Optimisation of rice straw and acacia biochar doses in two soils for phosphorus availability

Biochar (BC) is a soil fertility enhancer, regarded as a potential carbon sequester and nutrient leaching preventer. Various studies have shown better phosphorus (P) availability with adding biochar. However, limited reporting is available on absorption–desorption behaviour of added P and dose optimisation of biochar. Rice straw biochar (RSB) and acacia biochar (ACB), prepared by slow pyrolysis at optimised temperature and pyrolysis time, were incubated at different rates (0, 5, and 10 g kg−1) in loamy sand and clay loam soils to study P sorption compared to the unamended control. The data was observed to be best fitted into Langmuir adsorption isotherm for P adsorption. The effects on maximum P adsorption, bonding energy, and adsorption rate constants were lower in loamy sand compared to clay loam soil. The values of maximum P desorption (Dm) and desorption rate constant (Kd) decreased with the application of biochar. A linear regression equation optimised the amount of P addition for soil amendments. For the soil with inherent P content, rice biochar was the best option (5 g kg−1), while for soils lacking in P, ACB is optimised at 5 g kg−1 with external P input of 1 mg P kg−1 of test soil. This data can be recommended to the farmers as a best practice for the P addition, along with managing farm wastes as biochars to enhance nutrient availability.