Phosphate sorption characteristics of some floodplain calcareous and non-calcareous soils of Punjab

Understanding the P-sorption characteristics of floodplain soils is important for successful fertilizer management practices. The P-sorption characteristics of 16 soils that are representatives of the recent floodplain areas of the Punjab were described using Langmuir and Freundlich isotherm equations. The phosphate sorption maxima ‘b’ deduced from the Langmuir isotherm equation ranged from 232.6 to 572.5 μg g⁻¹ with an average of 410.9 μg g⁻¹. The calculated bonding energy ‘k’ for the Langmuir isotherm equation ranged from 0.052 to 0.410 ml g⁻¹ (mean= 0.132 ml g⁻¹, n=16). Freundlich ‘a’ and ‘n’ constants that represent the extent and rate of P-sorption ranged from 1.34 to 1.66 μg g⁻¹ and 0.71 to 0.86 ml g⁻¹, respectively. In soils of comparatively higher CaCO₃ content (S-9, S-10, S-11), isotherm inflection point that indicates the P precipitation in soil was distinct at high equilibrium solution P concentration. However, in majority of soils including non-calcareous soils, the P-sorption process also seemed to be controlled by strong affinity of non-carbonated clays (Fe and Al-oxides). The Langmuir adsorption maxima ‘b’ that represents the concentration at which precipitation begins to dominate the P-sorption process exhibited significant positive correlation with clay (r=0.59*), CaCO₃ (r=0.93**), Fe_AM (r=0.53*) and Fe_CDB (r=0.50*) but negatively correlated to soil pH (r=-0.93**).