The Joys of Cover Cropping
Part 1: The Miracle of Soil Fertility
I wrote the following article for a presentation titled “The Integrated Homestead” at the Airlie Foundation Center, Warrenton, Virginia, in April 2009. It was published in two installments in the Nov/Dec 2010 and Jan/Feb 2011 issues of Countryside & Small Stock Journal.
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The Miracle of Soil Fertility Cover Crop Strategies and Species
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Even the most beginner-level gardener soon learns that “cover crops” offer one of the best ways to improve the garden’s most precious asset, its soil. The more we learn about cover crops and cover cropping strategies, the more deeply we enter the mysteries of soil life and soil fertility.
A note on terminology: A distinction is sometimes made between “cover crops” (grown to cover and protect the soil) and “green manure crops” (grown to increase soil fertility). For my purposes here, I will use “cover crop” to mean either or both: A cover crop is a crop that we grow, not to use as food directly, but to protect, nurture, and “feed” the soil.
In order to appreciate the value of cover crops, imagine first of all a bare patch of soil exposed to the sun, wind, and rain. As temperature climbs with the advancing day, the soil surface dries. Soil particles cease to adhere, becoming instead a dry powder over the surface. Winds pick up these unconnected particles and blow them away (wind erosion), and heavy rains wash them away (water erosion). The longer this patch of ground remains exposed, the drier and more powdery it is, and the more soil particles are lost. If we leave it exposed long enough, there will be no soil left, and we’ll have to find another patch of ground if we want to grow something to eat.
Now imagine that same soil under a tight planting of clover. Its surface is cooler in the tight shade of the plants, its moisture less likely to be lost to evaporation. When the wind blows, its force is broken by the cover of plants (and in any case, the enhanced moisture under the cover means greater adhesion of soil particles, with much greater resistance to being picked up by the wind). Rain does not beat upon the surface—its force is broken by the leaves, causing a more gentle deposition of water on the soil, enabling it to absorb the rain, rather than wash away. Already we see how the cover crop is benefiting our soil, simply because it is preventing its physical removal from the site. Further, even a neophyte will intuitively sense that conditions under the cover (cooler temperatures, enhanced moisture, shade from the broiling sun) are more “soil friendly” than on bare ground.
And look, there’s an earthworm feeding at the surface before burrowing again into the depths. Here is a spider, there a pill bug, and there a ground beetle. Even at the visible level, we see that the environment under the cover crop is more alive than on the bare patch of ground.
Now let’s switch to X-ray vision and look beneath the surface. Suddenly the term “alive” takes on startling new meaning: The soil is teeming with bacteria, protozoa, nematodes, and shot through with fungal filaments. Looks like quite a party. To get a better idea of what they are doing there, we need to look again at the cover crop itself, and especially at what is going on inside its leaves.
The utility of cover crops for soil improvement starts with their ability to harvest the energy in sunlight and use it to drive chemical processes that synthesize complex sugars, proteins, and other molecules from carbon dioxide in the air, and water and minerals taken up from the soil. (In the terms used by the ancient Greeks to describe the physical world, solar energy is the “fire” that makes possible the fusion of the other three elements: earth, air, and water.) These molecules are used as food—that is, fuel for metabolic processes, and for growing new tissues. This harvesting of sun-energy directly for making their own food (photosynthesis) is something we animals cannot do. It is plants’ great contribution to other life forms, their work in the world.
Now let’s turn that cover crop under. We’ve been told that this “feeds the soil,” but exactly how? Remember all the species on and under the surface of the soil. They are not simply a random collection of life forms, but a cooperative community. We could also think of them first of all as an energy-exchange system. We’ve already seen how the cover crop plants convert solar energy and the mineral and water content of the soil into complex molecules that are in effect stored energy. When we kill the cover crop plants by turning them into the soil, the stored energy in their tissues is appropriated as an energy source (food) by other players in the soil system.
