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Many transitioning organic farmers worry about having sufficient nitrogen in the soil. In a true organic system, we depend on the biological activity of the soil for our main source of fertility. It is important to think of the soil/crop relationship as a living system where different inputs are closely related. Every change you make affects everything else. Most of the cultural weed control techniques also increase soil biological activity and therefore also improve soil tilth and fertility. We make frequent use of cover crops, either interseeded with a winter grain or planted after an early-harvested crop is removed. Legume cover crops particularly add valuable levels of nitrogen to the soil, thereby reducing need for purchased inputs. Cover crops also improve soil tilth, increase soil organic matter, increase soil microbial diversity and activity, and protect the soil from erosion. We also use moderate amounts of composted leaves, gypsum, composted poultry manure, and other approved organic fertilizers but we try to use as little purchased fertility materials as possible. Transitioning farmers should be very careful about using uncomposted manure, since it tends to increase weed pressure, especially when used at high rates. Raw manure and rock dusts, such as rock phosphate, are best applied to a cover crop. This allows time for the nutrients to be converted into a stable, available form before the main crop is planted.
Probably the most valuable 'inputs' that a transitioning farmer can purchase are good quality soil tests and the assistance of someone skilled in organic soil fertility management to help evaluate the results and recommend appropriate amendments. We have found that achieving a good calcium:magnesium ratio is essential for weed management and improving soil structure. When magnesium levels are high relative to calcium levels, weed problems and soil compaction are more likely to result. Many prevalent weed species in fields throughout the United States, such as foxtail and summer annual grasses, thrive in hard compacted soils, most often soils that are also low in calcium and high in magnesium. We have found that gypsum (calcium sulfate) is a good, inexpensive way to add both valuable calcium and sulfur without increasing magnesium, as some forms of lime will do. Other tough weeds, such as quackgrass and nutsedge, are much less common on fields that are cultivated and where the soil is loose.
Weed control presents THE primary challenge to organic crop farmers. Especially during the three year transition period before a field is fully organic, keeping the weeds from getting out of control often seems like an insurmountable task. After land becomes fully organic, weed pressure does dramatically decrease and the weed species change. Before any machinery is considered, weed control must start with cultural methods. It is a great deal easier to prevent weed problems than to kill them. Heavy reliance on chemicals in modern agriculture has made the past generation forget that farmers do have much control over the initial weed population in a field.
There are a number of cultural weed control concepts that must be considered. These include basic sanitation, that is, simply preventing new weeds from being introduced onto the farm. The use of clean seed, mowing weeds around the edges of fields or after harvest to prevent them from going to seed, and composting manure before application can greatly reduce the introduction of weed seeds and difficult weed species. Crop plants will become more competitive when soil drainage is improved. Tillage methods that aerate the soil can increase biological soil activity and plant growth. Selection of crop varieties that are vigorous and can shade the soil surface can prevent weeds from 'breaking through' the canopy and growing above the crop. Recent plant breeding has produced chemically dependent crop varieties that are shorter and less competitive. Some older crop varieties are actually much better suited to organic production.
One way that plants compete with each other is by releasing chemical substances that inhibit the growth of other plants. This is called allelopathy and should be viewed as one of nature's most effective ways that plants deal with competition. Species of both crops and of weeds exhibit this ability. Allelopathic crops include barley, rye, annual ryegrass, buckwheat, oats, sorghum, sudan-sorghum hybrids, alfalfa, wheat, red clover, and sunflower. Vegetables, such as horseradish, carrot and radish, release particularly powerful alleopathic chemicals from their roots. The allelopathic effect can be used to advantage when oats are sown with a new planting of alfalfa. Allelopathy from both the alfalfa and the oats will prevent the seeding from being choked with weeds in the first year. Buckwheat is also well known for its particularly strong weed suppressive ability. Planting buckwheat on problem weedy fields can be an effective cleanup technique. Some farmers allow the buckwheat to grow for only about 6 weeks before plowing under. This not only suppresses and physically destroys weeds, it also releases phosphorus and loosens the soil.
Crop rotation is essential to break weed and insect cycles. Monoculture and heavy herbicide use over a number of years effectively selects for a large weed population that is very well adapted to those conditions. Every year that such an environment is created, all of those well adapted weeds will produce thousands or even millions of seeds. Within a few years, existing herbicides are no longer effective. In a proper crop rotation, we change the environment each year, denying the weed seeds the previous year's favorable conditions. In general, we try to alternate legumes with grasses, spring planted crops with fall planted crops, row crops with close planted crops, and heavy feeders with light feeders. A typical rotation on our farm could include alfalfa for 2-3 years, corn for 1 year, 1 year of soybeans, followed by winter wheat interseeded with clover. The clover is then plowed down and planted to red kidney beans. This is followed by a winter grain such as spelt or triticale that is underseeded with clover or could go back to alfalfa. We are still actively learning about rotations that will work well on our farm. Since each crop changes the soil slightly, we always try to plant a crop after its most suitable predecessor. The 'rotation effect' has been documented to increase yield and vigor of the crop, therefore making the crop more competitive and reducing weed pressure. We try to plan rotations several years in advance to optimize soil conditions on each of our fields and to insure we have a diversity of crops on the whole farm each year.
Many farmers successfully take new land through transition by growing hay, especially legume hay. This helps restore soil biological activity, reduce weed pressure, and increases residual soil nitrogen. Harvest the hay judiciously though, it is possible to 'mine' essential nutrients out of long term hay fields. If a farmer intends to grow a fairly small acreage of high value crops, such as vegetables, it would be well worth transitioning approximately twice the land they think they will eventually need for production. This will insure that some of the land can be planted to soil building crops each year, such as small grains and clover, thereby maintaining long term soil fertility and tilth.
