Chemical fertilizers have ill effects e.g ., they are expensive and are manufactured from resources which are decreasing gradually e.g ., petroleum and coal. Also they are lost from the soil by surface run off and leaching. Biofertilizers are tiny organisms that helps in enriching the nutrient quantity of the soil.
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Manures include oilcakes, bone meal, urine, blood, Farm Yard Manure (FYM) (consists of cattle dung and crop residues), composted manure (consists of rotten vegetables and animal residues) and green manures. Some of the plants are used as green manures like Crotolaria juncea (Sun hemp), Sesbania aculeata(Dhaincha), Cyanopsis tetragoniloba (Cluster bean) Melilotus parviflora (Senji), Trifolium alexandrium (Barseem), Lens esculenta (Lentil), Medicago sativum (Leucerine).
Sources of Biofertilizers :
The main sources of biofertilizers are—
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(a) Bacteria (b) Cyanobacteria (Blue Green Algae), (c) Fungi.
1. Rhizobium is a bacterium living in the root of leguminous plants in symbiotic association. The root cells of leguminous plant contain a purple coloured pigment called leghemoglobin in which the bacteria float to fix atmospheric nitrogen.
2. Azospirillum lipoferum is another bacterium which lives in loose association with some grasses, rice, maize and rice.
3. Azotobacter, Clostridium, Aerobacter, Methanobacterium are free living bacteria which fix atmospheric nitrogen.
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4. The leaves of Azolla (an aquatic pteridophyte) have large number of plants of Anabaena (a blue green algae) which have the capacity to fix atmospheric nitrogen which is made available to Azolla.
5. Many free living blue-green algae (Anabaena, Nostoc, Aulosira) fix atmospheric nitrogen. Nostoc is placed in the rice crop and provides nitrogen to rice.
6. Mycorrhiza : It is the symbiotic relationship between roots of higher plants and fungi.
Mycorrhiza is of two types—
(a) Endomycorrhiza : Is important in phosphate nutrition of plants. It penetrates cortex. It is also called Vesicular Arbuscular Mycorrhiza (VAH)
(b) Ectomycorrhiza : It increases water and nutrient intake by plants. These occur on plants like pine. Eucalyptus, Ficus, oak etc. They absorb and store nitrogen, phosphorus, potassium, and calcium in fungus. They also convert complex organic molecules into simple, easily absorbable form.