The bacteria are among the first to start harvesting the stored energy. Though we cannot see bacteria, in healthy soil they are present in unimaginable numbers. Good prairie soil, for example, may contain up to thirteen tons of bacteria per acre. Bacteria specialize in fresh, green, recently dead plant residues, using their breakdown as an energy (food) source.
However, bacteria then serve themselves as food source for other members of the “soil food web”—for example, earthworms. And note that it is not just plant tissues, and creatures that eat them, that get eaten by members of the soil community—the “wastes” of soil community members (unused metabolic residues—say, the fecal pellets of crickets or pill bugs) contain residues of still-reclaimable energy. Wherever we look in the multiple pathways of energy exchange in the system (known as “trophic levels”), we see that “someone” has emerged with the capability for utilizing any energy residue as food. Thus all potential energy leaks out of the system are plugged. Since energy is always being added to the system in the form of sunlight, soil fertility necessarily increases over time.
The final residue of this complex breakdown process is humus—carbon compounds that resist further decomposition, and thus are no longer available directly as food energy. However, humus content in soils plays many important roles: It increases water retention, assists in the chemical bonding of nutrients with plant roots, and improves soil texture. (In sticky clay soils, for example, it helps with “aggregation” of tiny soil particles with little space between them, into larger clumps with larger pore spaces. The result is looser soil structure and increased oxygen—a more compatible environment not only for plant roots but for all members of the living soil.)
But it is not just the green parts of tilled-in cover crops that get consumed in the soil food web. The roots of dead cover crop plants are also a potential energy source—for anyone with the capability for utilizing them. In contrast to the soft green tissues, the roots have a much higher content of cellulose and lignin—high-carbon compounds that stiffen structural tissues like stems, branches, and roots—which resist breakdown by bacteria. However, soil fungi specialize in breaking the powerful chemical bonds of cellulose and lignin, as a way of harvesting their energy.
Though we think “mushrooms” when we think of fungi, the mushrooms we see are merely reproductive bodies grown by the actual fungi—thin filaments that form interwoven mats (mycelia) beneath the surface of soil, inside decaying logs, in the lower levels of leaf litter, etc. (There may be several yards of fungal hyphae—thread-like growing filaments—per teaspoon of garden soil, up to hundreds of yards in a teaspoon of prairie soil.) Though largely “out of sight, out of mind,” fungi play a critical role in the return of woody and cellulose-dense tissues (such as roots) to simpler compounds, suitable for uptake as nutrients by plant roots and other players in the soil trophic levels, and ultimately to additional soil humus.
In the case of the roots, the decomposition occurs “in place.” That is, the decay occurs along every inch of the extensive net of roots throughout the soil, opening up channels through which air and water, and earthworms and other soil web members, can move. Again, the result is more open, porous soil structure. More oxygen (as essential to most members of the soil food web as to you and me) enters the soil, and rain runs down into these channels—instead of running for the sea, carrying a burden of precious topsoil.
Given the depth to which roots grow, and in some cases their astounding mass and extent—alfalfa pushes its thick taproot eight to ten feet into ordinary soils; rye has an extremely dense root mass of countless fine roots, growing to four or five feet deep—the changes in soil texture following a cover crop can be quite dramatic.
So far we’ve been thinking about what happens after cover crop plants die. But let’s look at ways the soil changes while the cover crop is alive. We’ve become accustomed to asking what we can do to change the soil to benefit plants via their roots. Actually, plants themselves do a great deal to alter the nature of the soil they are growing in. They take the initiative to change it from mere “dirt” into something ever more complex and dynamic.
We often think of nature as “red in tooth and claw”—that is, as an arena of fierce competition, even combat. What a surprise when we learn how much natural systems depend on cooperation, on the forming of alliances between species in a given ecology for mutual benefit. Certainly there is fierce competition among soil community members for available resources, and some species can be pathogenic (disease creating) for others. But in truth, the more fierce becomes the competition, the more incentive to form alliances for access to resources, and for dealing with potential threats.
