Microorganisms – Biological nitrogen fixation has been taken advantage of for many centuries by growing leguminous crops, e.g. peas, beans, chickpea, in whose root nodules atmospheric nitrogen is fixed by rhizobial bacteria living in symbiotic association with the crop plant. It is now evident that there are a number of other microorganisms in or on the soil that can fix atmospheric nitrogen. The quantities fixed by these organisms are unlikely ever to be sufficient to supply the nitrogen requirement of high yielding crops, but they can in some circumstances supply significant amount of nitrogen to crops and allow economies in the use of nitrogen fertilizer.
However, it should be noted that nitrogen fixation is an energy consuming process and that the symbiotic nitrogen fixer obtain this energy from the associated crop, in principle resulting in some loss of crop production. Also, the non-symbiotic nitrogen fixing organisms appear to be able to operate effectively at high soil temperatures but (apart from Azotobacter) are ineffective in temperate conditions. They are :
Rhizobium Bacteria
Bacteria of various Rhizobium species live symbiotically in the root nodules of leguminous plants, use the carbohydrates of the host as an energy source and pass a proportion of the fixed nitrogen to the host plant. The rest remains in the bacteria and the nodules tissue. Most of it is mineralized and becomes available to plants after the nodules have sloughed off and died. There are a number of Rhizobium species each able to from nodules on one or more legume species. For symbiotic nitrogen fixation to take placa, the rhizobium species and the legume must be compatible.
Free-Living Bacteria
Azotobacter and related species of free-living bacteria are present and fix nitrogen in many soils, and t is commonly stated that the amount fixed in fertile temperate soils may be 10 to 20 kg/ha per annum. However, many soils especially rather acid soils, do not have active population of these bacteria and it has been widely (but not universally) found that the use of bacterial inoculants can increase yields. Apart from fixing nitrogen, Azotobacter produces a number of biologically active compounds and at least a part of the reported yield increases may be due to this factor.
Azospirillum
The bacterium Azospirillum is widespread in tropical soil and grows on the inside the roots of grasses and other graminaceous crops, where it is able to fix atmospheric nitrogen. At high soil temperatures, inoculation of soils with Azospirillum has increased cereal yields by providing fixed nitrogen, but it can apparently only provide part of the total nitrogen requirement of the crop. It may be particularly useful where crops such as barley, oats and millet receive little nitrogen fertilizer, and may thus have potential for the small and marginal farmers of the developing countries.
Blue-Green Algae
Blue-green algae have long been known to be free-living nitrogen fixer and the soil in waterlogged rice field provides a good environment for their growth. Besides fixing atmospheric nitrogen, blue-green algae excrete several vitamins and growth promoting substances which contribute to better growth of rice plants.
Azolla
Azolla is a floating aquatic fern with nitrogen fixing blue-green algae (Anabaena azolla) associated with it in cavities in the leaf surface. The merits of the Azolla-anabaena association lie in its ability to fix nitrogen and also to photosynthesize and give high yields of biomass, which can be used as green manure, particularly if Azolla is grown in rotation with rice or even in association with rice.
Other Microorganisms
Other microorganisms associated with plant that Mycorrhiza and Phosphate solubilizing microorganisms. The both of these microorganisms as endophyte fungi are common inhabitants of roots of a number of plants and help in the uptake of phosphorus.
However, it should be noted that nitrogen fixation is an energy consuming process and that the symbiotic nitrogen fixer obtain this energy from the associated crop, in principle resulting in some loss of crop production. Also, the non-symbiotic nitrogen fixing organisms appear to be able to operate effectively at high soil temperatures but (apart from Azotobacter) are ineffective in temperate conditions. They are :
Rhizobium Bacteria
Bacteria of various Rhizobium species live symbiotically in the root nodules of leguminous plants, use the carbohydrates of the host as an energy source and pass a proportion of the fixed nitrogen to the host plant. The rest remains in the bacteria and the nodules tissue. Most of it is mineralized and becomes available to plants after the nodules have sloughed off and died. There are a number of Rhizobium species each able to from nodules on one or more legume species. For symbiotic nitrogen fixation to take placa, the rhizobium species and the legume must be compatible.
Free-Living Bacteria
Azotobacter and related species of free-living bacteria are present and fix nitrogen in many soils, and t is commonly stated that the amount fixed in fertile temperate soils may be 10 to 20 kg/ha per annum. However, many soils especially rather acid soils, do not have active population of these bacteria and it has been widely (but not universally) found that the use of bacterial inoculants can increase yields. Apart from fixing nitrogen, Azotobacter produces a number of biologically active compounds and at least a part of the reported yield increases may be due to this factor.
Azospirillum
The bacterium Azospirillum is widespread in tropical soil and grows on the inside the roots of grasses and other graminaceous crops, where it is able to fix atmospheric nitrogen. At high soil temperatures, inoculation of soils with Azospirillum has increased cereal yields by providing fixed nitrogen, but it can apparently only provide part of the total nitrogen requirement of the crop. It may be particularly useful where crops such as barley, oats and millet receive little nitrogen fertilizer, and may thus have potential for the small and marginal farmers of the developing countries.
Blue-Green Algae
Blue-green algae have long been known to be free-living nitrogen fixer and the soil in waterlogged rice field provides a good environment for their growth. Besides fixing atmospheric nitrogen, blue-green algae excrete several vitamins and growth promoting substances which contribute to better growth of rice plants.
Azolla
Azolla is a floating aquatic fern with nitrogen fixing blue-green algae (Anabaena azolla) associated with it in cavities in the leaf surface. The merits of the Azolla-anabaena association lie in its ability to fix nitrogen and also to photosynthesize and give high yields of biomass, which can be used as green manure, particularly if Azolla is grown in rotation with rice or even in association with rice.
Other Microorganisms
Other microorganisms associated with plant that Mycorrhiza and Phosphate solubilizing microorganisms. The both of these microorganisms as endophyte fungi are common inhabitants of roots of a number of plants and help in the uptake of phosphorus.
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