Role of Bio-fertilizers in Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 198

pathway (Rodriguez and Fraga 1999 ). The plant obtains their phosphate require-
ments from the soil pool. It occurs in soil as inorganic phosphate, produced by
weathering of rocks that is unable to be utilized by the plants (Lee et al. 2005 ) or as
organic phosphate derived from decaying plant, animal or microorganisms (Rodri-
guez and Fraga 1999 ). About 15-20 % of applied phosphorus is recovered from the
crops and rest gets fixed in the soil and is not readily available to the plants. A group
of morphologically different microorganisms which have the property of solubiliz-
ing the fixed phosphorous by producing organic acids and enzymes and to make
them easily available to the crops are known as Phosphorous Solublising Microor-
ganisms (PSM). They include diverse species of Bacillus, Aspergillus, Pseudomo-
nas, Penicillium, Agrobacterium, Achromobacter, Burkholderia, Aerobacter, Er-
winia, Micrococcus, Flavobacterium and Trichoderma . These organisms solubilize
the fixed soil phosphorus thereby releasing the citrate and water soluble phosphorus
so as to help in mineralizing organic phosphate compounds that are present in the
organic wastes (Rodriguez and Fraga 1999 ). These microorganisms have the prop-
erty to bring phosphate solublization by secreting organic acids such as propionic
acid, lactic acid, formic acid, acetic acid, succinic acids etc. these acids lower the
pH and help to dissolve the phosphate bound (Rodriguez and Fraga 1999 ). They
also produce growth promoting substances e.g. IAA, GA etc. experiments conduct-
ed in field conditions in India have shown to replace 20-50 kg P 2 O 5 /ha in different
crops due to PSM's inoculation (Vora and Shelat 1996 , 1998, 1999). Improvement
in seed germination by application of PSB has been reported by Sharma et al. ( 2007 )
in Cicer arietinum . Various horticultural plants and vegetables were successfully
inoculated with P-solubilizing bio-fertilizers to obtain higher yields (Khan et al.
2010 ; Velineni and Brahmaprakash 2011 ). Field experiments demonstrated that P-
solubilizing bio-fertilizers in addition to improving the growth and quality of crops,
also reduced) the usage of chemical or organic fertilizers significantly (Young 1990 ;
Chang and Young 1992a , b ; Young et al. 1998a , b ; Young and Chen 1999; Chang
and Young 1999 ; Young et al. 2000; Liu and Young 2001; Young et al. 2003 ). Phos-
phate solubilizing bacteria has the capacity to convert inorganic unavailable phos-
phorus form to soluble forms like HPO 4 2− and H 2 PO 4 − with the help of processes
like organic acid production, chelation and ion exchange reactions and make them
available to plants (Chang and Yang 2009 ; Banerjee et al. 2010 ). Naturally occur-
ring rhizospheric phosphorus solubilizing microorganism (PSM) has a long history
and dates back to 1903 (Khan et al. 2007 ). Alam et al. ( 2002 ) pointed out that bac-
teria are more effective in phosphorus solubilization than fungi. Among the whole
microbial population in soil, phosphate solubilizing bacteria (PSB) comprise
1-50 %, whereas phosphorus solubilizing fungi (PSF) are only 0.1-0.5 %. (Chen
et al. 2006 ). Number of phosphorous solubilizing bacteria amongst total PSM in
north Iranian soil was around 88 % (Fallah 2006 ). Microorganisms concerned in
phosphorus acquirement include mycorrhizal fungi and PSMs (Fankem et al. 2006 ).
Among the soil bacterial communities, effective phosphate solubilizers ectorhizo-
spheric strains from Pseudomonas and Bacilli , and endosymbiotic rhizobia have
been described as (Igual et al. 2001 ). Strains from bacterial genera Pseudomonas,
Bacillus, Rhizobium and Enterobacter along with Penicillium and Aspergillus fungi