For example, we considered above the way in which plants synthesize complex compounds as usable food. So after all the hard work, you’d think they would hog all that good stuff for themselves, right? Interestingly, large quantities of the complex molecules synthesized by plants are not retained for their own needs directly, but are exuded through their roots—in the form of sugars, amino acids, organic acids, enzymes, proteins, and more—to feed their buddies in the soil. Among the many friends fed by exudates are bacterial colonies immediately around the roots, with increases in both overall numbers and diversity of bacterial species. Some strains of soil bacteria synthesize compounds that inhibit growth of pathogens, or colonize the surfaces of roots, providing a protective coating pathogens cannot penetrate. Some types also produce compounds that have a positive effect to boost growth of the plant. For example, certain bacteria help dissolve phosphorus (plentiful in most soils, though often in forms that are biologically inert), making it more available for uptake by plant roots. Protozoa and nematodes move in to graze on the bacteria. In the ensuing feeding frenzy, pathogenic organisms typically cannot attain the density levels that would cause disease.
We referred above to fungi that decompose (feed on) carbon-dense tissues of dead plants. Actually, that activity is carried out by a class of fungi called saprophytes, which are likely to be active anywhere there are accumulations of such residues on or under the soil surface. But there is an interesting class of fungi that form mutually beneficial relationships with living plant roots themselves, the mycorrhizae. Though some of our crop plants—for example crucifers (which include turnips, radishes, broccoli, and others) and chenopodia (spinach and beets)—do not form mutually dependent relationships with mycorrhizal fungi, most of them do. The fungi anchor on the plant roots and range out into the soil in their typical hyphal strands and mycelial mats. In effect, the fungus extends and enlarges the root system, resulting in greater water and nutrient uptake—for both plant and fungus—than either could manage alone. Root exudates feed the mycorrhizae, which in turn furnish roots with higher levels of phosphorus, nitrogen, and micronutrients.
The relationship between plant and fungus can be quite intimate, the hyphae penetrating the spaces between the cells of roots, or in some cases actually growing inside cell walls. Mycorrhizae benefit their hosts by suppressing diseases (including fungal diseases), with the potent antibacterial compounds they synthesize, or by parasitizing disease-causing nematodes.
When we think about sustainable soil fertility, let us remember this vision of a healthy, burgeoning community based on alliances—on the giving of good gifts from both sides to support and sustain a relationship of mutual benefit. This should be our guide as we try to boost the fertility of our soil—not the blandishments of agribusiness, eager to sell us soil fertility in a bag.
Let’s focus again on this fact: The greatest diversity and concentration of soil species is in the area immediately surrounding plant roots, the rhizosphere. That is, the more roots in the soil, the more alive is the soil; and as we have seen, a soil that is more alive is also more fertile. This may be counterintuitive, since we’ve probably been conditioned to think that crop plants take nutrients out of the soil (with the sly implication that we have to purchase nutrients—“fertilizers”—to replenish them). But here we see a case where soil fertility has increased, solely as a result of getting more plants (and plant roots) on the scene. Once we see the beauty of this concept—that soil fertility and maximum achievable aliveness of the soil are one and the same thing—we will never again be suckered into buying a dusty, odd-smelling powder in a bag to “fertilize” our soil (whether “organically certified” or not). This is not mere feel-good theory: In our own garden, we have not used any purchased “fertilizers” for well over two decades, a period in which the soil has done nothing but improve—and become more productive.
