Agriculture Reference
In-Depth Information
2.1
Introduction
Phosphorus (P) is one of the most essential plant nutrients which profoundly affect
the overall growth of plants (Wang et al.
2009
) by influencing various key meta-
bolic processes such as cell division and development, energy transport, signal
transduction, macromolecular biosynthesis, photosynthesis and respiration of
plants (Shenoy and Kalagudi
2005
; Ahemad et al.
2009
; Khan et al.
2009
). On
the contrary, unlike N, the atmosphere does not provide soluble P to plants. And
hence, the source of P is largely the primary and secondary minerals and/or organic
compounds. In comparison to other nutrients, P concentration in soil solution is
much lower and ranges from 0.001 to 1 mg/l (Brady and Weil
2002
). Broadly, P
compounds in soil can be placed into three categories: (i) inorganic compounds,
(ii) organic compounds of the soil humus and (iii) organic and inorganic P com-
pounds associated with the cells of living matter. Mineral compounds of P usually
contain aluminium (Al), iron (Fe), manganese (Mn) and calcium (Ca) and vary
from soils to soils. For example, P forms a complex with Al, Fe and Mn in acidic
soils, while in alkaline soils it reacts very strongly with Ca. However, under all
conditions, the types of soil P compounds are determined mainly by soil pH and by
the type and concentrations of soil minerals. Some of the most common P minerals
are presented in Table
2.1
. Of the total soil P pool, about 50 % of P is in the organic
forms (Richardson
1994
), which varies between 4 and 90 % in most soils (Yadav
and Verma
2012
). The organic P in plants includes (i) inositol phosphate (10-50 %
in soil) which represents a series of phosphate esters ranging from monophosphates
up to hexaphosphates. Phytic acid (inositol hexakisphosphate) is the main com-
pound that plants use to store P in seeds to support early seedling growth following
germination. Phytin (a Ca-Mg salt of phytic acid) is the most abundant of the
known organophosphorus compounds in soils. Other organic P in soils occur as
sugar phosphates, nucleotides (0.2-2.5 %), phosphoprotein (trace), phosphonates
(Tate
1984
) and phospholipids (1-5 %) (Yadav and Verma
2012
). Of the various
forms of P, plants take up only negatively charged primary and secondary ortho-
phosphate ions (H
2
PO
4
and HPO
4
2
) as nutrient. Indeed, the amount of plant
available P is very low relative to the total soil P. Moreover, majority of the soil P is
fixed, and only a small fraction of P is available for uptake by plants. Therefore, P
deficiency results in stunted growth, dark leaves, and inhibition of flowering and
root system development. In most plants, these symptoms will appear when P
concentration in the leaves goes below 0.2 %. And, hence, in many cases, phos-
phatic fertilizers which are quite soluble and manures that also contain P (soluble P,
organic P and inorganic P) are applied to overcome P deficiency in soils and to
provide adequate P to plants. The P of the phosphatic fertilizers or the manure reacts
very strongly with soil constituents and becomes unavailable to plants. The in-
soluble and inaccessible forms of P are hydrolysed to soluble and available forms
through the process of solubilization (inorganic P)/mineralization (organic P). The
immobilization in contrast is the reverse reaction of mineralization. During
immobilization, microorganisms convert inorganic forms to organic phosphate,