Role of Bio-fertilizers in Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 199

are the most influential P solubilizers (Whitelaw 2000 ). B. circulans, Bacillus
megaterium, B. subtilis, B. sircalmous, B. polymyxa, Enterobacter and Pseudomo-
nas striata , can be referred as the most important strains (Subbarao 1988 ; Kucey
et al. 1989 ). A fungus Arthrobotrys oligospora is also found to have the ability to
solubilize the phosphate rocks (Duponnois et al. 2006 ). Increased high percentage
of PSM is concentrated in the rhizosphere, and they are metabolically more active
than from other sources (Vazquez et al. 2000 ). By and large, 1 g of fertile soil con-
tains about 101-1010 bacteria, and their live weight may exceed 2,000 kg ha −1 . Soil
bacteria can be cocci (sphere, 0.5 μm), bacilli (rod, 0.5-0.3 μm) or spiral (1-100 μm)
shapes. Bacilli are common in soil, where as spirilli are very rare in natural environ-
ments (Baudoin et al. 2002 ). The PSB are cosmopolitan and vary in forms and pop-
ulation in diverse soils. Their population depends upon the physical and chemical
properties organic content and phosphorous content of soil and cultural activities
(Kim et al. 1998 ). Maximum populations of PSB are found in agricultural and
rangeland soils (Yahya and Azawi 1998 ). In north of Iran, the PSB count ranged
from 0 to 107 cells g −1 soil, with 3.98 % population of PSB among total bacteria
(Fallah 2006 ). Mineralization and solubilization potential for organic and inorganic
phosphorus, are also shown by bacterial populations (Hilda and Fraga 1999 ; Khiari
and Parent 2005 ). Phosphorus solubilizing activity is determined by the capacity of
microbes to liberate metabolites such as organic acids, which through their hydrox-
yl and carboxyl groups chelate the cation bound to phosphate, than are transformed
to soluble forms (Sagoe et al. 1998 ). Various microbial processes/mechanisms in-
cluding organic acid production and proton extrusion are used in Phosphate solubi-
lization. (Surange 1995; Dutton and Evans 1996 ; Nahas 1996 ). A wide range of
microbial P solubilization mechanisms exist in nature and much of the global cy-
cling of insoluble organic and inorganic soil phosphates is attributed to bacteria and
fungi (Banik and Dey 1982 ). Whitelaw ( 2000 ) suggested that Phosphorus solubili-
zation is also carried out by a large number of saprophytic bacteria and fungi acting
on sparingly soluble soil phosphates, mainly by chelation-mediated mechanisms.
Phosphate solubilizing microorganisms secrete organic acids and enzymes that act
on insoluble phosphates and convert it into soluble form, thus, proving P to plants
(Ponmurugan and Gopi 2006 ). Inorganic P is solubilized by the action of organic
and inorganic acids secreted by PSB in which hydroxyl and carboxyl groups of ac-
ids chelate cations (Al, Fe, Ca) and decrease the pH in basic soils (Kpomblekou and
Tabatabai 1994 ; Stevenson 2005 ). The PSB dissolve the soil P through production
of low molecular weight organic acids mainly gluconic and ketogluconic acids
(Goldstein 1995 ; Deubel et al. 2000 ), in addition to lowering the pH of rhizosphere.
The pH of rhizosphere is lowered through biotical production of proton/bicarbonate
release (anion/cation balance) and gaseous (O 2 /CO 2 ) exchanges. Phosphorus solu-
bilization ability of PSB has direct correlation with pH of the medium. In addition
to phosphorous solublization ability of PSB, they also can improve plant growth by
enhancing the availability of other trace element such as iron (Fe), zinc (Zn), etc.
Gull et al. ( 2004 ) suggested that PSB can solubilize the fixed soil P and applied
phosphates resulting in higher crop yields. According to Goenadi et al. ( 2000 ) direct
application of phosphate rock is usually ineffective in the short time period of most