The Influence of Compost Origin On Chemical and Biological Properties of Compost Extracts and Pak Choi (Brassica rapa Chinensis Group) Yield

There is growing evidence that compost extract can benefit plant health and yield but a consistency of the effect is a concern. Compost quality plays a major role in extract quality; however, little work has been done to evaluate the effect of compost sources on compost extract quality and subsequent plant response. The objective of this study was to determine chemical and biological properties of compost extracts produced from different types of compost and to examine their effects on plant growth. Five different composts: chicken manure based thermophilic compost (CT); green waste thermophilic compost (GWT); food waste vermicompost (FWV); aged chicken manure based vermicompost (ACV); fresh chicken manure based vermicompost (FCV); and their extracts were evaluated for chemical and biological properties. Aerobic aqueous extract of each compost was applied to pak choi ‘Bonsai’ grown in peat based media. Mineral nutrients, humic acid, and microbial activity present in each compost extract were generally representative of those properties of the respective compost sources. N and K content in all compost extracts were negatively correlated with C:N and C:K ratio of the compost sources.  Humic acid among compost extracts ranged 98.6 - 550.5 mg L^-1 in the order of GWT > ACV = FCV > FWV > CT while N content ranged 8.71 - 328 mg L^-1 in the order of CT > ACV > FWV > FCV > GWT. Active bacteria and active fungi were higher in the vermicompost sources compared to thermophilic compost. Indole-3-Acetic Acid was detected only in vermicompost sources. Except for the extract of GWT, all other compost extracts enhanced plant growth compared to control. However, the extracts of ACV and CT positively influenced plant growth to a greater extent than the other compost extracts. Although N content in CT extract was double that of ACV extract, plant dry weight, root biomass, total root length and root surface area was higher in ACV compared to the CT treatment. This suggests that compost extract effect on plant production is due to the combined contribution of mineral nutrients, organic acid and microbial activity.