Plant on a variety of soils had shown that

Plant roots take up phosphorus (P)
from soil mostly as the primary orthophosphate ion (H2PO4-),
but it is taken up also in the form of secondary orthophosphate ions by some
plants. With the increase in soil pH, the amount of secondary orthophosphate
form of P increases (Tiwari, 2012).The research on a variety of soils had shown
that soil pH influences on surface charge and it can be managed by liming
(Naidu et al., 1990; Rupa et al., 2001). In tropical regions, P
bioavailability could be increased by liming to raise the soil pH (Sanctez and
Uehara,1980), although the effect of liming on P sorption and bioavailability
were conflicting (Haynes,1982) in some cases.The increase (Pereira and De
Faria,1998) and decrease (Erani et al.,
1996) of P sorption with increasing pH were observed while others showed that
there was no significant influence by the pH (Arias and Fernandez,2001) of the
soils.The reduced P sorption may increase hydroxyl concentration and enhanced
competition between hydroxyl and phosphate ions on mineral surfaces for the
same adsorption sites (Sato et al.,
2005). Addition of liming, could decrease P sorption for the neutralization and
precipitation of Al3+ and hydroxy-Al as Al hydroxide, hence,
decreasing the number of P sorption sites (Smyth and Sanchez, 1980; Anjos and
Rowell, 1987; Naidu et al., 1990),
again with an increasing pH, the mineral surface is negatively increased
causing the greater electrostatic repulsion and thus could decrease P sorption
(Bowden et al., 1980 and Haynes, 1982) in soils. Chen and Barber (1990) showed
that if acid weathered soils is adjusted from pH 4.2 to pH 8.3, P sorption
increases up to pH of about 6.0 to 6.2, and then the sorption is decreased at
higher pH values.The adsorbing capacity of anions,  on certain mineral surfaces could vary at each
pH value of the medium (Hingston  et
al., 1967). Muljadi et al. (1966); Hingston et al. (1968,
1970); Obihara (1969) has shown that most of the experimental data can be
fitted to a Langmuir equation, which allows a maximum value for the adsorption
to be calculated. Such values are related to pH by curves known as ‘adsorption
envelopes’. For phosphate, the adsorption envelope commonly shows breaks in the
slope at pH values close to the second and thírd pK values of phosphoric acid.
Thís shape has been recently interpreted by Bowden et al. (1973) by
using the Stern double-layer theory. Phosphate is preferentially sorbed by soil mineral surfaces
as HPO2-4 rather than as H2PO4-  and the concentration of the divalent ion
(HPO42-) increases 10-fold for each unit increase in pH
from 2 to 7 (Bowden et al., 1980). This change partially offsets any decrease
in electrostatic potential (Haynes, 1984). Thus, P sorption may decrease
relatively slowly until pH 7; and then, above pH 7, the increase of HPO42–
concentration becomes progressively slower; whereas the decrease in surface
potential continued resulting in a more rapid decrease in P sorption (Haynes,
1984).

Besides, phosphate
sorption on ferrihydrite, decreased with pH ranging from 3.5 to 9.0,Arai and
Sparks (2001). Gjettermann et al.
(2007) found that with increasing pH both dissolved organic carbon and
dissolved organic phosphorus increased. Penn and Warren (2009) worked on the bioavailability and transport
potential of phosphorus in relation to phosphorus sorption in Georgia Kaolinite
at pH 4.3 and 6.3 to use ITC (isothermal titration calorimetry) technique and  showed that, in pH 6.3 Kaolinite exhibited
only exothermic reactions during P titrations. Based on sorption isotherms,
solution thermodynamic modelling and supporting ITC experiments, the exothermic
reaction indicated P sorption on to Kaolinite by ligand exchange and
dissolution or protonation of Kaolinite, while the endothermic reaction showed
Al phosphate precipitation. The adsorption of PO4 on aluminium oxides gave
maximum adsorption at pH 4, while PO4 adsorption on an Fe-oxide
(hematite) was maximized at pH 6.0 was attributed to dissolution of organic
coatings and increasing electrostatic repulsion. Liang et al. (2010), while studying with phosphate binding to Fe and Al
in organic matter as affected by redox potential and pH and it was observed
(Morris, 2011) that, changes in PO4 sorption across pH were
attributed to a combination of three predominant mechanisms: (i) changes in Fe,
Al, and PO4 protonation and resulting binding affinity (ii) changes
in bound Fe and Al concentration, and (iii) pH induced dissolved organic carbon
production at pH > 5.0.

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Very few
investigations are there on soil pH and P desorption and the results have been
conflicting; increased P desorption with increasing pH in some cases (De Smet et al., 1998) and with decreasing pH in
others (Barrow, 2002). Madrid and Posner (1979), Cabrera et al. (1981) studied on P desorption kinetics for synthetic
goethite as a function of suspension pH from 4 to 10. It was observed that
increased P desorption was there with increasing solution pH. This trend could
again be explained as above; as due to competition from hydroxyl ions and a
lessened attraction or an enhanced repulsion caused by increased negative
charge on the surface with increasing pH. Phosphorus desorption is a main
process which affect inorganic P bioavailability; and desorption also can
identify specific pools of bio available P. However, in order to use the
desorption process in nutrient uptake models such as described by Smethurst and
Comerford (1993), a P desorption isotherm is needed. An isotherm explained the
quantity of P on the solid phase relative to the solution P concentration with
the slope of this relationship being called the partition coefficient, (Kd.)
which is referred to as the rate of change of the quantity desorbed from the
solid phase more than the rate of change in solution concentration

Based on the above perspectives, the
present experiments were conducted with the hypothesis that phosphorus sorption
was a function of soil pH and the P desorption being increased with the
increasing pH of the soil.

Materials and Methods:

Collection and preparation of soil samples:

 Benchmark soils were collected from the five
locations (Bijir of Burdwan district, Gopalpur of Birbhum district, Narayanpara
of Hooghly district, Pundibari of CoochBehar district and Panchpota of Nadia
district) of West Bengal from 0-20cm depth of surface. The soil samples were
air dried and homogenized gently in a mortar to pass through a 2mm. sieve
before use.

 Physico-chemical properties of soil: Important
Physico-Chemical properties of soil samples (pH, EC, Organic carbon, soil
texture, Available-N, Available-P and Available-K, CEC, Ca, Mg) were measured
by the standard methods.

 Soil pH: pH of
soil samples (soil:water 1:2.5) was measured in suspensions by using pH meter
(Jackson, 1967).