Role of Legume Crops in Soil Improvement

Legume crops can fix atmospheric nitrogen is the most notable aspect that sets the legumes apart from other plants. By the Inoculation of legumes with the proper strain of Rhizobia spp., legumes can supply their nitrogen up to 90%.

Nitrogen Fixation by Legumes

Nitrogen fixation by legumes is essential to improve Nitrogen Availability in the soil at the global and regional scale. It is estimated in the USA that approximately 40 million kg of nitrogen in the soil was fixed by the presence of legumes including lentils (Lens culinarisMedik.), chickpeas (Cicerarietinum L.) and field peas (Pisumsativum L.) whereas, in Canada, food legumes such as dry bean, field pea, lentil, and chickpea could provide more than 170 million kg of nitrogen in the soil. This amount provided approximately fixed nitrogen of 7% in the soil which can be used for the next rotational crop. On a global scale, legumes are approximately cultivated on 250 million hectares and they can provide about 90 trillion kg  N per year.

This availability of legumes to provide nitrogen in the soil is the nodulation formation by the nitrogenase enzyme.  Nitrogenase is an important enzyme because it catalyzes dinitrogen to ammonia. The ability to fix N biologically makes the legumes an important candidate for cropping sequence to maintain the Nitrogen element in soils under agriculture and thus to improve soil physical condition and sustain the environmental balance.

Break pest cycle

Legumes reduce the incidence of diseases, pests and weeds. Therefore, with the developed agronomic practices like reduced tillage and with organic farming, the production of grain legumes is escalating.

Recycle nutrients by reducing soil pH.

It is estimated that the release of available P is highly dependent on soil characteristics such as pH. The organic acids like malate, oxalate, citrate, tartrate, and acetate released by the roots of grain legumes decrease the soil pH in the rhizosphere which helps in the conversion of unavailable P to available form. An enzyme phosphatase is released by grain legumes into the soil which helps in the breakdown of organic materials containing P. Legume litters contain K, P, and other nutrients which are recycled into the soil.

Biological diversity

Soil biological properties such as soil microbial biomass are generally used as an early indicator of changes in soil physicochemical properties because of soil management in agricultural ecosystems. During the process of biological nitrogen fixation, hydrogen gas is produced which encourages bacterial growth in the legume rhizosphere and results in higher microbial biomass C in the soil. The introduction of legume crop rotations had a significant influence on the microbial community structure and increased microbial diversity. Legumes also appear to promote AM (Arbuscularmycorhizea) colonization in low-input systems. Studies showed that AM has resulted in an increased flow of nutrients, the productivity of plants, and sustain the ecosystem.

Improve soil organic matter

Mucuna a legume plant with maize resulted in declining runoff and erosion, an upsurge in soil organic matter content and in the production of maize grains, and an improvement of the soil water regime. The quantity of nitrogen returned to the soil for succeeding non-leguminous crops depends on the following factors:

• The quantity of legume residue returned to the soil
• Nitrogen that symbiotically fixed in the residues
• Available nitrogen in the previous legume crop to the subsequent crop

Soil Erosion and Legume

The physical removal of soil by agents which provide the kinetic energy to move soil from one location to another is called soil erosion. Topsoil is the layer of soil where plants grow as it has the highest fertility than the other soil layers due to the presence of organic matter content, soil microorganism, and mineral nutrients. Legumes are known to use as a cover crop to control soil erosion. For example, legume shrubs (Coluteaarborescens, Dorycniumpentaphyllum, and Medicagostrasseri) grown as cover crops were found to reduce runoff and soil loss.