Adsorption Envelope on Phosphorus Sorption-Desorption in Some Soils of West Bengal

Abstract

A laboratory experiment was conducted to study the adsorption envelopes at the pH 4.0, 6.0 and 8.0 on the changes of the sorption capacity of phosphorus (P) in the soils treated with graded doses of P solutions. The P sorption was lowered down to 19.03 per cent in soils due to the change in pH from 4.0 to 6.0 and decreased up to 17.04 per cent when the pH was raised from 4.0 to 8.0 at 35 °C during the given sorption run. The Langmuir parameters (K1 and K2) were affected by the pH levels (i.e. pH 4.0, pH 6.0 and pH 8.0). During the desorption run, the percentage of P desorbed to the initially sorbed–P was increased (up to 40%) with the increasing pH (at pH 8.0) of the solution, suggesting the greater desorbability of P at the higher pH in soil. In general, the K1 values decreased at pH 6.0 in comparison to that at pH 4.0 during the desorption run. The trend in increase of the K2 values with the increasing pH was apparent. During the sorption run, the average values of the partition coefficient (Kd) in soil at pH 8.0 were lower (Kd = 812) in general than at the pH 4.0 (Kd = 1171). The calculated Kd values during the desorption run were decreased with the increasing pH (Kd = 4385 at pH 6.0 and Kd = 4153 at pH 8.0) over that of the initial solution pH of 4.0 (Kd = 4727). Hence, the decreasing Kd with the increasing pH signified the lower rates of change in the amount of sorbed P, rendering more availability of the given P in the solution phase.