All cover crops make their contribution to soil improvement, but most gardeners know that legumes as a class (beans and peas, clovers and alfalfa, and more) offer a bonus: They help “fix” atmospheric nitrogen in the soil. Actually, it is not the legume itself which performs this bit of alchemy: We shouldn’t be surprised to learn that it does so on the basis of an alliance with a class of rhizobial bacteria which live inside its roots—feeding on sugars provided by the plant and synthesizing nitrogen from the air into nitrates which are used by the legume itself, and which become available to other players in the rhizosphere after the plant dies. If you pull up a cover crop plant such as clover or cowpea, you will see the nodules of nitrates along the roots like beads on a thread. Their size and number make it obvious that the amount of nitrogen fixed in the soil is not trivial. In a field of alfalfa, for example, fixed nitrogen can amount to hundreds of pounds per acre per year.
Especially deep-rooted cover crops such as alfalfa “mine” the deep subsoil for minerals. When the plant dies (in the case of annuals) or sheds its leaves for winter dormancy (perennials), this enhanced mineral content becomes available to more shallow-rooted crops.
Remember that any cover crop that flowers helps support insect diversity; and that enhancing insect diversity is in the long term a better strategy for effective and ecologically sustainable limits on crop damaging insects than any program based on killing insects. Bird, amphibian, small mammal, and other populations also benefit from the additional ecological diversity produced by cover cropping.
Cover crops can do double duty as a source of feed for livestock. We can cut fresh green fodder such as grain grasses or alfalfa and carry to livestock such as pigs, poultry, and goats. We can dry such cuttings to make hay. (You might object that I’m “greying” the distinction here between cover crops and field crops. But that’s the name of the game in integrative cropping: In this case, a source of harvestable, storable livestock feed is also protecting and building fertility in the ground it covers.)
Do note the possibilities for allowing certain cover crops to mature, then turning appropriate livestock species in to self-harvest the resultant bounty. In earlier times, it was common to grow turnips, kale, even corn to maturity, then turn in pigs to “hog it down.” We can allow small grains and buckwheat to mature their seeds, then send in the chickens to do the harvest. Such strategies offer us an alternative for feeding our livestock with foodstuffs grown on our own ground—after all, growing these crops is the easy part—without the significant, maybe prohibitive, investment in equipment, labor, and storage for typical feed-grain crops.
As we work with cover crops, we should keep in mind the “feedback loops” that emerge as the seasons roll. The more cover cropping we do, the more life there is active in our soil, and the faster the organic matter we add to soil (whether in the form of composts, manures, mulches, or cover crops) is processed into plant nutrients—and ultimately humus. Earthworm populations soar under cover crops—and the more burrowing earthworms we have, the more rapid the changes they make to soil texture, and the more they bring up mineral riches from the subsoil depths (ten to thirteen feet) for deposition at the surface as castings (earthworm poops), which are teeming as well with beneficial soil microbes.
Cover crops, especially legumes, help enhance levels of mycorrhizal fungi spores, ensuring high rates of “infection” by beneficial fungi in subsequent crops.
Let’s come back to the idea, referred to above, of “turning in” the cover crops—that is, tilling in order to kill the cover crop and incorporate it into the soil. Against the background of all we’ve learned about the life community in the soil, it should be intuitively obvious that a community will be injured by serious disruption. Keep in mind the soil’s natural “profile,” with the denser, mineral-rich subsoil below and the more organic-matter-rich, biologically active few inches on the top. In addition to disrupting the pathways of the soil food web, most mechanical tillage—by plow or power tiller, even a spading fork if used to turn over forkfuls of soil—either inverts or mixes together these levels, in effect “diluting” the high-organic, more biologically active topsoil we’ve worked so hard to nurture. If you do need to loosen the soil, use a broadfork—a wonderfully ergonomic tool, a pleasure to use—which gently loosens the soil to a depth of ten inches or more, but does not invert its layers.
Of course, some soil disruption is inevitable in gardening. When I dig potatoes or sweet potatoes, for example, I’m aggressively going after that last delicious nugget, and massively disrupting the soil food web and mixing the layers of the natural soil profile. But if we rely heavily on routine and repeated mechanical disturbance (tilling) of the soil—so characteristic of modern agriculture—the next round of tillage may occur before the soil community has healed itself. In that case, the soil is on a downward spiral of decreasing tilth and fertility.
