Green cover crops

The choice of green cover is sometimes limited by the soil type. For example, vetches can only be sown on a clay or loamy soil. Rye is best sown on a loam or sandy soil.
Grasses are preferred on silt-sensitive, wet soils because of their intensive rooting to prevent structural damage.

The cost of cultivation is mainly determined by the seed costs and nitrogen fertilization. For optimal growth, a fertilization of 40 to 50 units of nitrogen when sowing the green cover crop is more than sufficient. Nitrogen is often not applied because the cover crops also aim to absorb the nitrogen present in the soil and to protect it from leaching. The seed price of vetches is higher than that of other cover crops. In contrast, vetches require little or no nitrogen fertilization. The chance of success (emergence, ground cover) is greater with grasses, grains and cruciferous crops.
With regard to the nitrogen subsequent delivery for the next crop, a subsequent delivery of a maximum of 40 kg N/ha is taken into account for grasses and cruciferous crops; for leguminous crops this is a maximum of 60 kg N/ha. The exempted quantities depend on the development of the cover crop, the weather and the crop to be grown in the following year.

By growing a green cover after the main crop, nitrogen leaching is prevented. White mustard has a fixation of 75 – 95 kg nitrogen per hectare in light soils; in the slightly heavier soils this varies from 120 to 160 kg nitrogen per hectare. Fodder radish has an even greater potential for nitrogen fixation above ground of 90 to 120 kg nitrogen per hectare on the lighter soils; on the heavier soils this is 200 to 240 kg per hectare.

White mustard, fodder radish, Italian and Westerwold ryegrass can be sown after an early potato crop (grown for either consumption or seed). After maize and late potatoes, grasses, winter rye and winter vetches can still be sown. In the following year, N fertilization must take into account the N released from the green cover. If not, the N fertilization will not be saved, burning can damage the crop and the chance of N leaching increases. Of all the green cover crops, grass is the best to limit N leaching, because it absorbs nitrogen both before and after the winter. The total aboveground N uptake by the grass is slightly lower than with mustard, because grass grows less quickly than mustard.

Rapid initial development is usually accompanied by good ground cover, which suppresses weeds. In addition, good coverage is important to prevent structural damage caused by heavy rainfall, drying out and wind erosion. In case of late sowing, it is recommended to increase the seed quantity; in this way a faster ground cover is obtained. Then preferably choose early varieties that generally have a fast ground cover.

Cruciferous crops have a deep taproot, which improves the permeability of the soil. Grasses have a more superficial intensive rooting, which improves the cohesion of the soil, especially after plowing. The root development of the green cover crop is important in soil prone to silting so the structure is preserved. For these soils, grass is preferred for sowing as a cover crop.

Once fully grown, all cruciferous green cover plants are more sensitive to frost. Early varieties are therefore more sensitive to frost than late varieties. Frost sensitivity is an advantage for a green cover, but only if the night frost does not come too early in the season. Frost makes plowing easier and decreases the chance of persistence in the following year (no seed setting). In contrast to summer vetches, winter vetches are less sensitive to frost.

Cruciferous plants (such as fodder radish and white mustard) are host plants for the white and yellow beet cyst nematodes. Planting non-resistant host plants can increase the nematode population. However, use of resistant host plants may lead to a faster decline in the nematode population, as the nematodes are lured to the roots but do not reproduce afterward. Sufficiently high soil temperatures are required to attract larvae; late summer has soil temperatures that are usually too low to effectively attract the larvae. Sowing after 1 August cannot therefore be expected to produce significant biological control of the beet cyst nematode. But in the interest of beet cultivation it is in any case undesirable to use green cover crops that are susceptible to the beet cyst nematode.
Clubroot-resistant cruciferous crops such as fodder radish can be used to prevent clubroot. Grasses are a less suitable green cover for grains because of the frit fly. Vetches are undesirable in the presence of the pea cyst nematode.