Phosphorus Adsorption and Phosphorus Use Efficiency in Calcareous Alkaline Soils Influenced by Humic Acid

The effect of lignitic coal derived humic acid (HA) was evaluated on phosphorus adsorption and desorption capacity in calcareous alkaline soils collected from Research Farm of the University of Agriculture, Peshawar, Pakistan. The P adsorption capacity was determined by adding 0, 10, 20, 40, 60, 120, 240 and 360 mg P L-1 (initially P applied, IPA) to 5 g soil with absence (alone P) or presences of 0.15% HA (P+HA) solution in duplicate. The mixture was shaken on horizontal shaker for 30 h continuously followed by 5 min centrifugation at 2500 rpm to collect the aliquot/supernatant. The soils were then added 45 mL distilled water followed by same shaking and centrifugation for desorption studies. The P+HA treatments yielded significantly higher EPC (equilibrium phosphorus concentration) and lower P adsorption (x/m) at each level of IPA than alone P rendering that HA reduced the P adsorption. The P adsorbed (x/m) expressed in percent of IPA (Xad) and the ratio of x/m to ECP (Kd) simply decreased with each increments of IPA in case of alone P but first increased and then decreased in case of P+HA indicating different behavior of P in presences of HA solution. The higher Xad and Kd in case of alone P revealed its comparatively higher affinity and more adsorption of P at any levels of IPA than the soil which received HA treatments. Similarly, the P+HA treated soils produced comparatively higher desorption of P at any IPA than the alone P treated soils suggesting that HA not only reduces the adsorption but can also increase their release from the soil surfaces into solution. The alone P was best fit to Langmuir modified two surface model whereas the P+HA was best fit to Freundlich isotherm models. These results indicated that humic acid application can improve the P use efficiency in field condition through changings in dynamics of P sorption and hence should be applied for better crop nutrient management.