Nutrient Cycling in Agroecosystems 59: 13–18, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 13 Effect of long-term application of sewage sludge to a grazed grass pasture on organic carbon and nutrients of a clay soil in Zimbabwe J. Nyamangara & J. Mzezewa Chemistry and Soil Research Institute, P.O. Box CY 550, Causeway, Harare, Zimbabwe e-mail: justice@csri.icon.co.zw Received 20 May 1999; accepted in revised form 4 January 2000 Key words: long-term application, sewage sludge, soil nutrients Abstract 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. Introduction Sewage sludge has been applied to agricultural land for centuries (Follet et al., 1981). In the European Community (EC) it is estimated that 29% of sewage sludge produced annually is used in agriculture (John- son & Corcelle, 1989). In the UK the proportion is 40% (450 000 tonnes dry solids) (Nyamangara, 1993). In Zimbabwe the use of sewage sludge and/or efflu- ent is limited to municipal farms and a few peri-urban farms. Typical sewage sludge may contain 3% N, 2.2% P and 0.3% K (Ott & Forster, 1977). Sewage sludge contains significant amounts of N, P, S, Ca and or- ganic matter. Nitrogen mineralisation rates of 50% in the first year and 30% in the second year have been reported in soils amended with sewage sludge (Cripps et al., 1992). In anaerobically digested sewage sludge, up to 70% of the N may be in the liquid frac- tion (Department of the Environment, UK, 1989) and therefore there is no initial period of N immobilisa- tion as in other organic fertilisers (Murwira, 1994). Phosphorus availability can be as high as 50% in the year of sewage sludge application (Department of the Environment, UK, 1989). The high organic matter content of sewage sludge improves the soil physical environment. The heavy metal content of sewage sludge, espe- cially from industrial areas, limits its application to agricultural soils. These metals are bioavailable but are not leachable by rain water (Follet et al., 1981; Williams et al., 1980). Zinc, Cu and Ni are phyto- toxic above certain thresholds. Cadmium accumulates in certain crops to levels deleterious to the consumer (Leeper, 1978: Sommers & Barbarick, 1986). Sewage sludge may also contain pathogenic microorganisms which are a health risk to humans, animals and plants. However, modern sewage sludge treatment, e.g., irradiation, reduces the number of pathogens to significantly low levels. The escalating cost of inorganic fertilisers has res- ulted in farmers exploring ways to use sewage sludge as a fertiliser. In Zimbabwe, limited research has been done to determine the contribution of sewage sludge or effluent to soil fertility. The aim of the this study was to assess the fertility status of a vlei clay soil grown