Soil Care Basics: Increasing Organic Matter
and Mineral Availability
The following helped lay the groundwork for my article “8 Strategies for Better Garden Soil”, published in the June/July 2007 issue of Mother Earth News.
The care of the Earth is our most ancient and most worthy, and after all our most pleasing responsibility. To cherish what remains of it and to foster its renewal is our only hope. ~Wendell Berry, The Unsettling of America
In Soil Ecology: The Basics of Fertility, I discussed soil ecology and concluded that our best soil-care strategy is the imitation of natural soil systems, which negatively means avoiding the destructive practices of industrial agriculture—monoculture, use of synthetic chemicals, and excessive tillage—and positively means increasing soil life diversity and population densities, feeding the soil from natural (and more home-grown) sources, and protecting soil structure. With those concepts as background, we can focus on specific practices to achieve these goals. Most soil-care strategies cluster around two key concepts: increasing organic matter and mineral availability—discussed in this article—and finding alternatives to the disruption of tillage—discussed in the companion piece “Protecting Soil Structure with Alternatives to Tillage”.
It has been said that organic matter in the soil consists of “the living, the recently dead, and the very dead.” The “living” portion consists of all the diverse forms of living organisms that make up the soil food web, but also plant roots themselves. It is a good thing to have lots of plant roots in our garden beds, because the most intense biological activity in the soil is found in the rhizosphere—the area in and immediately surrounding plant roots—largely because of important symbiotic relations between plant roots and soil organisms. “Recently dead” (or “active”) components include recently deceased soil organisms of all types, green plant material such as crop residues, and fresh manures. They decompose readily and make nutrients available quickly. The “very dead” portion is humus, the final residue of organic matter breakdown that is so important for soil structure, water retention, disease suppression, and nutrient-exchange pathways. All three forms of organic matter should be present in soil in goodly amounts, at all times, in order to cater to the specific “tastes” of the different classes of soil food web organisms, enlist their services in breaking down organic material into forms usable by plants, and improve soil structure. Our sources of organic matter should be as diverse as possible.
Manures of all domestic livestock—your own or a neighbor’s—can be valuable additions to soil. Their nutrients are readily available to soil organisms and plants. The easy-to-decompose organic matter in manures makes a greater contribution to soil aggregation by soil organisms than composts, which have already largely decomposed. The manures of ruminants contain more fiber, which breaks down more slowly and is thus available as a food source for longer periods, and makes a greater eventual contribution to soil humus.
Manure application must be done with care. To guard against contamination by possible pathogens in manure (far less likely in manures from homesteads and small farms than those from high-confinement livestock operations), it is best to allow a three-month interval between application and harvest of root crops or leafy crops like lettuce and spinach. (Tall crops like corn and trellised tomatoes should have no contamination problem.) Because nutrients from manures are so readily available in a big surge, they are more subject to leaching from the soil, where they are needed, into groundwater and streams, where they most definitely are not. If manures are overused, especially as the sole source of nitrogen, they tend to over-accumulate certain nutrients, especially phosphorus. Perhaps it is best to restrict fresh manures to heavy feeding, fast-growing crops like corn, and process additional manure by composting.
When thinking of manures, it is worth thinking a bit about our own. Flushing “humanure” away (ultimately) to the sea is highly problematic for water systems, and represents a net loss of potential fertility from agricultural soils. On the other hand, no manure requires more cautious management for safety than humanure. I recommend Joe Jenkins’ The Humanure Handbook, the “bible” on this subject. Don’t be hesitant about experimenting: A small proof-of-concept humanure composting operation consisting of two 5-gallon buckets and a compost bin can be very low-profile indeed. If you are nervous about using humanure compost on food crops (Jenkins has used his right in the vegetable garden for more than 15 years with no ill effects), you can use it to feed trees, shrubs, and “fertility patches” (more of which below).
Composting is the great recycler of almost any organic “waste.” It reduces the bulk of organic materials, stabilizes their more volatile and soluble nutrients, and speeds up the formation of soil humus. A properly made compost heap is assembled by layering more readily decomposable materials (wet, green, fresh, high-nitrogen) such as manures, crop residues, kitchen wastes, weeds, and other fresh green plant materials such as pasture cuttings—with less decomposable materials (drier, coarser, denser, more high-carbon) such as autumn leaves, straw, and residues of harvesting such as corncobs. The carbon to nitrogen (C:N) ratio is important—say an average of about 30 parts carbon to one of nitrogen—for efficient decomposition by the microbes that feed on the composting materials and break them down into simpler, more stable compounds. Heat is generated by the process. A temperature of 110° to 130° F is probably optimal for the thermophilic (heat-loving) microbes driving the decomposition, but such temperatures help kill weed seeds and disease organisms. Since water and oxygen are essential to the microbes, the pile should be moistened as it is assembled, but should not be sopping wet, which would decrease availability of oxygen and encourage growth of pathogenic organisms. The pile must be large enough to retain much of the heat of decomposition, but not so large that oxygen cannot penetrate to the center. Aeration is encouraged by mixing in the coarser elements throughout, ensuring plenty of air space. After the heat peaks, the heap should be turned—with the outer layers going to the inside and vice versa—to incorporate more oxygen and generate a new heating cycle. A third turning is sometimes necessary to complete the process.
