Biofertilizers: An ecofriendly way to replace chemical fertilizers
Increasing use of chemical fertilizers in agriculture make country self dependent in food production but it deteriorate environment and cause harmful impacts on living beings. Due to insufficient uptake of these fertilizers by plants results, fertilizers reaches into water bodies through rain water, causes eutrophication in water bodies and affect living beings including growth inhabiting micro organism. The excess uses of chemical fertilizers in agriculture are costly and also have various adverse effects on soils i.e. depletes water holding capacity, soil fertility and disparity in soil nutrients. It was felt from a long time to develop some low cost effective and eco-friendly fertilizers which work without disturbing nature. Now, certain species of micro-organism are widely used which have unique properties to provide natural products, and serve as a good substitute of chemical fertilizers.
What is biofertilizer?
A number of micro-organisms (bacteria fungi and algae) are considered as beneficial for agriculture and used as biofertilizers.
Biofertilizers are supposed to be a safe alternative to chemical fertilizers to minimize the ecological disturbance. Biofertilizers are cost effective, eco-friendly and when they are required in bulk can be generated at the farm itself. They increase crop yield upto 10-40% and fix nitrogen upto 40-50 Kg. The other plus point is that after using 3-4 years continuously there is no need of application of biofertilizers because parental inoculums are sufficient for growth and multiplication. They improve soil texture, pH, and other properties of soil. They produces plant growth promoting substances IAA amino acids, vitamins etc. They have 75% moisture and it could be applied to the field directly. Biofertilizers contained 3.5% - 4% nitrogen, 2% - 2.5% phosphorus and 1.5% potassium. In terms of N: P: K, it was found to be superior to farmyard manure and other type of manure (Mukhopadhyay, 2006).
Microbes used as Biofertilizer
Microbes are effective in inducing plant growth as they secrets plant growth promoters (auxins, abscisic acid, gibberellic acid, cytokinis, ethylene) and enhance seed germination and root growth. They also play a considerable role in decomposition of organic materials and enrichment of compost.
Nitrogen fixing Bacteria
1. Rhizobia: -
Legumes plants have root nodules, where atmospheric nitrogen fixation is done by bacteria belonging to genera, Rhizobium, Bradyshzodium, Sinorhizobium, Azorhizobium and Mesorhizobium collectively called as rhizobia. When rhizobial culture is inoculated in field, pulse crops yield can be increased due to rhizobial symbiosis (Dubey, 2001). Rhizobium can fix 15-20 N/ha and increase crop yield upto 20%.
It is a stem nodule forming bacteria and fixes nitrogen symbionts of the stem nodule also produce large amount of IAA that promotes plant growth.
Bradyrhizobium is reported a good nitrogen fixer. Bradyrhizobium strain inoculation with Mucuna seeds enhances total organic carbon, N2, phosphorus and potassium in the soil, increases plant growth and consequently plant biomass, reduction in the weed population and increased soil microbial population..
These are aerobic chemolithotrophs and anaerobic photoautotrophs. These are non nodule forming bacteria. They include numbers of the families:-
1) Azotobacteracae: e.g. Azotobacter:
They are the free living aerobic, photoautotrophic, non-symbiotic bacteria. They secretes vitamin-B complex, gibberellins, napthalene, acetic acid and other substances that inhibit certain root pathogens and improves root growth and uptake of plant nutrients. It occurs in the roots of Paspalum notatum (tropical grasses) and other spp. and adds 15-93 Kg N/ha/annum on P. notatum roots (Dobereiner et al., 1973). Azotobacter indicum occurs in acidic soil in sugarcane plant roots. It can apply in cereals, millets, vegetables and flowers through seed, seedlings soil treatment.
