Legume play a major role to maintain soil fertility and its Quality
Introduction
A legume is a plant in the family Fabaceae (or Leguminaseae).A legume fruit is a simple dry fruit that is called "pod", although pod is also applied to a few other fruit types, such as vanilla. Well-known legumes include alfalfa, clover, peas, beans, lentils, lupins, mesquite, carob, and peanuts. The farmers of our country are facing crucial problem in providing plants with required nitrogen due to inadequate supply of raw material, poor storage quality, indiscriminate uses and continuous hike in the prices of fertilizers due to increasing energy demand. Beside these, the soil health in terms of important physico-chemical and biological properties diminished and crops are showing less response in terms of yield to the added nutrients. Under such circumstances, there is an urgent need to shift chemical based modern agriculture into ecologically sound, viable and sustainable system i.e. integrated nutrient management, recycling of crop residues, and legume in crop rotation and intercropping. The ability of legumes to fix atmospheric nitrogen is perhaps the most notable aspect that sets them apart from other plants. In addition, legumes can provide a wide range of important soil quality benefits. The process whereby atmospheric nitrogen is reduce to ammonia in the presence of nitrogenase enzyme, produced by a group of microorganism is known as biological nitrogen fixation and it account for 175 x 106 metric tonnes per year at global basis. Out of this agricultural land accounts for 90 x 106 metric tonnes, forest and non-agricultural land 50 x 106 metric tonnes per year. Under the impact of ever increasing population and almost stagnated production of enriched pulses in India (varying from 13.4 to 14.2 metric tonnes for a long period reached to 14.3 metric tonnes during the year 2007-08. The consumption of pulses declined from 42 grams per capita per day (72 grams in 1956 -57) to 33 grams per capita per days during the same period.
(1)Nitrogen Fixation
Legume plant and seed tissue is relatively high in protein. This can be directly attributed to a legume’s ability to supply most of its own nitrogen needs with the help of symbiotic Rhizobia bacteria living in their roots. Inoculated with the proper strain of Rhizobia bacteria, legumes can supply up to 90% of their own nitrogen (N). Shortly after a legume seed germinates in the presence of Rhizobia bacteria in the soil, the bacteria penetrate the root hairs and move into the root itself. The bacteria multiply, causing a swelling of the root to form pale pink nodules. Nitrogen gas present in the soil air is then bound by the bacteria which feed on carbohydrates manufactured by the above-ground plant during photosynthesis. The bacteria produce ammonia (NH3) from the hydrogen acquired from the plant’s carbohydrates and nitrogen from the air. The ammonia then provides a source of nitrogen for the plant to grow. This symbiotic relationship between bacteria and legume allows them both to flourish and produce a high-protein seed or forage crop. Even though legumes can fix nitrogen from the atmosphere, they can take up large quantities of soil nitrogen if it is available. Generally speaking, the higher the protein content of a plant the more nitrogen it will return to the soil. Nitrogen is an important element for the formation of soil organic matter. Nitrogen release from a legume crop occurs as the above-ground plant residues, roots and nodules gradually decompose. Soil microorganisms decompose the relatively nitrogen-rich organic material and release the nitrogen to the soil when they die. Usually about two-thirds of the nitrogen fixed by a legume crop becomes available the next growing season after a legume in a rotation. In a perennial grass and legume mixture, legumes not only supply their own N, but approximately 36% of the N needs of the grass plants growing alongside them. If the crop is harvested as forage, the remaining stubble and roots will return 5 to 15 pounds of nitrogen to the soil as a function of each ton of forage removed. Tillage is not necessary to release legume-N into the soil. A study involving no-till corn after legumes demonstrated that the N benefits following a legume were the same whether the legume was killed with herbicide or with tillage.
(2.)Soil Quality Benefits of Legumes
Soil quality benefits of legumes include: increasing soil organic matter, improving soil porosity, recycling nutrients, improving soil structure, decreasing soil pH, diversifying the microscopic life in the soil, and breaking disease build-up and weed problems of grass-type crops.
(3.)Soil Organic Matter
As mentioned previously, legumes are high in protein, and therefore, nitrogen rich. Because most crop residues contain much more carbon than nitrogen, and bacteria in the soil need both, the nitrogen supplied by legumes facilitates the decomposition of crop residues in the soil and their conversion to soilbuilding organic matter.
(4.)Soil Porosity
Several legumes have aggressive taproots reaching 6 to 8 feet deep and a half inch in diameter that open pathways deep into the soil. Nitrogen-rich legume residues encourage earthworms and the burrows they create. The root channels
and earthworm burrows increase soil porosity, promoting air movement and water percolation deep into the soil.
(5.)Recycle Nutrients
Because perennial and biennial legumes root deeply in the soil, they have the ability to recycle crop nutrients that are deep in the soil profile. This results in a
more efficient use of applied fertilizer and prevents nutrients (particularly nitrate nitrogen) from being lost due to leaching below the root zone of shallower-rooted crops in the rotation.
(6.)Improve Soil Structure
Research in both the United States and Canada indicate improved soil physical properties following legumes. The improvements are attributed to increases in more stable soil aggregates. The protein, glomalin, symbiotically along the roots of legumes and other plants, serves as“glue” that binds soil together into stable aggregates. This aggregate stability increases pore space and tilth, reducing both soil erodibility and crusting.
(7.)Lower Soil pH
Because inoculated, nodulated legumes acquire their N from the air as diatomic N rather than from the soil as nitrate, their net effect is to lower the pH of the soil. In greenhouse studies, alfalfa and soybeans lowered the pH in a Nicollet clay loam soil by one whole pH unit. Legumes could lower the pH and promote increased plant-soil-microbial activity on soils with a pH above the range for optimum crop growth and development.
(8.)Biological Diversity
Legumes contribute to an increased diversity of soil flora and fauna lending a greater stability to the total life of the soil. Legumes also foster production of a greater total biomass in the soil by providing additional N. Soil microbes use the increased N to break down carbon-rich residues of crops like wheat or corn.
(9.)Break Pest Cycles
Legumes provide an excellent break in a crop rotation that reduces the build-up of grassy weed problems, insects, and diseases. A three year interval between the same type (grassy, broadleaf, cool season, warm season) crop is usually sufficient to greatly reduce weed, insect, and disease pressure.
Conclusion
All totaled, the soil quality benefits of legumes have been shown to be greater than the sum of their parts. Agriculture research in India that has been reported increases the yield of spring small grains of 7 to 14% after alfalfa. This yield increase due to improvements in moisture and nutrients measured in the soil. The Minnesota research attributes these yield increases to ‘rotation effects’ including improved soil physical properties, depression of phytotoxic substances, addition of growth promoting substances, and decreased disease pressure. Legumes can provide a multitude of benefits to both the soil and other crops grown in combination with them or following them in a rotation. Locally adapted legumes can be used in almost any conservation situation to improve soil quality.
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