Heavy metal accumulation in soils under long term fertilization

Investigations were made on the accumulation of DTPA extractable and total cadmium and lead in surface soils (0-15cm) from a ‘Long Term Fertilizer Experiment’ in progress since 1972 in a medium black soil at Coimbatore, India under irrigated condition. The experimental soil is a sandy clay loam with a pH of 8.2 and 2.0 percent calcium content. The organic carbon content is 0.3 percent. The treatments consist of NPK application at 50, 100 and 150 percent of optimal level, 100 percent optimal level of NPK without herbicidal addition through sulphur free sources, with zinc sulphate at 25 kg ha-1 and with cattle manure at 10 t ha-1. There are two other treatments in addition to unfertilized control, viz., application of NP alone and N alone at 100 percent of optimal level. Each treatment is being replicated four times in a randomized block design and has a net plot size of 200 m2. A fixed crop rotation of finger millet - maize –cowpea (since deleted from 2000) in a year is being followed. Soil samples are being collected after each crop, stored and evaluated for various parameters since 1992. The results of 1992, 2000 and 2008 revealed that the treatments effects on the concentration of DTPA extractable cadmium and lead were non-significant during 1992 but significant during 2000 and 2008. The results of 2000 and 2008 showed that the accumulation of DTPA and total Cd and Pb were the highest in the plots which received NPK at 150 percent. The least, in general, was in the control. Continuous fertilization for 36 years has not resulted in toxic concentrations of Pb and Cd.