Agriculture Reference
In-Depth Information
The incubation period also plays an important role in production of organic acids
which in turn affect the P solubilization process (Maliha et al.
2004
; Khan
et al.
2007
). Experiments have shown that P solubilization in culture medium
progressively increases with gradual increase in incubation periods (Ahmad
2014
). The decrease in P solubilization, however, occurs after certain period of
incubation which could be due to the depletion of nutrients, production of certain
toxic metabolites in the growth medium, or autolysis of cells (Khan et al.
2013
). In a
study, Stephen and Jisha (
2011
) reported maximum solubilization of P (30.44 mg/
100 ml) by
Burkholderia
sp. (MTCC 8369) after 18 days of incubation which
decreased thereafter with subsequent incubation days. This decrease in P solubili-
zation in some cases has been found to be due to increase in pH of the medium
caused by utilization of P and organic acids to run the various metabolic reactions
of the PSB (Tripura et al.
2007
). Similar variation in P solubilization with change in
time has been reported by others (Zaidi
1999
; Balakrishnan et al.
2012
; Panda
et al.
2013
).
3.2.7 Humic Substances and Organic Matter
Humic substances in soils are the dark brown, extremely versatile, and fully
decomposed remains of plant or animal organic matter. Humic compounds consist
of humic acid, fulvic acid, and humin fraction. In soils, humic substances play some
important roles in (i) soil conditioning and plant growth (Benedetti et al.
1996
);
(ii) improving nutrient uptake, especially P, S, N, and Zn; (iii) removing toxins
from both soils and animals; (iv) stimulating soil biological activity;
(v) solubilizing minerals; (vi) improving soil structure; (vii) protecting soil from
degradation; and (viii) enhancing water-holding capacity (WHC) for better drought
resistance and reduction in water usage. Humic substances also improves the
effectiveness of RP by causing the release of PO
4
from hardly soluble rock minerals
because of high total acidity and its ability to complex and chelate the resulting
solutions and to stimulate microbial metabolism. Of the different humic com-
pounds, humic acid (HA), a naturally occurring polymeric organic compound
(Schnitzer and Khan
1972
; Sposito
1989
) is an active constituent of organic
humus which improves soil structure and enhances the WHC of the soil. Apart
from these, HA affects the growth of useful soil organisms and serves as an
adsorption and retention complex for inorganic plant nutrients (Brannon and
Sommers
1985
). Moreover, HA can convert soil constituents into forms that are
suitable for uptake by plant due to its ability to form complexes (Vaughan and
McDonald
1976
). Since HA contains 51-57 % organic C, 4-6 % N, and 0.2-1 % P,
it improves crop yields by supplying N and P to the plants together with the
improvement in the physico-chemical and biological characteristics of the soils
(Hajra and Debnath
1987
).
The application of humic compounds in the presence of PSB increases the pH
and available P and decreases the exchangeable ions. The presence of sodium
