Fractions, uptake and fixation capacity of phosphorus and potassium in three contrasting soil orders

A controlled greenhouse experiment was conducted to study the fraction, uptake and fixation of phosphorus (P) and potassium (K) in three soils with contrasting characteristics. The experiment was carried out in two phases, viz., sorghum-Sudangrass depletion and test crop experiment with maize with three levels of fertility status (depleted, original and fertilized with 40 mg kg-1 P and K) and two levels of rhizospheric status (rhizosphere and non rhizosphere). Clay was separated by ultrasonic vibration followed by fractionation into colloidal and non colloidal clay and short-range order (SRO) minerals, P and K fixation were measured in these clays. Phosphorus and potassium fractions and uptake were measured in soil samples. The results showed that, available and non-exchangeable potassium was highest in Vertisol and Inceptisol, respectively. Calcium-P was the dominant fraction in soil and highest in Inceptisol. Maximum K-fixation capacity for both the clay fractions was found in Vertisol (32.57% for colloidal clay and 37.94% for non colloidal clay), depleted soils (30.58% for colloidal clay and 31.04% for non colloidal clay) and rhizosphere (28.34% for colloidal clay and 29.59% for non colloidal clay). Phosphorus fixation was highest in Alfisol (58.72% for colloidal clay and 67.26% for non colloidal clay), depleted fertility status (53.41% for colloidal clay 55.45% for non colloidal clay) and non-rhizosphere (52.53% for colloidal clay and 54.26% for non colloidal clay) for both the clays. Phosphorus fixation was positively correlated with different pools of iron and aluminum compounds whereas, potassium fixation showed positive significant correlation with amorphous ferri-alumino silicate content.