Chapter 7

Crop Rotation on Organic Farms

Crop Rotation Considerations

Crop rotations are designed to maintain crop and soil health to ensure long-term sustainability. Crop sequences deal with the effects of previous crops on current crop choice. A successful crop rotation encompasses many management components in organic farming system, including economics, fertility, erosion protection, soil biology, insect, disease, and weed considerations. The following addresses some of the key issues when planning crop rotations.

Botanical Family

Knowing what family a plant belongs to can be useful in making decisions about rotating plants for managing pests. Plants in a family (See Table 8.2) are genetically related, so they have similar characteristics and susceptibilities to various pests such as diseases, insects and/or nematodes, and weeds. In general, it is not recommended that a field be planted with plants of the same family in succession to avoid the buildup of pests. As an example, even though broccoli, cabbage, turnips, and mustard greens appear very different from another, they all belong to the mustard family (Brassicaceae). Therefore, they all share some common disease problems. Rotating between these plants will not reduce any disease problems that may be occurring. In fact, it will increase the chance of problems with soil-borne diseases such as black leg, black rot, Fusarium yellows, and clubroot, which these crops all have in common.

Growing Cool and Warm Season Crops

Knowing the time of year, a plant grows and matures is important for developing a conservation crop rotation. Cool-season crops require cool soil and air temperatures, tend to be shallow rooted, are susceptible to drought, and are grown for their leaves or roots. They are grown in spring or autumn. Warm-season crops require warm soil and air temperatures to germinate, grow, and mature.

Crop Rooting Depth

Organic farmers often base their crop rotations on whether various plants in the rotational lineup are shallow-rooted or deep-rooted. This is particularly important for nitrogen, especially in lighter soils since it can be leached below the rooting depth of some crops. As an example, organic growers often will follow a shallow-rooted crop like onions or carrots, with a deeper-rooted crop such as corn to recover nutrients that were not used or may have been leached to lower depths in the soil profile. Deep-rooted crops are typically grown after fallow periods if the leaching of nutrients was higher than normal.

Crop Nutrient Demands

Crop nutrient requirements are important when planning a crop rotation. Organic vegetable growers often base their crop rotations on whether various plants in the rotational lineup are light or heavy feeders (See Table 8.4) since crops differ in their ability to extract water and nutrients from the soil. As a general rule heavy-feeding crop should follow a medium-feeder, and one or two years of light-feeders or a soil building cover crop like hairy vetch.

Crop Rotation Periods

As previously mentioned, susceptible crops should be rotated at intervals to inhibit the buildup of specific pests. Flexibility can be built into longer rotations. For example, rotation periods of several years may be required to suppress soil borne pathogens. Longer rotations also allow the addition of perennial crops, such as grass or legume hay, resulting in healthier soil by building organic matter and improving soil aggregation.

Cover Crops

Cover crops are often overlooked when planning a crop rotation as the focus is typically on cash crops. Cover crops can be established in a pure stand or in a mixture with other crops or as a full season crop to maximize its beneficial properties. Selecting the right cover crop(s) is critical to maximize the benefits of including them in the crop rotation. The key to selecting the right cover crop is matching it to the next cash crop, watching the weather, and timing termination to minimize negative impacts on soil moisture and temperature. Selection of cover crops will depend on the goals of the organic farmer, which may include providing biological nitrogen, organic matter, insect habitat, weed management, erosion protection, or combinations of these.

Carbon-to-Nitrogen Ratio

All organic matter is made up of substantial amounts of carbon combined with lesser amounts of nitrogen. The balance of these two elements in an organism is called the carbon-to-nitrogen ratio (C:N ratio). The course of decomposition of organic matter is affected by the presence of carbon and nitrogen. Based on calculations of how quickly a particular cover crop will decompose when incorporated into the soil, organic farmers may choose to follow the crop with either a light- or heavy-feeding crop.

Potential Problems with Cover Crops

The use of cover crops is not without some potential problems with crop rotations. Many annual cover crops must be mowed before they produce viable seeds, which could become a weed. Some cover crops have allelopathic properties that can have detrimental effects on the cash crops that follow. Large amounts of cover crop residues can cause significant problems during seeding of the next crop. Precision seeders are particularly sensitive to seedbed conditions with excessive crop residues.

Manure Application

Manure is a critical part of an organic grower’s nutrient management program, and it is important to consider when manure can be applied in the rotation. NOP rules restrict manure application prior to crop harvest, requiring incorporation of manure into the soil 90 to 120 days before harvest, depending on the crop.

Organic Matter Contribution

A well-planned rotation of crops and cover crops ensures diverse sources of organic matter, and is an important strategy for increasing overall organic matter content of soil. Low carbon-to-nitrogen (C:N) ratio materials, such as legume residues, decompose quickly because they contain relatively large amounts of nitrogen, but they contribute very little to the building of humus.

Tillage Practices

Various tillage practices can make an impact on crop rotations. For instance, farmers must consider how different types of tillage systems can influence a rotation. For example, if management plans call for a period of no till, incorporation of crop residues will be delayed and the selection of crops for a rotation in this instance may be more limited.

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