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

A study was carried out to evaluate the effect of ammonium lignosulphonate (NH 4 LS) addition on the retention and desorption of P in a clay soil. High rates of NH 4 LS and diammonium phosphate (DAP) were added in four experiments: (1) simultaneous NH 4 LS and DAP addition, (2) DAP addition as a pretreatment followed by NH 4 LS addition, (3) NH 4 LS addition as a pretreatment followed by DAP addition, and (4) incubation of the soil with NH 4 LS for 15 d as a pretreatment followed by DAP addition. Both NH 4 LS and DAP increased Ca dissolution (CaD) when added alone, but DAP decreased CaD in the presence of NH 4 LS. Simultaneous addition of NH 4 LS and DAP, or incubation of NH 4 LS 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 NH 4 LS 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 NH 4 LS was added either before or simultaneously with DAP. Phosphate desorption increased with increased P retention and was greater with simultaneous additions of NH 4 LS and DAP. In summary, NH 4 LS improved the efficiency of P fertilizers through increasing P solubility. Key words: Phosphate precipitation, Ca dissolution, phosphate potential, line potential