Role of Bio-fertilizers in Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 190

Above all neither toxic exudates of these microbes, nor microbes themselves
are accumulated in the food chain, self-replication of microbes curtails the need for
repeated application and target organisms rarely build up resistance as is observed
when chemical agents are used to get rid of the pests detrimental to plant develop-
ment (Mahdi et al. 2010 ).
Agricultural land deprived of essential nutrients gets impoverished after long
term cultivation, to provide or nourish the soil nutrient content under conventional
farming system, farmers use apply elevated doses of chemical fertilizers which in
turn contaminate the ecosystem. Thus to implement the agricultural land a balanced
and accountable use of organic agriculture is required. The principles of organic
farming also outline the similar concepts where the soil health and biodiversity
is built up to sustain the plant growth in longer term (Mahdi et al. 2010 ). Various
beneficial microbes and their products found in rhizosphere are useful to plants
by means of promoting growth or by acting as bio-control agents or both and are
termed as Plant Growth-Promoting Rhizobacteria (PGPR) (Akhtar et al. 2012 ;
Faramarzi et al. 2012 ). Rosenblueth and Martinez ( 2006 ) described several endo-
phytic bacteria from different plant species mainly belonging to genera Rhizobium,
Azospirillum, Bacillus, Pseudomonas, Azotobacter, Burkholderia, Herbaspirillum,
etc. play beneficial roles e.g. endophytic N-fixation, increased P-uptake, improve
photosynthesis and plant vigor, tolerance to biotic as well as abiotic stresses and in
addition to these properties they act as insecticides and help in phytoremediation of
polluted soils. Bio-fertilizers application can be used on crops prior to planting i.e.
directly to soil, as a side dressing or as a foliar spray because it does not pollute and
it adds humus to the soil (Raj 2007 ; Venkatashwarlu 2008 ). Co-inoculation of some
Pseudomonas and Bacillus strains along with effective Rhizobium spp. is shown
to stimulate chickpea growth, nodulation and nitrogen fixation (Mohammadi et al.
2010 ). Findings of Mohammadi et al. ( 2010 ) showed that the highest sugar, protein,
starch contents, nodule weight and seed nitrogen, potassium, phosphorus of chick-
pea were obtained from combined application of phosphate solubilizing bacteria,
Rhizobium and Trichoderma fungus. The Bio-fertilizers fix nitrogen in the soil that
benefits the plant to overcome the nutritional stress. Appropriate doses of phospho-
rus, potassium, zinc, iron, molybdenum and cobalt along with fertilizers mitigate
the stress and the legume starts responding directly to the nutrient. Usually most
of the nitrogen fixed passes directly into the plant whereas some of it gets leaked
into the soil for non-legume plant. However, after death, decay of these legumes by
micro-organisms nitrogen eventually returns to the soil.
Types of Bio-fertilizers
A variety of recognized microorganisms for nitrogen fixation are also used such as
Azorhizobium caulinodans and is effectively utilized in rice and maize. Likewise
Acetobacter and Sinorhizobium have been used for sugarcane and soybean crop.
Respectively microbes like Thiobacillus and Thiooxidans are known for sulphur
and iron oxidization.