Alternative methods for using organic materials composting vs. adding directly to soil

Abstract

Composting organic wastes before applying them to the soil helps to produce a relatively homogeneous, odor‐free, easy‐to‐handle material. Also, it may destroy or deactivate weed seeds and many plant and animal pathogens. Decomposition of residues before they are added to the soil will presumably have different effects than allowing them to decompose in the soil, but effects on soil fertility and microbiology have been little studied. This paper describes research on the influence of composting on soil nitrogen availability, cation exchange capacity, organic matter content, and structure. Sewage sludge was added to the soil either directly, or first mixed with sawdust, or first composted with sawdust, in each case at rates equivalent to 150, 300, and 900 ppm N. The amended soils and a no treatment check soil were placed in pots that were either planted to ryegrass or allowed to incubate. Ryegrass yields were greatest for the direct application of sludge, while either composting or mixing with sawdust before application caused enough immobilization of N to depress yields. Compared to sludge mixed with sawdust, composting led to lower soil organic matter (3.04% vs. 4.11%), lower total N (0.12% vs. 0.14%), and lower CEC (15.2 vs. 17.3 me/100 g soil). Total porosity, as indicated by moisture retention curves, was highest with the sludge‐sawdust mixture (77%), compared to 55%, 52%, and 50% for composting, direct addition, and no amendment, respectively. The shape of the curves indicated a very large increase in macropores and medium size pores, which help in water transmission and water retention by the soil. These results indicate that adding organic materials directly to the soil may be as good or better for soil fertility than first composting them.