4 Foods that Boost Soil Nitrogen

Plants in the legume family have the ability to fix nitrogen in your farm’s soil, but these four crops will feed you, too.

by Lisa Munniksma

When you plant peas, lentils, beans and peanuts, your farm benefits from more than just a crop of protein-dense foods. The legume family—Leguminosae or Fabaceae—also works with soil bacteria to deposit nitrogen from the atmosphere into the soil.

Plants that make nitrogen available in the soil to other plants rather than removing it from the soil are appealing in crop rotations, companion planting and soil-building efforts. Legumes draw nitrogen from the air, convert it to a form usable by plants, and fix it into the soil via nodules on their roots, with the help of soil bacteria.

Many legume seeds benefit from inoculation with the proper rhizobia to boost their nitrogen-fixing benefits. Peas, lentils, dry shell beans and green beans need Rhizobia leguminosarum; soybeans require R. japonicum; and cowpeas and peanuts call for Bradyrhizobium sp. Read inoculant labels to ensure you’re using the correct strain for your crop. Organic Materials Review Institute-approved inoculants are readily available for certified organic farmers. Inoculant comes in a peat, granular or liquid form that you add to the soil or coat the seeds with before planting. The bacteria are living and need to be handled with care, kept cool and applied soon before planting.

Not the first food crop most small-scale farmers think of, legumes provide nutrition for both people and the soil and add an interesting mix to your crop rotation and market offerings. Of the three subfamilies of legumes—Papilionaceae, Caesalpiniaceae and Mimosaceae—this article concentrates on Papilionaceae, which includes peas, lentils, beans and peanuts you can grow on your farm as a source of food and income.

1. Beans

Beans are a warm-season crop with varieties suited for every climate of the U.S. Direct-plant seeds after danger of frost has passed and the soil temperature exceeds 50 degrees F (65 degrees F for lima beans and cowpeas). Plant every two to three weeks until mid-summer for a continual bean harvest. If you’re planting dry shell beans, do so all at once, concentrating the harvest labor.

Plant most beans 1 inch deep in heavy soils or 1½ inches deep in light soils, except lima beans and cowpeas, which should be planted 1/2 inch deep in heavy soils and 1 inch deep in light soils. Plant bush beans 2 to 4 inches apart in rows spaced 2 to 3 feet apart and pole beans 6 to 10 inches apart in rows spaced 3 to 4 feet apart. Train vines to climb a trellis, stake or wire.

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Beans require 1 inch of water per week and can be mulched for weed control. Days to maturity depend on the type and variety. Harvest green beans, lima beans and any other beans you plan to eat fresh when the edible beans fill the pods; this is usually eight to 10 days after flowering. Don’t allow the beans to get too big, as they become tough and tasteless. Plants will continue producing as you harvest, so leave developing blossoms intact.

Harvest dry shell beans after the plants turn brown and the pods have dried—this can be done after the first frost. Use a combine to harvest and thresh; cut and windrow the beans to dry further before threshing; or harvest by hand, pulling the plants up and hanging them in a protected place to dry for another few days before threshing. The University of New Hampshire Cooperative Extension recommends using the “bite test” to ensure dry beans are dry enough for storage: If you bite one and it dents, it has too much moisture.

2. Peanuts

Peanuts originated in Bolivia and, as a result, are suited for warm and tropical climates. They require at least 110 frost-free days and light, well-drained soil with a pH of 5.8 to 6.2. Peanuts need a lot of water, between 1/4 and 2/5 inch per day. Plant them in soil 65 degrees F or warmer, 1 to 1½ inches deep and 6 to 8 inches apart. Space rows at 24 inches apart for bunch types and 36 inches for runner types.

Unlike other legumes, peanuts’ edible portion grows underground. The stalk curves downward after fertilization and penetrates the soil for pod and seed development. Peanuts are ready to harvest when plants begin to yellow. Use a spade to hand-dig them or, for large operations, a mechanized peanut digger or combine. When hand-digging peanuts, shake off dirt and hang the plants to cure in a warm, dry, pest-free place for one to two weeks. Remove the pods and air-dry them for another one to two weeks, then roast them.

Runner, Virginia, Spanish and Valencia are the four market classes of peanuts in order of popularity in the U.S., according to the National Sustainable Agriculture Information Service.

3. Lentils

Lentils are believed to be one of the first agricultural crops, first cultivated in the Near East. Lentils are a drought-hardy, cool-season crop. They do best in sandy loam soils that drain well, are high in phos­phorus and potassium, and have a pH of 7.0. Lentil seeds should be planted in early spring, 1½ inches deep in moist soil to 2½ inches deep in dry soil. Because the seeds require good seed-to-soil contact, they’re best seeded with a grain drill.

Harvest lentils on a hot, dry day, when plants begin to turn yellow and the lower pods turn brown and rattle when shaken. Small-scale harvest can be done by hand, pulling and piling the plants in the field to dry before threshing. Larger crop fields can be harvested with a mower or combine, windrowed and mechanically threshed when dry.

4. Peas

Plant peas in the spring, when you’re able to till the soil 8 to 10 inches deep. As cool-season vegetables, peas can tolerate light frosts, especially when protected with light row cover. Peas make good fall crops, too, when planted eight to 10 weeks before the first-expected frost. Plant pea seeds 1 to 1½ inches deep, 1 inch apart, with rows 18 to 24 inches apart.

There are peas for eating in the hull (snow peas and snap peas) and hulled (green peas). Harvest them—hand-picking on a small-scale farm—about five to seven days after flowering. Snap-pea pods are 2½ to 3 inches long at maturity; snow peas are harvestable when pods are 2 inches wide and up to 4 to 5 inches long, when peas begin to form but before they increase in size; and green peas are ready at the size of BB pellets. Peas left to ripen for too long will become fibrous and tough.

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