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

The retention of phosphate (P) in soil limits its mobility through both diffusion 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 study examined the one-dimensional diffusion of applied phosphate and water-extractable Al in a gibbsite-rich acid soil as affected by liming. The soil used was a loamy sand, a Typic Hapludox (US Taxonomy), obtained from a site 150 km south of Perth, Australia. Soil was limed at 5 g kg(-1) to raise the soil pH from 5.5 to 7.1. KH2PO4 was applied to the surface of unlimed and limed soil 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 of unlimed soil (0 to 0.2 cm) increased from 0 in the original soil to 22.3 mumol g(-1), 3 d after the P application. However, the WE-P decreased with incubation period to a value of 9.7 mumol g(-1) on day 28. Similar WE-P concentration patterns were observed with the limed soil, but the WE-P concentration near the surface decreased to a much lower value under limed (7.6 mumol g(-1)) than unlimed (9.7 mumol g(-1)) conditions. Liming also reduced P diffusion. After 28 d of incubation P diffused to a depth of 2.1 cm in the unlimed soil, but to only 1.5 cm under the limed soil. Acid-extractable P (AE-P) also increased from 0 to 50 mumol g(-1) in the surface soil layer for both unlimed and limed soil as a result of P fertilizer application. There was little change in the AE-P for both treatments from day 3 of incubation to day 28. The pH and Al and Fe concentrations in the soil solution were elevated near the soil surface where K and P ions interacted with soil matrices. Liming significantly reduced the total amount of WE-P and P diffusive movement in the soil, but reduced Al toxicity since Al ions shifted to less toxic species with increases in pH. P fertilizer application also increased soil pH under both unlimed and limed conditions. This would reduce Al toxicity, benefiting crop production.