The soils of Bijir, Gopalpur, Narayanpara and
Pundibari under the districts of Burdwan, Birbhum, Hooghly and Coochbehar
respectively were acidic (pH 5.26-6.46) in reaction, while the soil sample of
Panchpota of the district Nadia was alkaline (pH 8.12) in nature The soils were
non-saline, EC (dS m-1) ranging from 0.06-0.25 having the variation
in oxidisable organic carbon from 0.37 to 0.58 in percentage. The CEC of the
soil under acidic range varied from 5.37 to 6.5 cmol (p+) kg-1
,while the same was 18.2 cmol (p+) kg-1 for the soil of Panchpota.
The available N, P and K (kg ha-1) of the given soils varied from
114.15 to 159.31, 16.63 to 30.14 and 106.58 to 149.18 respectively. The
dominance of sand (44.1% to 59.7%) was there in all the soil samples over that
of silt (19.8% to 34%) and clay (20.5% to 21.9%) content.

The adsorption envelopes were studied at the pH 4.0, pH 6.0
and pH 8.0 for the given soils (Tables 2 to 7) to understand the effect of
different pH on the changes of the specific sorption/desorption isotherms
(Figures 1-2) of ‘P’ in soils.

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It was observed that the phosphorus sorption was a function
of initial soil pH where the sorption of P decreased with the increasing pH at
the equilibrium phosphorus concentrations (Figure 1). The phosphorus was
desorbed upto 40.78% at pH 8.0 over that of 21.82% at pH 4.0 and 36.07% at
pH6.0 for the soils of Bijir (Table1) due to the increase in pH in soil. Again
the percentage of P desorped to the P initially sorbed varied from 39.80 -22.70
at pH 8.0 over that of 19.51-15.73 at pH 4.0 and 34.82-21.16 at pH 6.0 of the
soils of Gopalpur and that from 27.47-30.81 at pH 8.0 than the pH 4.0
(16.84-20.78) and at pH 6.0 (25.29-29.36) for the soils of Panchpota. sorption
decreased upto 19.03% in soils due to the increase in pH from 4.0 to 6.0 and
decreased (Table 1) also when the pH was increased from 4.0 to 8.0  during the given sorption run. It was
reported that (Chen and Berber, 1990) that for an acid weathered soils of pH
4.2 to 4. 6, when adjusted upto a ph 8.3, sorbed P increased upto pH 6.0 to
6.2, then decreased at higher pH values. The initial increase in P sorption was
explained as the formation of amorphous hydroxyl Al with highly active sorbing
surfaces.The subsequent decrease in P sorption was attributed to increased
competition of hydroxyl with phosphate for sorption sites. Similar trend
wasobtained in lime treated acid soils(Mokunye,197) and in a Al – organic
matter complexes(Haynes and Swift,1989).  This was in support for the weathered acidic
soil (Naidu et al., 1990) resulting an
increased electrostatic repulsion due to the increased negative surface charge
(Haynes, 1982), accomplished by increasing pH. The higher amount of hydroxyl
ions’ concentration over that at the lower range of soil pH, could compete
effectively with phosphate ions for the same sorption sites on the mineral
surfaces (Smyth and Sanchez, 1980).The Langmuir equation fitted the sorption
data with variable success (r2 = 0.02 to 0.65) . The trend in
Langmuir K with increasing pH suggested a reduced ‘P’ affinity for the specific
sorption surface (Table 2).

The Langmuir equation was fitted to the given P sorption/desorption
data for all the three pH levels (Table 2 and Table 3) with the Langmuir
parameters such as K1 and K2 values being affected by pH
levels (i.e. pH-4.0, pH-6.0 and pH-8.0). With the increase of the pH in
general, the K1 values were increased and K2 values were
decreased under the given soils (Table 2 and 3).

            The Kd values was defined
by the rate of change in the amount sorbed ‘P’ relative to the amount of P in
solution on addition of graded doses of phosphorus in the solution phase. The
soil pH was found to govern the changes in partition coefficient (Kd)
values with respect to equilibrium P concentration (Fig 3). The Kd
values at the higher pH levels were lower in general than at the pH- 4 during
sorption run (Figure 3). Hence, while assessing the lowering of Kd
with the increasing pH, could relate to the decreasing rate of change in the
amount of ‘P’ sorbed and thus more of the P-available pools was found in the
soil solution, although, at higher equilibriums (?3?g P/ml), no
differences  obtained if pHs under
isothermal condition (Sato,2005).

The desorption isotherms was found
to be depended on soil pH and the amount of initially added P (Fig. 3), where,
the percentage of P desorbed to the P initially sorbed increased with
increasing pH of the solution suggesting the greater desorbability of P at the
higher pH in soil (Table 1). The desorption data were fitted to the Langmuir
equation at different pH levels with variable success (Table 3) under the
different P concentrations . The variation in soil pH (Table 3) influenced the
Langmuir parameters (K1, K2) with the K1
values decreased with the increasing pH in comparison to pH 4.0 in general. The
increase in K2 values (Table 3)with the increasing pH was apparent.
As a general consensus, the increased P desorption with increasing pH was due
to the increased competition with hydroxyl ions for the same sorption sites and
due to the change in electrostatic potential of the surface which drew attention
for the further study.

            The calculated Kd during
the desorption run decreased in general with the increasing pH (Figure 4)
although, the higher Kd at pH 8.0 was apparent over that of the
initial solution pH of 4.0. and pH 6.0. Hence, the decreasing Kd
with the corresponding pH signified the decreasing rates of change in the
amount of sorbed P, rendering more availability of the given P in the solution
phase.

Conclusions: The investigation mainly focused on
the effect of soil pH on phosphorus desorption in soils having different pH
levels. It was found that, change in soil pH from acidic (pH 4.0-6.0) to
alkaline range (pH 8.0) affected the amount of available P sorbed in the soils.
The percentage of the desorbable- P varied with the increasing level of pH of
the given soils. The decreasing trend of the partition coefficient with the
increasing pH signified the pattern of change of the sorbed (lower in this
case) affecting status of the available pool in the soil solution. The P
desorption isotherm will help to understand the desorption processes in
nutrient uptake models where liming in acid soils may be an important aspects
for P management.