Composting is at least as much art as science. More than anything I know, it is something to “learn by doing.”
Composts are great for improving soil quality and water retention, disease suppression, and humus content (though less decomposed materials such as manures and green plants give a more immediate “flush” of nutrients when applied directly). However, they are quite labor-intensive. The older I get, the more interested I am in an easier alternative. Fortunately, there are three.
In classic composting, we layer the more high-nitrogen, easily decomposed elements with the high-carbon, difficult to break down materials. Suppose instead we keep these two types of compost materials separate, and simply apply them in two layers directly to the garden bed. This is the concept of “layer composting” or “sheet mulching”—it has even been called “lasagna gardening.” The more moist, volatile, high-nitrogen materials go down first, in direct contact with the soil and the microbial populations ready to feed on them, while the drier, coarser, high-carbon elements are used as a cover to keep the first layer from drying out or losing its more volatile elements to the atmosphere.
An effective—and fun—alternative to labor-intensive classic composting is vermicomposting, using earthworms to convert nutrient-dense materials such as manures, food wastes, green crop residues, etc. into forms usable by plants. You can buy manufactured worm bins in which the worms grow and convert what you feed them, or you can easily make your own. You can also buy composting earthworms. Note that the species used is usually Eisenia foetida or related species—“red wrigglers” or “manure worms” (the sort you will find in an aging manure pile, which indeed you could use to “seed” a population in your bin), rather than species that burrow deep into soil like “night crawlers.” I started out with a 3×4-ft worm bin, then last year converted the center of my greenhouse to a 4×40-ft series of bins, 16 inches deep. My worms process horse manure by the pick-up load from a neighbor who breeds and boards horses, and the earthworm castings are a major part of our fertility program. Not only do castings help feed plant roots, they also carry a huge load of beneficial microbes that boost the soil organism community.
Another fun alternative to composting is chicken power. I use electric net fencing to manage my chickens, rotating them from place to place on pasture. When needed, however, I “park” them on one of my garden spaces, onto which I have simply dumped whatever organic materials I would once have used to assemble compost heaps. The chickens happily do what they love best—scratch ceaselessly through that material looking for interesting things to eat, in the process shredding it and incorporating it into the top couple inches of soil, the zone of most intense biological activity. Their droppings, scratched in as well, give a big boost to the soil microbes.
As said in “Soil Ecology: The Basics of Fertility”, it may be necessary initially to add slow-release sources of minerals such as rock powders to correct mineral deficiencies. In the long run, however, we can supply needed minerals with less reliance on purchased inputs. The organic materials we add to our soil bring with them, in addition to nitrogen to feed growing crops and carbon to boost humus, most of the minerals healthy crops need. In addition, however, planting “fertility patches” allows us to grow a lot of our own mineral supplementation. Certain plants can function as “dynamic accumulators.” That is, their roots grow down into the deep subsoil, “mining” it of mineral reserves made available out of the parent rock itself, and making them available to more shallow-rooted crops. The roots of comfrey, for instance, can grow eight to ten feet into the subsoil. Stinging nettle may have a bad reputation among gardeners who have felt its sting, but it is an extremely useful dynamic accumulator. Both nettle and comfrey, in addition to high mineral content, are high in protein (nitrogen), and can be used to “fire” a compost heap or for mulches. (More on mulches below.) And both will benefit from massive infusions of organic fertility, in any form you can throw at them, even raw poultry manure.
In addition to their role in bringing mineral content up from the depths, fertility-patch plants can be used to correct mineral imbalances. For instance, if overuse of manures has led to excessive levels of soil phosphorus, alfalfa—which benefits from high levels of phosphorus—can be grown as a “sponge” to take up excess phosphorus in the soil. When cut and used in composts or mulches, it makes the phosphorus available elsewhere on the homestead where it is needed.
If you have some pasture, think of it as well as fertility patch par excellence: Especially when growth is fast and lush in the spring, you should be able to take one or two cuttings, perhaps even more, for use in composting or as mulches. If you do not have any pasture, consider using parts of your lawn instead, perhaps those less visible if you are nervous about a neighborhood outcry. I have begun overseeding my lawns each fall with the same sort of grass/clover mix I use on the pasture. In the spring, I allow some areas to grow about eight or ten inches before cutting with the scythe for fertility applications elsewhere.
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.
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.
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 grassroots 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 crops 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.
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 for 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.
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!)
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.
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.