Soil and Crop Micronutrient Concentrations as Modified by Wastewater Sludge

Abstract

Soil extracts are used to provide a measure of plant nutrient bioavailability, but their predictive capacity is variable. In a greenhouse experiment, soil was amended with sewage sludge from textile manufacturing wastewater. We investigated the correspondence between maize (Zea mays L.) shoot uptake and concentrations in soil extractions for plant essential micronutrients concentrated in the sludge: B, Cu, Mn, and Zn. Hot water extract was used for B, whereas diethylene triamine pentaacetic acid (DTPA) extract was used for Cu, Mn, and Zn. Separate pots with NH₄NO₃ amendment, but no sludge, allowed comparison of soil micronutrient extraction and plant uptake under conditions of elevated mineral N fertility similar to those with sludge amendment, but in the absence of sludge. Addition of sludge did not change soil extractable concentrations nor shoot concentrations of B and Zn. B was abundant in the sludge but likely remained strongly bound as organic complexes, whereas the quantity of Zn was not high enough to register difference. Shoot Mn was increased by both sludge amendment and an NH₄NO₃ effect, likely from pH decrease associated with nitrification of the ammonium in the sludge and in the fertilizer. DTPA-extractable Mn increased in sludge-amended plots. DTPA-extractable Cu increased strongly following sludge amendment, but, in contrast to Mn, shoot Cu concentration was not changed. Thus, selectivity of roots to avoid Cu is not revealed by DTPA. The correspondence between extraction-based estimation of availability and the actual plant uptake varies among micronutrients due to effects of soil conditions and selectivity of roots.