Legumes grown in cropping systems as pulse crop, forage crop, green manure , cover crops( inter row crop or after the main crop) can add to soil a substantial nitrogen 50- 200kg N/ha and organic matter. In addition to nitrogen ,the legumes can mobilize the other essential plant nutrients in soil-plant system and help their utilization by the crops.They also protect the soil by reducing erosion and promoting aggregation and and improve the soil health/quality. By abundantly introducing and promoting legumes, how much fertilizer nitrogen can be reduced or saved by farmers , get the benefit of cost reduction on inputs and safeguard the environment and human health.
@ Rao, on an average legumes can contribute 28-84 kg N/ha/year under natural ecosystem which can enhanced to 200 kg N /ha/year in a cropping system, provided you have proper soil bacteria. The nitrogen produced by legume roots can be used as fertilizer nutrient.
The crop rotation with Forage legumes, such as alfalfa and sweet clover, levels of available nitrogen in the soil so high that biological nitrogen fixation in legumes largely inhibits.the biological nitrogen fixation process is the most efficient way to supply the large amounts of nitrogen needed by legumes to produce high-yielding crops with a high protein content. legume plants must enter into a "symbiotic" or mutually beneficial partnership with certain bacteria called rhizobia., rhizobia present in the soil or added as seed inoculum invade the root hairs and move through an infection thread toward the root. The bacteria multiply rapidly in the root, causing the swelling of root cells to form nodules.the rate of fixation is fully " synchronized" with the rate of plant growth. This fine tuning between nitrogen supply and demand is another reason for the high efficiency of symbiotic nitrogen fixation.
Nitrogen fertilization is one of the most commonly used practice to increase crop yields throughout the world because of abundant availability of N fertilizers and their great effectiveness to increase yields compared with other organic fertilizers, such as manure and compost.
But also keep in mind that this N is not free: you must add lime to prevent acidification
"In the fallow year after winter pea harvest, farmers have experienced higher-than-expected rates of nitrogen mineralization. In a study conducted in 2016 with four farmers in east-central Washington, fallow after winter pea had significantly higher residual nitrogen levels and averaged +76 lb nitrogen/ac more than the winter wheat-fallow of each farmer (Fig. 7)"
https://www.uidaho.edu/-/media/UIdaho-Responsive/Files/cals/departments/entomology-plant-pathology-and-nematology/graduate-research/Re-inventing-Austrian-Winter-Pea.pdf
20 - 30 per cent of the total nitrogen in the soil can be saved through contribution of growing legumes.The soil factors,moisture level, microbes and climatic conditions in an situation also adds to the degree of contribution.
nearly 20-25 percent of nitrogen not only saved in the legumes crop but the amount of nitrogen also being added to the soil by incorporation of leaf litters, that maintain the nutrients content for succeeding crops.
Dear colleagues/friends thank you for your responses to my question.We understand from the literature that legumes can fix and add to soil around 20-25 kg N/ ha to even upto 200 kg N/ha. Forage legumes may fix more nitrogen and add to soil than the grain legumes (which they may themselves use part of the fixed N for grain production).The amount of N fixed may also differ among the different legumes and the duration of crop or the period they are allowed to remain in the field. We can grow legumes as main crops like peanut, chickpea and soybean ; they may be grown as green manures before the main crop like rice, as intercrop and also after the main crop as a catch crop or cover crop . Based on the previous literature under field conditions, can we work out how much N can be biologically fixed by the legume and appropriately reduce the fertilizer N to be added in a cropping system or inter cropping system? This will help the farmers to save on N fertilizer and also save on P fertilizer by mobilizing the residual P from the previous crop.
I do agree with Dr. Annangi Subba RaoIn fact growing the legume crops at least once in a season will help in increasing soil fertility as they have the capacity to fix atmospheric nitrogen through their root nodules. This reduces the use of chemical fertilizers like urea and ammonium nitrate. Legumes improve soil fertility through the symbiotic association with microorganisms, such as rhizobia, which fix the atmospheric nitrogen and make nitrogen available to the host and other crops by a process known as biological nitrogen fixation (BNF). Adding legumes to a crop rotation has many benefits, including reducing the need for external nitrogen input. Legumes, with the proper soil bacteria, convert nitrogen gas from the air to a plant available form. Therefore, they do not need nitrogen fertilization, and can even add nitrogen to the soil. Moreover, the leguminous plant increases the soil ripeness due to the Rhizobium bacteria in the root knobs of the plants. These microscopic bacteria changes over atmospheric nitrogen to a shape which can be utilized by plants.
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