Sewage Sludge-Soil Interactions as Measured by Plant and Soil Chemical Composition

The utilization of sewage sludge on agricultural lands requires prior knowledge of the interactions among sludge, soil, and plant. A green house experiment was conducted on three mineralogically different soils: a limed, volcanic ash-derived Andept (Akaka series, thixotropic, isomesic Typic Hydrandepts), an alkaline Vertisol (Lualualei series, very-fine, montmorillonitic, isohyperthermic Typic Chromusterts) and a limed, manganiferous Oxisol (Wahiawa series, clayey, kaolinitic, isohyperthermic Tropeptic Eutrustox). Each soil received four rates (0, 45, 90, and 180 Mg ha⁻¹) of a Honolulu sewage sludge, and an N-P-K fertilizer treatment. Lettuce (Lactuca sativa L.) was grown as a test crop. Concentrations of plant Cu and Fe were not significantly affected by sludge rate; plant Cd and Zn concentrations increased as sludge rate increased regardless of soil type. Plant Ni remained essentially constant as sludge rate increased in two of the soils; in the Oxisol, plant Ni increased with increasing sludge rate. Manganese in the soil solution drastically increased with sludge rate, and became phytotoxic (plant Mn exceeded 700 mg kg⁻¹, a level above which significant yield reduction occurred) with increasing sludge addition in the Oxisol, and with 180 Mg ha⁻¹ in the Vertisol. The Andept, with its high adsorption capacity, retained most of the heavy metals added by the sludge; this soil also retained phosphate so strongly that P was the growth-limiting factor in all treatments as indicated by low dry-matter yields, low plant P (<1.4 vs. 4.0 mg kg⁻¹ for maximum yield) and by low soil-solution P concentration (<0.08 vs. 0.40 mg P L⁻¹). In terms of heavy metal accumulation in plants, it appeared that the Andept could tolerate the highest sludge loading rate, and the Oxisol the lowest. Contribution from Dep. of Agronomy and Soil Science, College of Tropical Agric. and Human Resour., Univ. of Hawaii, Honolulu. Journal Series no. 3175 of the Hawaii Inst. of Tropical Agric. and Human Resour.