Unfortunately, excessive mechanical tillage is an inherent part of most modern industrialized agriculture. The negative impacts on the soil are not trivial. Remember the discussion about erosion above—the loss of soil to wind and rain. There is, however, a third form of erosion which has taken a disastrous toll on the nation’s soils: oxidative erosion. When soil is exposed too frequently and too excessively to oxygen, its carbon (humus) content oxidizes—that is, the oxygen bonds with the carbon to make carbon dioxide. Yes, that’s right, that carbon dioxide, the greenhouse gas. Though we usually think of climate-changing CO2 as coming out of the tailpipes of our infernal combustion engines, it is also emitted on a colossal scale by mechanical tillage in industrialized agriculture. (Note that chemical fertilizers and use of toxic pesticides and fungicides also lead to destruction of soil carbon, which always results in its release to the atmosphere. And remember that, while we’re focusing for the moment on the increase of atmospheric carbon dioxide resulting from loss of soil carbon, the loss of humus in the soil also means loss of fertility and tilth—the loose, open structure so important to soil quality—and reduced water retention.)
Remember as well, however, the discussions above of practices and processes that add more carbon to the soil (cover crop roots decomposing in place, fungi “eating” woody tissues and leaving humus residues, etc.). These processes sequester carbon in the soil—that is, bind carbon into compounds that remain in the soil for as long as we are willing to practice sustainable agriculture. Whatever “Big Ag” is doing to exacerbate climate change, in our small way we can start to reverse those changes—by building up, rather than dispersing, soil carbon. (The amount of carbon in play is not trivial: It has been estimated that every one percent increase of carbon in a garden’s soil is equivalent to the weight of all the carbon in the atmosphere above that garden, right out to the vacuum of space.)
Ah, but here’s the rub: If we are striving to minimize soil life disruption by minimizing tillage, how do we manage to kill the cover crop in order to make its fertility available to following crops? A power tiller is the usual answer to this conundrum. Not in my garden, however. Not only does a tiller have a destructive effect on the “crumb” texture we’ve worked so hard to help create in our soil, it is something of an illusion that it is “labor saving.” When you factor in the time (and the manufacturers of these machines hope you won’t) spent on maintenance and repair, and on frequent untangling of tough cover crop stems and roots from the tines, the tiller may not be “faster,” “easier,” or “more efficient” than more human (or livestock) powered alternatives. A tiller is as well enormously more expensive, both for purchase and for upkeep, than well made hand tool alternatives. It is noisy, stinky, and jarring—forms of stress on the body and nervous system which add to the work of the tillage task. Perhaps for a large market garden a power tiller is a necessity, but—especially as the condition of our soil improves and it is more easily worked—a power tiller for the home gardener is a case of overkill, of wretched excess. There are several alternatives.
The most straightforward way to kill the cover crop is to pull it up by the roots (and lay the plants on the bed as a mulch). This is somewhat labor intensive, to be sure. It becomes less so, however, as the soil “mellows” (becomes more friable) as a result of cover cropping and other soil nurture practices. (You will be amazed at how much faster and easier many time-consuming garden tasks, such as weeding and transplanting, become as the soil loosens and deepens.) If the soil is not loose enough, loosen it with a broadfork—plants will pull out much more easily. Also, time this task following a good rain (or water thoroughly the day before), when the moist soil will yield up plant roots more easily.
We know by now the benefits to the soil as roots from killed cover crop plants decay in place, so you might worry that hand pulling denies the soil this benefit. That is true to some extent, of course, but be assured that a large number of smaller roots break off and remain in the soil as you hand pull the cover crop plants.
