Effect of long-term application of sewage sludge to a grazed grass pasture on organic carbon and nutrients of a clay soil in Zimbabwe

A study was conducted to assess the nutrient status of a Zimbabwean vlei clay soil grown to a Kikuyu (Pennisetum clandestinum Chiov.) grass pasture which had been amended with sewage sludge for 19 years. There was a significant (P <0.05) accumulation of organic C, mineral N, resin extractable P, and exchangeable K, Ca, Mg and Na in the top soil horizon. Organic C increased from 2.5 to 8.7% and 1.8 to 4.5% in the 0-5- and 5-10-cm horizons, respectively. Addition of sewage sludge resulted in a 19- and 57-fold increase in extractable P in the 0-5-5-10-cm soil horizons, respectively. Exchangeable Na significantly (P<0.05) increased from 0.88 to 4.10 cmol/kg and from 1.04 to 3.06 cmol/kg in the 0-5- and 5-10-cm horizons, respectively. It was concluded that sewage sludge is a valuable source of nutrients and also provides an opportunity to increase soil organic matter.