Urban Orchards : Grow Your Own Fertilizers

Grow Your Own Fertilizers: Cover Crops and Green Manures

The practice of growing dynamic accumulators and letting them be is called cover cropping. Turning the accumulator into the soil is green manuring. Cover crops and green manures improve the soil by:

Greatly increasing the organic matter content of the soil.
Increasing the availability of nutrients and decreasing leaching.
- The roots of cover crops and green manures are a safety net catching nutrients that otherwise leach away. (In older books, another term for cover crops is catch crop.) In addition, the thousands of foraging root hairs liberate nutrients from mineral form and assimilate them.
Improving the soil's tilth.
- Green manures and cover crops decompose, forming humus, which acts like a glue, binding together particles of soil in a way that opens pore spaces. The crumbly, granular texture so typical of a loamy soil that is full of worms results in part from humus. Soil structure is also improved by the action of the roots. As roots grow and die each day, tiny channels are left for the exchange of gases, the percolation of water, and the movement of worms and bacteria. This accounts in part for the spongy, soft texture of an unspoiled forest soil.
Reducing "weeds."
- A dense cover crop helps choke out undesirable plants. Tall, bulky cover crops, such as fava or bell beans, vetch, and Sudan grass, produce so much foliage and shade that plants germinating after the cover crop wither and die. Even rye and oats, which grow only 6 to 8 inches tall before being tilled under as a green manure, can smother unwanted "weeds." Tillage also helps destroy "weeds' that have already sprouted (be careful, however, to seed or transplant soon after green manuring, because tilling brings "weed" seeds to the surface to germinate.
Reducing soil pests.
- The roots of a cover crop and the decomposing foliage of green manures such as Mexican marigold (Tagetes rninuta), ornamental marigolds (to a lesser degree), and the crotolaria legumes exude chemicals, which help suppress soil nematodes. Avoid cover crops and green manures similar to the intended crop; they may harbor pests. A cover crop of mustards can allow the cabbage root maggot to flourish, which would endanger a following crop of cabbage. Likewise, avoid grasses for green manures if you plan to grow grains.
Enhancing the soil's biological activity.
- The shade of cover crops and green manures moderates the extremes of heat and moisture that bare soil suffers. Soil life is healthier. Green manures also provide raw food for soil life.

Establishing A Cover Crop or Green Manure
Whether you plan to grow a cover crop or green manure, start with a seedbed. While I have tossed seed into wild grass, the resulting germination and growth were very poor. You'll get better results with a rotary tiller, some time, muscle, and a soil that is neither too dry nor too wet.

See Rodale's Chemical-Free Yard and Garden for a chart that will help you select the best green manure for your orchard site. Choose a cover crop or green manure seed that prospers in your soil and climate and is suited to the coming season. Mix at least one type of legume with several herbal or herbaceous plants, and add a grass if none grows on the site.

Some plants are a little wild and woolly for an ornamental landscape-for example, vetch, Sudan grass, sorghum, and kale. Others look like a verdant lawn during the first portion of the season - rye grass, oats, clovers, millet, and barley are examples. Some cover crops make an impressive floral display. During the midsummer heat, a swath of buckwheat with its pure white flowers in full bloom is a spectacular sight (and buckwheat attracts beneficial insects, an extra bonus for your landscape). One of my favorite spring blossoms is the potent red of crimson clover. In a cover crop, crimson clover provides a colorful highlight as other early spring flowers fade. Its dramatic display lures the eye away from more mundane cover crops; it is a visual camouflage. Other cover crops with nice floral displays include bird's-foot trefoil, yellow and rose clovers, crotalaria, lupines, mustards, sesbania, and sunflower.

Except with the leguminous plants, all you need to do is work the soil to form a seedbed, rake in the seed, apply a light mulch, and water. If you have a new house surrounded by exposed subsoil, you will be better off adding soil amendments (fertilizers and organic matter) and topsoil to improve the raw subsoil. Even though some cover crops do quite well in harsh conditions, it is worth the expense and effort to provide a better soil horizon at the start. Improving subsoils solely with cover crops and green manures takes years.