I only pull cover crops up by the roots when I have to. Vigorously growing grain grasses, for example, will simply regrow if cut, so I pull them up if I have to prepare the bed for planting immediately. But mowing will kill many cover crop plants, which is certainly less disruptive to soil life, and of course leaves the entire network of roots in place. For example, it is easy to kill cowpea plants by cutting them off at ground level. Sometimes cutting a cover will kill it in one part of its life cycle, whereas cutting it earlier only stimulates regrowth. For example, rye as a cover crop is notoriously difficult to kill. If you mow it when it flowers, however (when the anthers expand and pollen can easily be shaken off), it will die. The cut plants can be used for mulch, either on the same bed or elsewhere, or used in compost heaps. Oats die after maturing their seeds. Thus another strategy for utilizing a cover crop without turning it in: Grow the oats as a nurse crop for clover or alfalfa; cut the mature seed stalks for feeding livestock; and allow the nursed crop to surge as more sunlight reaches it through the oat stubble.
Putting down a heavy, tight mulch will smother whatever it covers. Thus another way to kill a cover crop—say, a stand of clover, another plant that regrows vigorously after cutting—is to cover it with what I call a “kill mulch.” A puny little mulch will not do the job—a lot of mulch is required. I often start with a layer of newspaper and/or cardboard, which creates a more smothering barrier to growth, and cover thickly with whatever organic mulch I can get my greedy hands on. (I have read in numerous sources that modern newsprint is not polluting of garden soil. I do send the slick colored paper off to the public recycle bins, though supposedly even that is okay to use. I’m a little less confident of the purity of cardboard, though have read that all American and European cardboard is safe to use for mulching. Cardboard manufactured in other parts of the world may or may not be. My compromise is to use cardboard as the bottom layer of mulches over permanent paths, and for wide-area, one-time kill mulches in the “forest garden.”)
You can even do lazy-gardener composting by laying down all the materials you’d normally use in a compost heap (the denser, more moist, more nitrogenous materials on the bottom; the coarser, drier, more carbonaceous stuff as a protective layer on top) to smother a cover crop and initiate its consumption by the soil food web. (This technique has been called “layer composting” and even “lasagna gardening.” The soil food web converts the organic materials to fertility, but without the labor of assembling and turning compost heaps.) Having put the kill mulch in place—and again, you’ll need to use a lot for this technique to work—you can transplant large crop plants like tomatoes and peppers into holes through the mulch, but you’ll have to put off growing small-seeded, direct-sown crops until the mulch has been largely “digested” by the soil.
There is one “power tiller” which, based on long years of its use, I can recommend without reservation: Gallus gallus domesticus. That’s right, your chicken flock is your best “lazy gardening tiller.” Having too many times in the past attacked the tough, massive root system of a rye cover with a power tiller, I’m more than happy to hand off that chore to my hard-working chickens instead. My preferred method is simply to net the area with electric net fencing (“electronet”) and let them have at it.
On a smaller scale, you can make a “chicken tractor,” a small, movable poultry shelter sized to fit the width of your garden beds. Eight or so chooks inside will eagerly till in a cover crop on a single bed, without disturbing adjacent beds with harvest crops. When tillage is complete, simply move the shelter one length down the bed.
The benefits of tilling with chickens are numerous: Their tillage is only inches deep, so they have no effect on the soil profile, and disruption in the “action” zone in the top few inches of soil is minimal. Such disruption as occurs is more than offset by the load of poops laid down and worked in by the birds—a tremendous boost to soil microbes and fertility. They enjoy a bountiful harvest of nutrient-dense foods—the cover crop plants, slugs and snails, earthworms, and more—as a bonus for their services; and your wallet enjoys a bonus as well, from the savings on feed costs. (About those earthworms: Yes, we should try to maximize earthworm populations in our garden soil. But I’ve never noticed a drastic reduction in worm populations following a pass by my chickens. Indeed, their droppings, which boost bacteria and other microbes, prime foods for earthworms, probably lead to a concomitant increase of worm populations in the long term.)