Crops That Improve the Nitrogen Content of Soil

Rather than allowing your garden to lie fallow throughout the off-season, develop a cover crop that will increase the soil’s nitrogen degree. Select a legume, or member of the bean family, to accomplish this, since beans have nitrogen-fixing bacteria in their roots which take free nitrogen and fix it to compounds readily available for plant use. Nonlegume cover crops help enhance the dirt, but they don’t enrich the nitrogen. Since cover plants are tilled into the soil, they also enhance soil permeability, structure, organic matter and nutrient-holding ability.

Annual Clovers

Clovers have benefits for home gardeners since they’re quick-growing annuals which are easy to cut down before they’re added to the soil. Berseem clover (Trifolium alexandrinum), a Mediterranean native, grows in mild winter areas as a cool-season crop as well as a summer annual in areas with cool summers. It increases soil nitrogen by 2.6 percent. The 24-inch-tall clover doesn’t volunteer and does well in most soil types. Shorter rose clover (Trifolium hirtum) reaches 3 to 18 inches tall and weighs two percent nitrogen. A cool-season crop, rose clover tolerates any well-draining soil type.

Vining Vetches

Vetches (Vicia spp.) Possess a vining habit. The annual plants are cool-season plants, sown in fall and cut down and dug beneath before they blossom in spring and until stems become hard. Hairy vetch (Vicia villosa) is 12 to 20 inches tall and has many cultivars, like “Lana,” “Oregon” and “Auburn.” Common vetch (Vicia sativa) is approximately 22 inches high and is also referred to as spring vetch or Oregon vetch. It does nicely in mild winter areas. Both these species contribute 4 percent nitrogen into the soil.

Bean and Pea Crops

Field peas (Pisum sativum) and bell beans (Vicia faba) are cool-season annuals and, as with other cover crops, aren’t allowed to blossom or form fruit until they’re tilled under. Bell beans have large seeds and large, fleshy leaves. They require fertile, well-drained dirt and enrich the soil with 1.2 percent nitrogen. A vigorous grower, field pea requires well-draining dirt conditions. The cultivar “Austrian Winter” is most common, but two newer kinds, “Miranda” and “Magnus,” mature earlier. Field peas enrich soil nitrogen by about 3 to 4 percent.

Warm-Season Crops

A warm-season, annual cover crop, sunnhemp (Crotalaria juncea) grows 39 to 117 inches tall. It tolerates drought and infertile soil. Sown in summertime, it grows quickly, suitable for tilling beneath in 60 to 90 days, contributing 1.7 percent nitrogen enrichment. Cut down the plant before stems become woody. Another warm-season annual, cowpea (Vigna unguiculata) grows between 19 and 24 inches tall, usually tilled under approximately 60 days after sowing. A number of cultivars exist, and soil nitrogen increases 1.4 to 1.5 percent. It grows well in light or heavy soil types, but requires good drainage.

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