Synthetic zeolites as amendments for sewage sludge-based compost

The effects of incorporating a synthetic zeolite (Zeolite P) in a range of concentrations (0.1–1.0 w:w) into an experimental horticultural compost, derived from sewage sludge, have been investigated. The impact of zeolite treatment on time-related changes of the labile zinc, copper, iron and manganese pools within the compost was compared to lime incorporation (5% w:w) and to a proprietary unamended peat-based compost. Addition of 0.5% and 1.0% zeolite significantly reduced labile zinc over a 90 day period. The highest zeolite treatment was more effective than liming; 0.5% zeolite was as effective as lime. Plant growth trials measuring transfer of metals to ryegrass (Lolium perenne L. cv Elka) in successive harvests demonstrated that both 1.0% zeolite and 5% lime treatment caused significant reduction in total metal transfer from soil–plant over a 116 day growth period. It is concluded that the use of synthetic zeolite as an amendment for compost of this type significantly reduces potential for soil metal mobility and soil–plant transfer.