SEWAGE-SLUDGE EFFECTS ON GROWTH AND NITROGEN-FIXATION OF SOYBEAN

Previous studies on the effects of sewage sludge on the growth of soybeans (Glycine max L. Merr.) have failed to resolve whether land application is an acceptable practice. To study this problem, two sludges were incorporated into soil at four rates (based upon the N requirement of the crop, i.e. 0, 78, 156, and 312 kg available N ha-1) and at two locations. The growth, nodulation, and nitrogen fixation of soybeans were monitored. Although the electrical conductivity (EC) of the soil, which is an indication of the soluble salt content, was increased immediately after addition of the sludge, no long-lasting, adverse effects on plant growth were noted. When the EC of soil was determined at harvest, it had declined to background levels for ali treatments. Although the microsymbiont of soybean (Bradyrhizobium japonicum) is relatively sensitive to the toxic effects of soluble salts, nodulation was not affected by the application of the highest rate of sludge. In the first year, the sludge significantly increased the foliar N content of the crop. The rate of nitrogen fixation was inversely related to the amount of sludge-borne N added to soil. At the highest rate of sludge addition, N fixation provided approximately 14% of total plant N with approximately 50% of total foliar N obtained from fixation at the control rate. When the same parameters were measured during the second year, sludge application had no significant effect on any of the parameters examined. These results indicate that the application of sewage sludge to land cultivated to soybeans may be an acceptable practice.