THE EFFECT OF LIMING ON HEAVY-METAL CONCENTRATIONS IN WHEAT, CARROTS AND SPINACH GROWN ON PREVIOUSLY SLUDGE-APPLIED SOILS

Liming is often recommended to minimize the plant uptake of potentiall y toxic elements from sludge-amended soils. In outdoor experiments con ducted during 1989-91 in a rural location, near Brentwood (UK), wheat, carrots and spinach were grown on soils from a wide range of sites pr eviously amended with heavy applications of sewage sludge. The objecti ve of these studies was to examine the effect of liming on the accumul ation of sludge-borne metals in the crop plants. The results showed th at liming the soils to pH 7 prior to sowing significantly reduced meta l concentrations in carrots and spinach, although the reduction appear ed to be greater for Cd, Ni and Zn than for Cu and Pb. The wheat crop was grown on soils which had been limed 2 years previously, and the av erage pH of these soils was 6.5 compared to a pH value of 5.95 in the unlimed soils. This comparatively small pH difference between limed an d unlimed soils (6.50 - 5.95) generally had little influence on metal contents in wheat. These results suggested that maintaining the soil a t pH 7 is better than pH 6.5 for minimizing the accumulation of potent ially toxic elements from soils which have received relatively high le vels of sludge application over many years. The data for winter wheat suggested either that metal uptake into the grain was not sensitive to differences in soil pH or that a relatively small residual effect of past liming was not high enough to reduce metal uptake.