Chemical and physical properties of two tropical soils treated with sewage sludge compost

Abstract

The effects of sewage sludge compost applications on a Mollisol and an Ultisol of Puerto Rico were evaluated. Experimental rates were control (0 compost), 37, 74, and 148 t/ha/yr, which were to be applied during a three year period. In addition, a treatment consisting of a single application of 445 t/ha was included to assess the impact of single massive applications vs. continuous applications of compost. Results here presented pertain exclusively to the project's first year. The compost was obtained from the sewage sludge compost facility of Puerto Rico's Solid Waste Management Authority in Arecibo. The material was predominantly inorganic and exhibited a high soluble salt content, which diminished its quality. Compost additions caused significant pH increases in both soils. The effects were more noticeable on the Ultisol (Corozal clay), where pH increased from 4.55 in the control to 6.45 with the lowest compost treatment. The electrical conductivity of both soils increased considerably with compost additions, sometimes approaching limits considered detrimental to support crop growth. The organic matter content of both soils also increased with compost additions. This increase had a positive effect on their water retention capacity. Nitrogen contributions from the compost were minimal. However, significant increases in the levels of phosphorus were observed in both soils. Compost additions caused significant increases in the levels of EDTA extractable metals (i.e., Cu, Zn, Cd, Cr, Fe). However, elements regulated by the U.S. Environmental Protection Agency (e.g., Cd, Pb, Cr) were added in amounts well below the established limits, and thus were not a reason for concern.