Agriculture Reference
In-Depth Information
(Solaiman and Rahbbani
2006
) including the major element phosphorous (P). Even
though the vegetables require high amounts of P for its luxuriant growth and
development, the accessibility of P to such crops is restricted due to its rapid
fixation ability (Khan et al.
2007
; Bushman et al.
2009
). Therefore, the deficiency
of soluble P in soil has become one of the most limiting factors in crop production in
different agroclimatic regions. The plants obtain their P requirements from the soil
pool where it occurs as inorganic P, produced as a result of weathering of parent
rock or as organic P derived from decayed plants, animal remains, or microorgan-
isms. Mineral forms of P present in soil are apatite, hydroxyapatite, and oxyapatite,
while organic P occurs chiefly in the form of inositol phosphate. Other organic P
compounds in soil are in the form of phosphomonoesters, phosphodiesters (includ-
ing phospholipids, nucleic acids), and phosphotriesters (Paul and Clark
1988
).
Although P is present in soil in abundance, yet it is the least soluble and majority
of it is immobilized and rendered unavailable for plant uptake. Plants acquire P
from soil solution as phosphate anions which are extremely reactive and are
immobilized through precipitation with cations such as Ca
2+
,Mg
2+
,Fe
3+
, and
Al
3+
. And hence, the soluble fraction of P within the soils is usually very low
relative to other mineral nutrients.
Deficiency of P is a common and quite widespread problem among many soils
including the Indian soils, because of which, the growth of vegetable suffers
heavily. Phosphorous, therefore, needs to be applied frequently and externally in
the form of phosphatic fertilizers in order to maintain a lavish crop growth. The
chemical fertilizers when used, however, also become rapidly immobilized soon
after application and thus remain unavailable to the plants (Sanyal and Datta
1991
;
Rodriguez and Fraga
1999
). In this context, soil microorganisms play an important
role in phosphate solubilization by mineralizing the organic P in the soil and thus
making it available to the plants. Some microbes, isolated from various
rhizospheric soils including those of vegetable rhizospheres, popularly known as
the phosphate-solubilizing microorganisms, have this ability of solubilizing insol-
uble mineral P by various mechanisms (Sung-Man et al.
2010
; Varsha et al.
2010
;
Sagervanshi et al.
2012
; Sharma et al.
2012
; Alia et al.
2013
; Onyia and Anyanwu
2013
; Karpagam and Nagalakshmi
2014
). Some of the important genera of
phosphate-solubilizing bacteria include
Achromobacter
,
Aerobacter
,
Alkaligenes
,
Bacillus
,
Pseudomonas
,
Serratia
, and
Xanthomonas
(Li
1981
; Sharma et al.
2005
;
Chen et al.
2006
; Ivanova et al.
2006
). Besides providing P, PSM also facilitate the
growth of vegetables by other mechanisms (Jeon et al.
2003
; Lucy et al.
2004
;
Egamberdiyeva
2005
; Calvo et al.
2010
; Kang et al.
2010
; Sung-Man et al.
2010
;
Dastager et al.
2011
; Sagervanshi et al.
2012
).
