Cadmium and zinc availability in soil irrigated with sludge containing a cationic conditioner

Abstract

Tomato plants (Lycopersicum esculentum Mill, ‘Red Cherry Small’), grown in 15 cm diameter plastic pots with a standard greenhouse medium (1:1:1, by volume, soil:peat:sand) were irrigated for 15 weeks with liquid sewage sludge containing a liquid cationic conditioner (Petroset SB, Phillips Petroleum Company, Bartlesville, Oklahoma) to determine the effect of the conditioner on Cd and Zn availability. Half of the plants received 50 ml week−1 liquid digested sludge with no conditioner and half of the plants received 50 ml week−1 sludge containing 0.25 ml conditioner (200:1, by volume, sludge:conditioner). Plant height was measured weekly. Plants were harvested 3, 6, 9, 12, and 15 weeks after sludge treatments began and separated into roots, shoots, and fruits for dry weight determination and Cd and Zn analyses. Soil and sludge crusts were sampled at the same times and analyzed for extractable concentrations of Cd and Zn. Dry weights of plants grown with conditioned sludge were similar to those grown with nonconditioned sludge. Plants with conditioner flowered and fruited one and two weeks earlier, respectively, than plants without conditioned. Six weeks after treatments began, when the plants had grown to their greatest height, Cd concentrations in sludge crusts, soil, and roots receiving conditioner were 2.0, 1.5, and 2.1 times greater, respectively, than crusts, soil and roots not receiving conditioner. After the six weeks sampling time, Cd concentrations in crusts, soil, and roots receiving conditioned sludge were similar to those in crusts, soil, and roots receiving nonconditioned sludge. At the third-week sampling time, shoots of plants grown with conditioner had 2.6 times more Cd than shoots of plants grown without conditioner. Cadmium concentrations in shoots from both treatments were similar at later sampling dates. Cadmium content of fruits was the same for both treatments all sampling times. Zinc content of roots, shoots, fruits soil, and sludge crusts was no affected by the conditioner. Results showed that a cationic conditioner, added to sludge, increased the availability of Cd, but not of zn, for tomato plants until maximum height was reached.