The Needs of Young Legumes
While legumes are, in nature, colonizers of disturbed soils, they need certain conditions to sprout and flourish. For initial growth, they require a minimum level of available phosphate. Colloidal phosphate provides only 2 percent of its weight as phosphate in the first season, and only 18 percent over many years. The chemical forms of phosphate offer more available phosphorus the first season. Superphosphate is 20 percent soluble phosphorus and triple superphosphate has 46 percent of the desired element. Being very soluble and concentrated, they ensure the sprouting legumes a healthy start. Their judicious use makes sense in sites with poor soil. Like any synthetic fertilizer, however, superphosphates can do harm. For one thing, the higher concentration makes superphosphate easier to misuse. Also, beneficial, decomposing fungi and beneficial bacteria are reduced with repeated applications. And even though the soil "locks up" a lot of superphosphate, leached phosphorus from commercial applications has polluted streams and rivers, something that will not happen with the slow-release action of rock powders. There's no harm, however, in using a small amount for the initial growth of legumes. A single application of super- or triple-superphosphate should not be considered debilitating.

Legumes tolerate a slightly acidic soil (pH 4.8 to 6.3). If your soil supports good, healthy blueberry or camellia bushes, the pH is too acidic. To adjust the pH, lime (calcium carbonate) will be needed. Besides adjusting the pH, lime increases the growth of young leguminous plants and makes phosphates more available.

Crop-yield data available also support the conclusion that the liming of more acid soils (with a pH below 6.5) over a period of time renders the phosphorus of the soil more available to plants . . . the fixation of nitrogen from the atmosphere by legumes is more effective where high levels of calcium are present in available form. The root nodule bacteria of alfalfa and sweetclover do not persist well in a soil when its pH is below 6.5. (USDA 1938 Yearbook)

Legumes and Inoculants
If you sow legumes, be prepared to use rhizobial inoculant, a powder containing the strain of nitrogen-fixing bacteria that forms root nodules on your legumes. If your site has wild or "weedy" legumes, the proper bacteria may already be present. Look for bur clover (Mendicago hispida), lupines (Lupinus sp.), and beggarweed (Desmodium purpureum), among others. To be safe, buy the right inoculant when you buy seed and follow the directions on the packet.

Green Manures-An Investment in Your Soil's Fertility
There are many times during the season, and at different stages of plant growth, when you can till under a cover crop. Each time has virtues and drawbacks, and choosing a time is much like picking a savings plan. You can invest wisely or squander your capital, but no single plan is "the best" - for protection, use a number of plans, each for a specific situation.

Short-Term Investments, Rapid Return
Succulent young plant tissue has the highest amount of readily available nutrients. For a quick return on your investment, grow dynamic accumulators until just before they bloom, then turn them under as a green manure. They will release nutrients quickly for the crop that follows. However, you must wait three to four weeks before planting, while the soil digests the green manure. If you plant earlier, your crop will get less nitrogen, since much of the nitrogen will be tied up in the bodies of microorganisms. Once they digest the manure, large numbers die and decompose, releasing nitrogen for the crop.

Long-Term Investments, Slow Payback
Dry, dead plant material does not have as much nitrogen as green foliage, and the nitrogen is firmly bound up in a form that does not easily decompose. This can be a liability or an asset - depending on your investment strategy. If you need plenty of nitrogen in the next few months, do not till under dead cover crops. The soil's bacteria will be so busy trying to digest what you tilled under that little, if any, nitrogen would reach your crops.

As a long-term deposit, however, tilling dead plants under is an ideal way to keep organic matter and nitrogen in the soil. In fact, nature uses this investment scheme often - the forest is littered with the branches and trunks of these "long-term accounts." The slow decay and the billions of bacteria tying up nitrogen in their bodies ensure that this precious element does not leach or vaporize.

