2019 ASHS Annual Conference

Compost extract (CE) is a suspension of soluble and particulate material from compost in water intended as an agricultural fertility input or biological inoculum. Popular practice and theory around CE suggests that it can promote crop health beyond its nutrient value, presumably by influencing microbial communities in the phytosphere. We review this body of knowledge and identify a need for predictors of CE efficacy. Little is known about the range of microbial composition of compost extracts from different parent composts or how CE may be predicted to influence plant growth when applied to seeds or to residues before soil incorporation. We completed a series of biological tests to characterize 10 compost extracts, including total microbial abundance by fatty acid methyl ester (FAME) profiles, microscopic counts, and the Soil Microbiometer® system, a lettuce seed phytotoxicity bioassay, and a Pythium sp. damping-off suppression survey on cucumber (Cucumis sativus) seedlings. Three compost extracts advanced to a greenhouse experiment testing effects on lettuce growth in residue-rich soils. Agreement of microbial abundance measures with FAME analysis was fair and poor, for microscope counts and the Soil Microbiometer® system, respectively. One CE retarded lettuce seed germination, two CE reduces damping off in cucumber, and the interacting effects of residue and CE influenced lettuce growth. High carbon residues reduced growth and CE had no effect in these treatments; however, inoculation of alfalfa residue with yardwaste and chicken manure CE increased lettuce growth 53%. These growth-promoting extracts contained less microbial biomass and microfauna than the vermicompost extract, which performed comparably to the controls. Results suggest that certain CE can enhance lettuce growth when applied to high nitrogen residues before soil incorporation. We underline that available testing is inadequate for predicting whether a particular CE will successfully deliver any of several possible benefits.