The influence of aeration system, temperature and compost origin on the phytotoxicity of compost tea
Numerous studies have evaluated compost teas’ capacity to suppress plant disease, but no study has been made of possible compost tea phytotoxicity. In this study, 24 assays were carried out on compost tea produced from three different composts: one produced from bovine manure (M) and two others made from municipal solid waste but differing in the degree of stability (stable: MSW I and unstable: MSW II); using two aeration systems: continuous and discontinuous aeration, and at three temperatures: 10, 20 and 30ºC. The aim was to evaluate the influence of production conditions on the phytotoxicity of tea. In addition, the evolution of phytotoxicity over the period of extraction was also studied, as well as the influence that dilution of the tea before its application has on phytotoxicity. Phytotoxicity was evaluated by means of Zucconi’s germination test using lettuce seeds and in addition a growth test was carried out using two teas: M-N-10i and MSW I-A-20, using two types of seeds: barley and lettuce. Compost teas were chemically analysed for pH, EC, NH4+-N and heavy metals in order to explain differences in phytotoxicity. According to Zucconi’s test, all the teas, independently of the dilution rate, caused negative effects on root length of lettuce seeds. However, growth tests showed more positive results, with growth inhibited only at very high tea concentrations. A growth test is likely to be more reliable than a germination test because application conditions are closer to real conditions. Aeration and temperature influenced compost tea phytotoxicity, with aerated teas produced at low temperatures causing less phytotoxicity. Furthermore, tea phytotoxicity was also dependent on the stability of the compost from which the tea was produced; unstable compost produced compost teas of greater phytotoxicity.