LEGUMES, commonly known as beans and pulses, are plants of the family Fabaceae, also known as Leguminosae. Legumes are second to cereals in providing food for the world. In comparison to cereal grains, legume seeds are rich in protein, providing a highly nutritional food resource. The major staple foods such as beans, soya, lentils, peas and chickpeas are all legumes. Many legumes are used as food plants.
There are literally hundreds of varieties of different species of legumes, including beans and peas with different sizes, colours, shapes and growing habits. These represent a major component of the world’s vegetables. In addition to those legumes cultivated for human consumption, many yield important fodders, green manures and forages, e.g. berseem, alfalfa, clover etc.
The legumes are one of the largest groups of flowering plants with about 18,000 species. The species within the family range from dwarf herbs of arctic and alpine vegetation to massive trees of tropical forest. The legume is modified in many ways to facilitate its dispersal by animals, wind and water.
Apart from other plants, legumes have ability to fix atmospheric nitrogen making the land fertile for the next crop. As these are relatively high in protein which shows their ability to supply their own nitrogen needs. Legumes fix atmospheric nitrogen by small out growths or nodules on their roots. These nodules contain modified cells of a soil bacterium ‘Rhizobia’. The bacterium is medium sized, about two microns long and 0.7 micron wide bacterial cells. The legume plant gains nitrogen that is fixed by the rhizobia.
Nitrogen release from a legume plant occurs as the above ground plant residues and below ground as roots and nodules which decompose gradually. After decomposition nitrogen become available to the next crop.
The amount of nitrogen fixed by an individual legume crop is strongly linked to their biomass productivity. Research shows that each tone of legume biomass fixes 50 kg of nitrogen per hectare on an average. There is no need to till the soil to release legume nitrogen into the soil. Different legume crops have different ability to fix atmospheric nitrogen according to their biomass production and amount of nodules on their roots. For example, sweet clover, peas, lentil, soybean and chickpea can fix 105, 80, 60, 60 and 50 kg nitrogen per acre, respectively.
Legumes improve soil quality by adding organic matter to the soil with nitrogen and more carbon. Legumes facilitate decomposition of crop residues in the soil and their conversion to soil building organic matter through the soil microorganisms. Legumes are faster growers and produce greater biomass which ultimately adds organic matter to the soil.
Legume plants help increase diversity of soil flora and fauna lending a greater stability to the total soil life. These provide additional nitrogen which helps soil microbes to break down carbon rich residues of different crops like wheat.
Legumes also improve physical properties of the soil. This improvement is attributed to increase in stable soil aggregates. Soil particles bind together into stable soil aggregates by the protein, glomalin, secreted by legume roots which serve as binding agent. This aggregates stability increases soil pore space and improve soil tilth which reduces soil erosion and compactness.
Legumes root deeply into the soil and thus they have ability to recycle the plant nutrients from deeper soil profile. This makes possible for plants to use nutrients which are leached down beyond the root zone. This also increases the fertiliser use efficiency of the crop. By the penetration of legume roots deeper into the soil, soil porosity is also increased which increases air movement and water percolation in the soil.
Legumes can be used efficiently in crop rotation. There are also substantial benefits for soil and the following crop. Studies show that after harvesting of chickpea, about 42 kg of nitrogen is readily available for the sowing of the next crop.
Using in crop rotation, legumes reduce the grassy weed problems. The legume plants break the life cycle of many insects and diseases which is important for integrated pest management. Rotation of leguminous crops especially with grasses is very helpful for the control of soil erosion problems and for moisture conservation particularly in dry areas.
Inoculation of legume seeds before sowing with rhizobia is a common practice which increases the ability of legume plants to fix atmospheric nitrogen. Inoculated legumes are well grown and green, signifying functioning, having more nitrogen fixing nodules under reasonable seasonal conditions. The same species without inoculation are likely to be poorly grown and with low ability of nitrogen fixation. Inoculated legumes may produce double or even triple quantity of nitrogen resulting in more biomass production.
Inoculated legumes acquire their nitrogen from the air as diatomic nitrogen rather than from the soil as nitrate. As a result pH of the soil becomes low which is helpful for optimum crop growth and development. The principle of inoculation is based on the ability of an isolate rhizobium to form nodules in a limited genera or species of legumes.
Supply of some nitrogen fertiliser may be advantageous with respect to fast growth and development. In addition to all other traits, many legumes are highly drought tolerant and grow well even in small amounts of water.
Legume trees are utilised for a variety of other purposes including timber, medicine, tannins and gums. Various species are used as an insecticide, fish poison or molluscicide. Some legume trees yield valuable resins, used in varnishes, paints and lacquers and others are the source of dyes.
Leguminous shrubs and trees have also been used as ornamentals. Ornamental legumes include acacia, prosopis, cassia and erythrina species.