2) Spirillaceae: e.g. Azospirillum and Herbaspirillum:
These are gram negative, free living, associative symbiotic and non-nodule forming, aerobic bacteria, occurs in the roots of dicots and monocot plants i.e. corn, sorghum, wheat etc. It is easy to culture and identify. Azospirillum is found to be very effective in increasing 10-15% yield of cereal crops and fixes N2 upto 20-40% Kg/ha. Different A. brasiliense strains inoculation in the wheat seed causes increase in seed germination, plant growth, plumule and radicle length. Herbaspirillum species occurs in roots, stems and leaves of sugarcane and rice. They produce growth promoters (IAA, Gibberillins, Cytokinins) and enhance root development and uptake of plant nutrients (N, P & K).
3) Acetobacter diazotrophicus:
Another diazotroph is Acetobacter diazotrophicus occurs in roots, stem and leaves of sugarcane and sugar beat crops as nitrogen fixer and applied through soil treatment. It also produces growth promoters e.g. IAA and helps in nutrients uptake, seed germination, and root growth. This bacterium fixes nitrogen upto 15kg /ha/year and enhance upto 0.5 – 1% crop yield (Gahukar – 2005-06).
Cyanobacteria (Blue green algae):
Nostoc, Anabaena, Oscillatoria, Aulosira, Lyngbya etc. are the prokaryotic organisms and phototropic in nature. They play an important role in enriching paddy field soil by fixing atmospheric nitrogen and supply vitamin B complex and growth promoting substance which makes the plant grow vigorously. Cyanobacteria fixes 20-30 Kg/N/ha and increase10-15% crop yield when applied at 10 Kg/ha.
Azolla – Anabaena symbiosis
It is a free floating, aquatic fern found on water surface having a cyanobacterial symbiont Anabaena azollae in their leaves. It fixes atmospheric nitrogen in paddy field and excrete organic nitrogen in water during its growth and also immediately upon trampling. Azolla contributes nitrogen, phosphorus (15-20 Kg/ha/month), potassium (20-25 kg/ha/month) and organic carbon etc. and increases 10-20% yield of paddy crops and also suppresses weed growth. Azolla also absorbs traces of potassium from irrigation water and can be used as green manure before rice planting. Azolla spp. are metal tolerant hence, can be applied near heavy metal polluted areas.
Phosphate Solubilising Bacteria
Pseudomonas fluorescens, Bacillus megatherium var. phosphaticum, Acrobacter acrogens, nitrobacter spp., Escherichia freundii, Serratia spp., Pseudomonas striata, Bacillus polymyxa are the bacteria have phosphate solubilising ability. ‘Phosphobacterin’ are the bacterial fertilizers containing cells of Bacillus megatherium var. phosphaticum, prepared firstly by USSR scientists. They increased about 10 to 20 % crop yield (Cooper, 1959) and also produces plant growth promoting hormones which helps in phosphate solubilising activity of soil.
Phosphate solubilizing fungi
Some fungi also have phosphate dissolving ability e.g. Aspergillus niger, Aspergillus awamori, Penicillium digitatum etc.
Plant Growth Promoting Rhizobacteria (PGPR)
They are also called as microbial pesticides e.g. Bacillus spp. and Pseudomonas fluorescence. Serratia spp. and Ochrobactrum spp. are able to promote growth of plants. Pseudomonas fluorescence application to the Black pepper enhanced uptake of nutrients which increased plant biomass. Fluorescent rhizobacteria improve the growth of H. brasiliensis.
Mycorrhizas are developed due to the symbiosis between some specific root inhabiting fungi and plant roots and used as biofertilizers. They absorb nutrients such as manganese, phosphorus, iron, sulphur, zinc etc. from the soil and pass it to the plant. Mycorrhizal fungus increases the yield of crops by 30-40% and also produces plant growth promoting substances.
VAM fungi or Endomycorrhiza
They occur commonly in the roots of crop plants. VAM fungal hyphae enhance the uptake of phosphorus and other nutrients that are responsible for plant growth stimulation including roots and shoot length. VAM also enhances the growth of black pepper and protects from Phytophthora capsici, Radopholus similis and Melvidogyne incognita (Anandraj et al., 2001). VAM fungi enhance water uptake in plants and also provide heavy metals tolerance to plants.
Deepali and Kamal K. Gangwar