Andhra Pradesh Agricultural University, Hyderabad, A. P.
Online published on 25 January, 2012.
Different legumes fix different amounts of nitrogen from the atmosphere, the species of legume and conditions of development of the crop affecting the result. In perrenials not less than one third of the fixed nitrogen is concentrated in the root mass while in annual legumes, when ripe for harvesting, practically all the nitrogen assimilated from the atmosphere is in the part above the ground. Lucerne fixes about 400 kg N per hectare per year while pulses fix only 50 to 60 kg N per hectare. The gain of nitrogen by soil after harvesting is about 100 kg per hectare in the case of lucerne while in the case of pulses the loss of nitrogen in the soil may be from 5 to 15 kg N per hectare. While USA, Australia, UK and Newzealand show a positive nitrogen balance and add every year substantial nitrogen to the soil, we have a negative nitrogen balance to the extent of 8.2 lakh tonnes per year. The position can be improved by proper use of forage legumes along with other procedures. Foreign and Indian data show that in specific cases the yields of succeeding cereal crops are increased substantially when legumes are grown. About 200 species of non-legumes that can fix atmospheric nitrogen in symbiosis with microorganisms were listed. They usually form nodules on the root systems and sometimes on the leaves of the plants. They are mainly trees and shrubs and are therefore of interest in forestry or agriculture. However, the nitrogen of leafy fodder from these trees fed to cattle eventually goes to the soil as manure. The asymbiotic nitrogen fixing organisms enrich the soil only to a small degree. However, they (specially Azotobacter) are known to synthesise growth promoting substances and enhance the vigour of the crop leading to better utilisation of the added nitrogenous fertilisers. Blue green algae play an important role in fixing nitrogen in rice fields. It was observed that even with high yielding varieties of paddy where heavy doses of nitrogenous fertilisers are required the return in yield per unit nitrogen was more when fertiliser addition was accompanied by blue green algae inoculation than when applied alone Photosynthetic bacteria like Rhodopseudomonas capsulatus have important role in nitrogen fixation m tropical soils like India and these are not fully exploited as yet The relevant data are discussed in the paper with suggestions for expanded utilisation of biological nitrogen fixation in agriculture.
