Materials of biological origin commonly used for maintaining and improving soil fertility belong to two categories:
(i) Green Manures and (ii) Bio-fertilizers.
Manures supply practically all nutrients required by crops. Farm yard manure consists of rotten vegetable and animal refuse.
1. Green Manure:
Quick growing leguminous crops are cultivated and ploughed into the soil while still green, commonly used green manures in India are Sunnhemp (Crotalaria juncea), Dhaincha (Sesbania aculeata), Cluster bean (Cyamopsis tetragonoloha), Senji (Melilotus parviflora), Cow pea (Vigna sinensis), Horse gram(Macrotyloma uniflorum), Berseem (rrifolium alexatidrium), Lentil (Lens esculenta). These plants used as green manure are fast growing, and add both organic matter as well as nitrogen to the soil. They also check soil erosion and leaching. Green manuring increases crop yield by 30-50 percent.
2. Bio-fertilisers:
Bio-fertilisers are the organisms which can bring about enrichment of soil.
The main sources of bio-fertilisers are:
(i) Bacteria,
(ii) Cyanobacteria;
(iii) Fungi.
Out of 35000-37000 million tons of nitrogenous fertilizers required all over the world, about 175 crore tons are available free of cost naturally in the form of bio-fertializers. Rhizobium (bacteria) present in the root nodules of leguminous plants fixes atmospheric N2. Rhizobium cultures are now raised in the laboratory and used for inoculating seeds of legumes to improve chances of nodulation and nitrogen fixation.
The leaves of Azolla house as many as 80,000 blue-green algae belonging to Anabaena azollae which have the capacity to fix atmospheric nitrogen and make it available to Azolla. When the plant dies and decays in the soil, nitrogen becomes available to plants. Application of 10 tonnes of fresh Azolla biomass in one hectare adds as much as 100 kg nitrogen. Azolla pinnata is an excellent bio-fertiliser for rice.
Azospirillum, a non-symbiotic bacterium, can fix nitrogen independently in the soil. It is particularly suited for crops like maize, jowar, bajra. ragi and sugarcane. Johannah Dobreiner observed a loose association between Azospirillum lipoferum with roots of some Brazilian grasses and maize. Free-living bacteria like Azotobacter and Bacillus polymyxa fix atmospheric nitrogen and make it available to corp. Saving of 10-25 kg/ha of nitrogen can be made by growing Azotobacter with cotton, rice, maize, jowar.
Cyanobacteria like Anabaena, Nostoc, Aulosira, not only fix atmospheric nitrogen, but also derive energy for this by carrying out photosynthesis. It makes them more important. These have the potential to supply 7-8 lakh tonnes of fixed nitrogen equivalent to 15-17 lakh tonnes of urea.
Phosphobacterin:
Bio-fertilizer containing Bacillus megatherium; it can convert native unavailable phosphorus into available form by solubilizing it. Ectomycorrhizae increase water and nutrient uptake, plant vigour, growth and yield. These occur commonly on the roots of trees like pine, oak, peach and eucalyptus.
These absorb and store nitrogen, phosphorus, potassium and calcium in their fungal mantle. These also convert complex organic molecules into simple, easily absorbable form. Endomycorrhizae, found in many herbaceous species like Orchids and some woody plants, are important in phosphorus nutrition of plants.