Plant Availability of Heavy Metals in a Sludge‐Treated Soil: I. Effect of Sewage Sludge and Soil pH on the Yield and Chemical Composition of Rape

Abstract

AbstractThis study investigated the effects of sewage sludge and soil pH on yield and chemical composition, and heavy metal uptake of fodder rape (Brassica napus L. ‘Kenton’). Plants were grown in the greenhouse on a sandy soil amended with an anaerobically stabilized sewage sludge. Lime needs of the soil to attain pH levels of 5.6, 6.0, and 7.5 were determined in a separate incubation study. The rates of sewage sludge applied were 0, 50, and 100 t/ha on a dry weight basis. Application of sewage sludge in the incubation study decreased the soil pH, and consequently increased the lime needs of the soil. The soil pH also decreased with increasing period of incubation. Plants were harvested 5 weeks after germination, and again 5 weeks after the first harvest. For each harvest, the application of sludge increased the total dry matter yield of rape significantly at all pH levels. Maximum yields were obtained at pH 6.0. The concentrations of all measured macro‐nutrients generally increased with increased rate of sludge, but the concentrations were invariably lowest at pH 6.0.The concentrations of Zn, Cd, and Ni in fodder rape generally increased, and those of Mn, Cu, and Co generally decreased with increased rate of sludge application. The pH level influenced the concentrations of the heavy metals studied, with the exception of Pb. The concentrations generally decreased when the pH increased. Although concentration of heavy metals was higher in the second harvest, in general, total amounts recovered were higher in the first harvest. The recovery of metals in rape, calculated as a percentage of 0.2N HCl‐extractable metals from the soil, decreased as the levels of sludge and soil pH increased.