Ramlal has been cultivating wheat crop year after year in the same field. Recently he has observed decline in the yield despite best inputs. Agriculture inspector of the area suggested him to sow some legume crop (e.g., pea, chick pea or soybean) for one or two years before again using the field for wheat crop. What is the logic behind this suggestion.

Sowing the same crop in the same field year after year results in reduction of nutrients from the particular depth of the soil, increase in populations of soil borne pathogens and pests of wheat crop and growth of weeds, infections and infestations.

Growing some root – nodules containing legume crops such as pea, chick pea, soyabean, etc., in the field will improve soil structure and fertility as it will draw water and nutrients from different levels of soil and add nitrogen salts to the soil Weeds, soil borne pathogens and pests of wheat crops will be eliminated as they do not find their host. This will increase the overall fertility of the soil and hence crop production

Biological Nitrogen Fixation

Legume plants are involved in biological nitrogen fixation. Nitrogen of air is converted into organic nitrogen only by prokaryotic cells: bacteria and cyanobacteria. A group of Gram negative bacteria, the rhizobia, form symbiotic association with numerous legume host plants and produce root nodules where nitrogen fixation occurs. Root nodules of soybean (Glycine max) has Sinorhizobium Meliloti (old name Rhizobium meliloti). While root nodules of pea (Pisum Sativum) and chickpea (Cicer arietinum) have Rhizobium leguminosarum.

In the process of nitrogen fixation, dinitrogen (N₂) of air is taken by endosymbiotic bacteria of root nodules and is converted into ammonia (NH+) which diffuses out of bacteria. Inside tissues of root nodules, this ammonia is converted either in amide amino acid (glutamine, e.g., pea) or ureides (e.g., soybean) which are exported via xylem tissue to leaves of host plant. In leaves amides and ureides are used in the synthesis of various amino acids.

If legume plants grown in the presence of nitrogen fertilizers (e.g., nitrate-NO₃^-), they will use this fertilizer as a source of nitrogen nutrition and they will not form symbiotic root nodules (i.e., there will be no biological nitrogen fixation)