Timing
As with trading stock on Wall Street, timing is everything. To maximize gains, you must turn under the accumulator at the right time. The cooler the soil, the longer decomposition takes. Mid-spring warmth signals the beginning of the green manuring season. The hottest part of the summer is a "down market," too hot for healthy decomposition. The cooling soils of early fall are again ready for green manuring.

While the standard guideline calls for waiting three to four weeks after green manuring before seeding or transplanting, the wait is much, much longer if you tilled in brown, or mature, plants. The warmer the soil, the sooner you can plant. You will have to green manure for a number of seasons to discover the timing that works best with your climate, soil, and accumulators, but patience pays dividends.

Depositing Your Green Capital
With tall and bulky green manure plants such as fava beans, vetch, mustards, kale, Sudan grass, and sunflowers, a rotary tiller is difficult to use. Bastard trenching is the best way to proceed. First, cut the plants above ground and chop up the stems and foliage with a machete. Then, open up a trench, fill it half full with chopped plants, and cover with the soil taken from the next trench.

If you prefer tilling, grow the accumulators only 6 to 12 inches tall. A tiller can easily chop and till under short green manures. By immediately seeding a new green manure and tilling again when it reaches the same height, you can greatly speed up the improvement of your soil. You can grow a rapid succession of green manures throughout the summer. Choose plants that thrive in summer heat - buckwheat, rye grass, crotolaria, millet, sesbania, sorghum, Sudan grass, and sunflower. During the drought of the late 1970's in northern California, I pot my clients' flowerbeds into a winter and fall green manuring program. After two years, clients were calling to say thanks for the wonderful improvement in the soil - though nature had done most of the work.

References
Hills, Lawrence. Comfrey. New York: Universe Books, 1976. The definitive book on this important, though tenacious, herb. Mcleod, Edwin. Feed the Soil. Graton, CA: Organic Agriculture Research Institute, 1982. A basic review of nitrogen-fixing plants and cover crops, with an encyclopedic reference. I dislike the opening dialog between rabbits and a worm, but I use the encyclopedia frequently. Pieters, Adrian. Green Manuring. New York: John Wiley and Sons, 1927. The best and most comprehensive book on the subject, with lots of scientific studies. Required reading for serious students of green manuring. Out of print - look for it at university agricultural libraries. Raymond, R., and Altha, R. Improving Garden Soil With Green Manures. Charlotte, VT: Garden Way Publishing, 1979. A brief, excellent review of cover crops and green manures. Containing a very good table that outlines the virtues and limitations of many plants. Turner, Newman. Fertility Pastures. 2nd, ed. rev. Pauma Valley, CA: Bargyla and Gylver Rateaver, 1975. My favorite book on plants as indicators of soil fertility and plants as soil improvers in cover crops and herbal leys. Written for dairy farmers, but much of the information relates to edible landscaping, as well. U.S.D.A. Soils & Men: Yearbook of Agriculture 1938. Washington, DC: U.S. Dept. of Agriculture, 1938. This rare volume was written before chemical fertilizers had an iron grip on commercial agriculture and when farmers still grew most of their own fertilizer. An excellent review of soil preserving and soil building techniques. Includes William Albrecht's best paper on nitrogen fixing and green manures, Loss of Soil Organic Matter and Its Restoration. Walters, Charles, Jr., and Fenzau, C.J. An Acres U.S.A. Primer. Raytown, MO: Acres, U.S.A., 1979. A long winded but interesting survey of a genre of organic farming found throughout the Midwest. Though some of their conclusions are hard to accept, these farmers get good results and yields.

Reprinted with permission from:
Designing and Maintaining Your Edible Landscape Naturally, by Robert Kourik. Metamorphic Press, 1986. To order write to Box 1841, Santa Rosa CA, 95402. OR call (707) 814-2606. $50 + $5 shipping.