Chemical and physical properties of two tropical soils treated with sewagesludge compost

The effects of sewage sludge compost applications on a Mollisol and an Ulti sol of Puerto Rico were evaluated. Experimental rates were control (0 compo st), 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. c ontinuous applications of compost. Results here presented pertain exclusive ly to the project's first year. The compost was obtained from the sewage sl udge compost facility of Puerto Rico's Solid Waste Management Authority in Arecibo. The material was predominantly inorganic and exhibited a high solu ble salt content, which diminished its quality. Compost additions caused si gnificant pH increases in both soils. The effects were more noticeable on t he 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 appreaching limits considered detrimental to support crop growth. The organic matter co ntent of both soils also increased with compost additions. This increase ha d a positive effect on their water retention capacity. Nitrogen contributio ns from the compost were minimal. However, significant increases in the lev els of phosphorus were observed in both soils. Compost additions caused sig nificant increases in the levels of EDTA extractable metals (i.e., Cu, Zn, Cd, Cr, Fe). However, elements regulated by the U.S. Environmental Protecti on Agency (e,g., Cd, Pb, Cr) were added in amounts well below the establish ed limits, and thus were not a reason for concern.