soybeans or cowpeas can fertilize beds that will be planted to fall crops that like plenty of nitrogen, such as broccoli or fall-planted garlic and shallots. For a quick-growing “filler” between spring and fall crops, nothing beats buckwheat, the “instant cover crop” (thirty days from seed to flower), for suppressing weed growth, and shading the soil and thus protecting soil life and retaining moisture. Don’t forget winter—an opportunity not to be missed for adding organic matter to the soil in the form of cover crops, which also protect soil life from winter’s assault. Grain grasses again are an excellent choice. A mix of hairy vetch and rye (cereal rye, the sort of rye used to make bread, not perennial rye or annual grass rye) will start later in the cold season than any other choice.


Growing mulch-in-place

One of my favorite winter covers is a mix of oats and “field pea” or “winter pea” (Pisum arvense, a close relative of Pisum sativum, the common garden pea): Both oats and field peas are cold-hardy enough to sail through the early frosts, attaining lush knee-high growth, but then reliably winter-kill when the ground freezes—and lie down into the most beautiful mulch-in-place you can imagine, ready for spring transplants right through the mulch, and with the fertility “bonus” from the nitrogen-fixing peas. (I say “reliably” with reference to my own conditions—I grow in Zone 6b—you Florida and California homesteaders are on your own on this one!)


Undersown cover

Since most garden beds are given over to food crops most of the growing season, how do we find opportunities to “shoe-horn” in the cover crops? Fortunately, we can utilize undersown cover crops to grow soil-building covers together with harvest crops. For example, we can sow Dutch white clover in a bed we are planting to tall crops with a small “footprint” such as trellised tomatoes or pole beans. Dutch white comes up fast and establishes a tight cover which suppresses weeds, retains soil moisture, increases bioactivity in the topsoil (remember that the zone of greatest activity is in the rhizosphere), and adds biomass and nitrogen to the soil for future crops. Since it is low-growing, it does not interfere with managing or harvesting the taller crops above it. What more could you ask of one plant?

Another strategy comes to mind for establishing an overwinter cover. A crop like broccoli is quite cold-hardy, so—by the time its harvest is complete—we have lost the opportunity to start an overwinter cover in its place. Rye and vetch, on the other hand, are great winter covers, but rather slow to get started. It’s a marriage made in heaven: When transplanting, we undersow the broccoli with the rye and vetch. They take some time to establish, so do not overwhelm or shade out the broccoli. By the time we harvest the last of the broccoli, however, the rye/vetch cover is coming on strong—for protection of the ground over the winter, and a big boost to fertility in the coming spring.


Yellow Dock

A final thought about cover crops: Perhaps many homesteaders are a bit too paranoid about “weeds.” Some weeds are deep rooted, and can be used like comfrey as dynamic accumulators to bring minerals up from the deep subsoil. An example is yellow dock (the name is sometimes given to Rumex crispus, sometimes to the closely related and functionally similar Rumex obtusifolius), against which gardeners usually feel compelled to make war. Why not allow some yellow dock to grow here and there, in edges and corners where it is not in the way? When the plants start to make seed heads, cut them off just above the crown, to prevent huge numbers of seeds loose in the garden, and use them in mulches or composts. The deep, persistent roots will quickly replace the lost growth. You could say that we are using the dock as a “pump” to bring up additional minerals from the deep subsoil for use near the surface.