Nitrogen Fertilizer from Legumes

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The importance of legumes as a green fertilizer has been recognized for centuries. As early as the third century b.c.e., the Greek philosopher and scientist Theophrastus recommended that beans should be planted to enrich farm soils. Then, as now, a common agricultural practice was to rotate cereal crops, such as corn or wheat, with legumi nous crops, such as clover or alfalfa (Medicago sativa). The legumes used as green fertilizer not only contribute nitrogen to the soil but also can be harvested as animal feed.

The ability of legumes to enrich soil nitrogen content stems from their symbiotic relationship with Rhizobium and other nitrogen-fixing bacteria, which enter and colonize the root hairs of certain legume seedlings. The cell walls of the root hairs respond by curling to form a nodule that houses and protects the colony of nitrogen-fixing bacteria. The bacteria fix molecular nitrogen of the atmosphere into nitrogen-containing compounds useful to the plant, which in turn provides the bacteria with water, minerals, and carbon-based products of photosynthesis.

Legumes as Crops

For centuries, people around the world have depended on a combination of grains and legumes for a healthy and sustaining diet. Legumes were among the earliest of crops cultivated in the Fertile Crescent of the Middle East (the region between the Tigris and Euphrates Rivers) some eleven thousand years ago, where farmers supplemented their wheat and barley crops with lentils and peas.

Legumes can be recognized by their distinctive fruits, which are typically elongated pods that contain and protect the seeds. The fruit is an ovary that is dehiscent, splitting along two sides at maturity, as do these green beans.

Legumes still rank among the most important of all staple food crops, especially the pulses (edible seeds), such as peas and beans, chickpeas, and lentils. Legumes are also important as cover plants to hold and stabilize soils, as nutrient-rich feed for livestock, as timber products, and as green fertilizer. Some species are also valuable because they can be grown in poor soils or in areas of low rainfall. Other derivatives include medicines, food flavorings, tannins, gums, resins, and dyes. An extract from Lonchocarpus and Derris called rotonene is the active ingredient in fish poisons, molluscicides, and insecticides.

Nutritionally, legumes are especially good sources of proteins, carbohydrates, minerals, vitamins, and fiber. Especially important as protein sources in areas where animal protein is scarce, legumes contribute about 18 percent of the total plant protein consumed by humans. Legume proteins contain large amounts of some essential amino acids, such as lysine, but are low in the sulfur-containing amino acids methionine and cystine. Legumes' carbohydrate content varies from 13 to 65 percent, of which half or more is starch. Many legumes are also sources of iron, calcium, phosphorus, zinc, copper, and magnesium. They are particularly high in B vitamins, such as thiamin, riboflavin, niacin, folic acid, and pantothenic acid, as well as vitamins C and E. Legumes are also valued for their low fat content.

Among the most common legumes found in the human diet are the following:

Garden pea. Native to Asia, the garden pea (Psum sativum) is a cool-season crop that is widely cultivated throughout the world, with China and Russia being the most important producers of dried peas and the United States and Great Britain leading the production of green peas. The garden pea also deserves mention as the plant used by Gregor Mendel in his experiments that defined the science of genetics. The garden pea is marketed as frozen, canned, dried peas, or as snow peas and sugar peas, which represent the harvested pods.

Common bean. Another extremely important legume crop, the common bean (Phaseolus vulgaris)is a warm-season crop native to South America but is now one of the most widely cultivated legumes in the world, thanks to its introduction into Europe and Africa by Spanish and Portuguese during the sixteenth century. The common bean remains the most important pulse crop in tropical Africa and America, especially in Brazil. Common beans are harvested in the podded stage (snap beans or string beans) or as shell beans. Shell beans are by far the largest crop and include various types such as navy, kidney, French, string, pinto, and yellow-eye beans. Common beans are the primary ingredient in many staple and well-known foods, including Boston baked beans and chili con carne. Because of their relatively high pectin content, common beans must be soaked in water prior to consumption.

Lima bean. Also known as the butter bean or Madagascar bean, the lima bean (Phaseolus lunatus) is native to Central America, where it was cultivated at least seven thousand years ago. Like the common bean, the lima bean has been introduced throughout the world and is now grown in the warmer regions of Africa, Asia, and North America. The seeds are harvested and marketed canned or frozen in North America, as a bean paste in Japan, and ground into bread flour in the Philippines. Although rich in proteins and carbohydrates, lima beans contain glycosides, which can produce toxic prussic acid, and must therefore be thoroughly prepared by soaking and boiling in frequent changes of water.

Peanut. One of the most nutritious legumes, the peanut (Arachis hypogaea) is native to Brazil but has been widely introduced, particularly in the United States, where it has long been an important cash crop along the southern tier of states. The plant initially pushes a flowering stalk above ground, but following fertilization the stalk is pushed into the ground, where it matures, giving rise to the name ground peanut.

Soybean. Soybeans (Glycine max) are an important food crop that have been cultivated in the Far

East for thousands of years, often in combination with rice crops. The high protein content of about 45 percent is a major reason for their importance as a food crop. An oil extracted from soybean seed is used in the manufacture of cooking oils and some margarine. Other important soybean products include tempeh, miso, tamari, and tofu.

Lentil. One of the oldest of crops, lentils (Lens culinaris) were domesticated as early as 8000 b.c.e. in the Fertile Crescent region of the Middle East, between the Tigris and Euphrates Rivers. Lentils are still cultivated widely in the Old World; their highest production rates are in India, but lentils also remain the most important pulse crop in Nepal and Bangladesh. Harvested lentil seeds are used in the production of flour, soups, and as a dried snack food, while the plant is used as high quality straw feed for livestock in the Middle East. Lentils are considered an excellent pulse crop because of their high protein content as well as being excellent sources of vitamins A and B, potassium, and iron. Nutritionally, they are also valuable because they lack fat content and cholesterol.

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