Agriculture Reference
In-Depth Information
one or more minerals required for the growth and development of plants
(Gyaneshwar et al.
2002
). In order to maintain a good health, plants require these
minerals sufficiently and regularly. And hence, the plant nutrients in the form of
chemical fertilizers are applied from external sources. Such chemical fertilizers,
however, pose health hazards and cause pollution problems, when applied exces-
sively in soil. Besides, chemical fertilizers are quite expensive. Moreover, the usage
of chemical fertilizers alters the structure, composition and function of beneficial
soil microorganisms (Whitelaw
2000
; Reena et al.
2013
).
One of the most important problems in tropical agriculture is the low-soil-P
availability. Many of the tropical soils are highly weathered and have a high P
fixation capacity that makes their management more difficult. Sanchez and Logan
(
1992
) in a study estimated that 1,018 million ha in the tropics have a high P fixation
capacity. In tropical America, there are 659 million ha affected, 210 in Africa and
199 in Asia. The term “P fixation” is used in reference to a series of complex
reactions that remove bioavailable soil P from the soil solution, where roots directly
take up plant nutrients (Barber
1995
). Additionally, P is one of the essential
nutrients and is classified as macronutrient because it is required in large amounts
by the plants (Bushman et al.
2009
). On the contrary, about 98 % soils have
inadequate supply of available P (Hansan
1996
) and hence plants suffer heavily
from P deficiency. Most of the soils contain the substantial reserves of total P; large
part of it relatively remains inert and only
10 % of soil P enters the plant-animal
cycle (Kucey and Leggett
1989
). When P is added as fertilizer to the soil, it gets
rapidly fixed. Therefore, P is one of the three major nutrients which are generally
added to soil in agronomic practices.
<
4.3 Mechanism of P Solubilization: A Brief Account
The major microbiological means by which phosphate compounds are mobilized is
the production of low molecular weight organic acids (Goldstein
1995
) accompa-
nied by acidification of the medium. These organic acids are the source of biotical
generated H+ ions, which dissolves the mineral phosphate and make it available for
the plants (Bhattacharya and Jain
2000
). The type of organic acid produced and
their amounts, however, differ with different organisms. Among them, glucuronic
and
-ketogluconic acids are the most frequently secreted organic acids causing
mineral phosphate solubilization (Song et al.
2009
). Other organic acids such as
acetic, citric, succinic, propionic, glycolic, oxalic, malonic, fumaric and tartaric
acid have also been identified among P solubilizers (Ivanova et al.
2006
). Ryan
et al. (
2001
) in a study reported that the ability of different carboxylic anions to
desorb P decreased with a decrease in the stability constants of Fe- or Al-organic acid
complex in the order:
α
citrate
oxalate
malonate/malate
tartrate
lactate
>
>
>
>
>
gluconate
formate. Tri- and dicarboxylic acids are more effective as com-
pared to monobasic and aromatic acids-aliphatic acids which have also been found
significant
acetate
>
>
in P solubilization compared to phenolic, citric and fumaric acids
