Phosphate sorption and desorption as affected by addition sequences of ammonium lignosulphonate and diammonium phosphate in a clay soil

Abstract

A study was carried out to evaluate the effect of ammonium lignosulphonate (NH4LS) addition on the retention and desorption of P in a clay soil. High rates of NH4LS and diammonium phosphate (DAP) were added in four experiments: (1) simultaneous NH4LS and DAP addition, (2) DAP addition as a pretreatment followed by NH4LS addition, (3) NH4LS addition as a pretreatment followed by DAP addition, and (4) incubation of the soil with NH4LS for 15 d as a pretreatment followed by DAP addition. Both NH4LS and DAP increased Ca dissolution (CaD) when added alone, but DAP decreased CaD in the presence of NH4LS. Simultaneous addition of NH4LS and DAP, or incubation of NH4LS with soil decreased dissolution of Ca. Formation of Ca-P compounds accounted for part of the increases in P retention especially at high rates of P additions. Additions of NH4LS reduced P retention, except when DAP alone was added initially to the soil. This suggests that lignosulphonate competed with phosphate ions for adsorption sites when NH4LS was added either before or simultaneously with DAP. Phosphate desorption increased with increased P retention and was greater with simultaneous additions of NH4LS and DAP. In summary, NH4LS improved the efficiency of P fertilizers through increasing P solubility. Key words: Phosphate precipitation, Ca dissolution, phosphate potential, line potential