PHOSPHORUS, NITROGEN, AND RADIONUCLIDE RETENTION AND LEACHING FROM A JOEL SAND AMENDED WITH RED MUD GYPSUM/

The leaching of phosphorus (P), nitrogen (N), and radionuclides (Th-23 2, Ra-226, (228)R, and K-40) from Joel sands amended with red mud/gyps um (RMG) at 9 rates (0, 2, 4, 8, 16, 32, 64, 128, and 256 t/ha) was me asured using columns. Intense leaching conditions (34 mm/day for 12 da ys) and a high rate of applied P (320 kg/ha as superphosphate) and N ( 680 kg/ha as ammonium nitrate) were used to simulate extremes of irrig ated vegetable production on the Swan Coastal Plain. Addition of the h ighest rate of RMG (256 t/ha) reduced leaching of fertiliser P and amm onium-nitrogen (NH4-N) by 85% and 50%, respectively, compared with 0 t /ha after 12 days. At 64 t RMG/ha P leaching was reduced 50% compared with 0 t/ha. Nitrate-nitrogen (NO3-N) leaching was not affected by add ition of RMG. Reduced leaching of NH4-N was attributed to an increase in cation exchange capacity of the soil with the addition of RMG. Bica rbonate-extractable P in the soil increased with rate of RMG to >50 mu g P/g soil at 256 t/ha. This indicates that soil testing of residual P could be used to reduce P inputs to vegetable crops after soils were amended with RMG. This would further reduce the impact of vegetable p roduction on the water systems of the Swan Coastal Plain and extend th e period of effectiveness of RMG amended soils. The increase in Th-232 specific activity in Joel sand amended with RMG was well below statut ory limits even at the highest rate. Neither K-40 nor Ra-226 Were dete ctable in RMG amended sands up to 256 t RMG/ha. There was no evidence of leaching of Ra-226 or Ra-228 at any rate of RMG. These results sugg est that the use of RMG amendment on commercial horticultural properti es on the Swan Coastal Plain could be feasible.