Tools & Practices (continued)

Planned Crop Rotation

Essentially a tool for annual cropping systems, crop rotation refers to the sequence of crops and cover crops grown on a specific field. Particular sequences confer particular benefits to long and short-term soil fertility, and to pest management.

Agronomic operations are especially dependent on crop rotations that include forage legumes. These provide the vast majority of the nitrogen required by subsequent crops like corn, which are heavy consumers of that nutrient. Even when livestock enterprises are present to generate manure, the animals are largely recycling the nitrogen originally fixed by legumes in the system. An example of a basic agronomic rotation, typical of that found on Midwestern organic farms, is shown in Figure 2.

The basic Midwestern rotation demonstrates the elegant way in which a whole farm system can be derived and function:

• Legumes fix nitrogen in the soil, providing for subsequent non-legumes in the rotation.
• Several insect pest cycles are interrupted, especially that of the northern and western rootworm species, which can be devastating to corn.
• Several plant diseases are suppressed, including soybean cyst nematode.
• Weed control is enhanced as perennial weeds are destroyed through cultivation of annual grains; most annual weeds are smothered or eliminated by mowing when alfalfa is in production.
• Livestock manures (if available) are applied just in advance of corn, a heavy nitrogen consumer.
• All crops can be marketed either as is, or fed to livestock on-farm and be converted into value-added milk, meat or other livestock products.

Ralph and Rita Engelken, widely respected organic pioneers in the 1970s and 1980s, used a similar rotation that suited their hilly northeast Iowa farm and supported their main livestock enterprise—backgrounding beef cattle. (Backgrounding is confined or semi-confined feeding of young range stock to increase their size before final finishing in a feedlot.) The feed ration the Engelkens relied on consisted mostly of haylage, corn silage, and ground ear corn. The 6-year rotation/crop mix that allowed them to produce virtually all their own feed on 410 acres was:

Another example of an agronomic crop rotation—this one suitable to drier, western climates—is typified by the Quinn Farm in North Central Montana and presented in Figure 3.

Bob and Ann Quinn’s rotation begins with the most reliable cash crop, hard red winter wheat, fall-seeded after alfalfa. Weeds are controlled following harvest and the land reseeded to lentils, kamut or durum wheat the following spring. Switching from a winter grain to a spring grain helps to break weed cycles and optimizes soil moisture. In the next year another spring grain or buckwheat is planted and undersown with alfalfa.

If the alfalfa survives the winter, it is managed as a hay crop for a year and incorporated in April prior to seeding winter wheat. If the alfalfa is winter-killed, peas are planted in spring, followed by winter wheat in the fall—shortening the rotation by one year (18).

In vegetable crop rotations, nitrogen fixation and carry-over is also important, though it plays second fiddle to pest management concerns. The well-known market gardener, Eliot Coleman, recommends an 8-year rotation as shown in Figure 4.

The rationale for Coleman’s 8-year rotation follows. Since he gardens in the Northeast, some of the details reflect those constraints.

Potatoes follow sweet corn…because research has shown corn to be one of the preceding crops that most benefit the yield of potatoes.

Sweet Corn follows the cabbage family because, in contrast to many other crops, corn shows no yield decline when following a crop of brassicas. Secondly, the cabbage family can be undersown to a leguminous green manure which, when turned under the following spring, provides the most ideal growing conditions for sweet corn.

The Cabbage Family follows peas because the pea crop is finished and the ground is cleared [early] allowing a vigorous green manure crop to be established.

Peas follow tomatoes because they need an early seed bed, and tomatoes can be undersown to a non-winter-hardy green manure crop that provides soil protection over winter with no decomposition and regrowth problems in the spring.

Tomatoes follow beans in the rotation because this places them 4 years away from their close cousin, the potato.

Beans follow root crops because they are not known to be subject to the detrimental effect that certain root crops such as carrots and beets may exert in the following year.

Root Crops follow squash (and potatoes) because those two are good “cleaning” crops (they can be kept weed-free relatively easily), thus there are fewer weeds to contend with in the root crops, which are among the most difficult to keep cleanly cultivated. Second, squash has been shown to be a beneficial preceding crop for roots.

Squash is grown after potatoes in order to have the two “cleaning” crops back to back prior to the root crops, thus reducing weed problems in the root crops (19).

Georgia growers Ed and Ginger Kogelschatz use a somewhat simpler rotation scheme that divides most garden crops into four basic classes that are then sequenced for a 4-year cycle (20). They have adapted this concept from Shepherd Ogden, the author of Step By Step Organic Vegetable Gardening (21). Ogden’s basic rotation scheme is: