Chemical retardation of phosphate diffusion in an acid soil as affected by liming

The retention of phosphate (P) in soil limits its mobility through bothdiffusion and mass flow. Soil P retention is a complex reaction with aluminum(Al) and iron (Fe) playing major roles in P retention in acid soil. This studyexamined the one-dimensional diffusion of applied phosphate andwater-extractable Al in a gibbsite-rich acid soil as affected by liming. Thesoil used was a loamy sand, a Typic Hapludox (US Taxonomy), obtained from asite150 km south of Perth, Australia. Soil was limed at 5 gkg −1 to raise the soil pH from 5.5 to 7.1.KH 2 PO 4 was applied to the surface of unlimed and limedsoil columns, which were then incubated in a constant humidity chamber for 3,7,14 and 28 d. Water-extractable P (WE-P) concentration in the surface ofunlimed soil (0 to 0.2 cm) increased from 0 in the original soilto22.3 μmol g −1 , 3 d after the Papplication. However, the WE-P decreased with incubation period to a value of9.7 μmol g −1 on day 28. Similar WE-Pconcentration patterns were observed with the limed soil, but the WE-Pconcentration near the surface decreased to a much lower value under limed (7.6μmol g −1 ) than unlimed (9.7 μmolg −1 ) conditions. Liming also reduced P diffusion. After28 d of incubation P diffused to a depth of 2.1 cmin the unlimed soil, but to only 1.5 cm under the limedsoil.Acid-extractable P (AE-P) also increased from 0 to 50 μmolg −1 in the surface soil layer for both unlimed and limedsoil as a result of P fertilizer application. There was little change in theAE-P for both treatments from day 3 of incubation to day 28. The pH and Al andFe concentrations in the soil solution were elevated near the soil surfacewhereK and P ions interacted with soil matrices. Liming significantly reduced thetotal amount of WE-P and P diffusive movement in the soil, but reduced Altoxicity since Al ions shifted to less toxic species with increases in pH. Pfertilizer application also increased soil pH under both unlimed and limedconditions. This would reduce Al toxicity, benefiting crop production.