Agriculture Reference
In-Depth Information
6.2 Phosphorus Status and P Dynamics in Rhizosphere:
Current Perspective
Globally, rock phosphate (RP) is an abundantly available and inexpensive form
of P, and therefore, about 80 % of mined RP is used for agricultural fertilizer
(Tirado and Allsopp
2012
). Worldwide, there is a huge RP deposit in countries such
as Morocco and China, while other regions particularly Asian countries depend
heavily on import of phosphates for intensive agriculture production systems. From
a consumption point of view, China is the largest consumer of P fertilizers account-
ing for about 34 % of the total world consumption, whereas India ranks second with
19 % of global consumption (FAOSTAT
2012
). Similarly, the uptake of P by plants
varies greatly (Fig.
6.1
). Regrettably, mining of RP and its use as fertilizers pose a
serious human health risk (Zhang and Shan
2008
; Pan et al.
2010
; Tirado and
Allsopp
2012
). Considering the declining P resources, cost of P-fertilizer produc-
tion, and direct or indirect human health problems due to their excessive applica-
tion, scientists around the world have directed their attention toward exploring the
natural and inexpensive P resources which could serve as an eco-friendly and
economical alternative for chemical fertilizers in sustainable agriculture. In this
context, the phosphate-solubilizing actinomycetes have provided some solutions to
the expensive P problems.
Further, the chemical and biological processes in the rhizosphere play an
important role in soil nutrient availability and crop productivity (Zhang
et al.
2010
) as presented in Fig.
6.2
. Plants acquire P in the form of orthophosphate
anions (mainly H
2
PO
4
and H
2
PO
4
2
) which they obtain from different soil P
reserves including inorganic P (Pi) and organic P (Po). Broadly, on the basis of
plant accessibility, soil P has been grouped as follows: (a) solution P, present in the
soil solution and immediately available for plant uptake; (b) active P, adsorbed on
active sites of soil yet readily available and in equilibrium with solution P; and
(c) fixed P, strongly adsorbed and least soluble with slow conversions to active P
over a vast period, even years (Syers et al.
2008
; Shen et al.
2011
). Generally,
concentration of plant-available P in soil is below the critical level required for
plant growth and yields (Raghothama
1999
). Thus, the synthetic phosphatic fertil-
izer or organic manure is applied to overcome the P deficiency to plants. Of the total
P applied to soils, only 15-25 % of it is available for uptake by plants and the
remainder is fixed (Shen et al.
2011
). The P dynamics and availability in soil is,
therefore, controlled by several factors: (1) the ability of plants to form extensive
root systems, (2) impact of microbial colonization onto the development of plants,
(3) soil microflora affecting biogeochemical cycling of elements, and (4) physico-
chemical properties of soils supporting plants and microbial life in soils.
