VARIATION OF COMPOST PHYTOTOXICITY DURING A COMPOSTING PROCESS

Phytotoxicity is an important indicator of compost maturity and it may be determined by germination bioassays using sensitive species to phytotoxic metabolites. In a compost made of straw, rapeseed cake and shredded sorghum, the phytotoxicity was assessed three, six and nine months after the start of composting, by using two sensitive species to phytotoxic compounds: Sinapis alba and Raphanus sativus. Four dilutions (1:5, 1:10, 1:50, 1:100) of compost in distilled water and a control (only distilled water) were used and germination percentage and root length were recorded. Relative Germination Percentage (RGP), Relative Root Growth (RRG) and Germination Index (GI) were calculated for each dilution and each species on each date, as well as RGP_global, RRG_global and GI_global for each species on each date. Germination percentage showed significant differences between dates for both species and root length was significantly different between dates for S. alba. Germination percentage and root length were significantly different between the two species depending on the date. For both species RGP values were close to 100%, indicating that compost hardly affected seed germination of either species. RRG values were higher than 100% in all cases (except for the dilution 1:5 on date 1 and date 3 for both species and for dilution 1:10 on date 3 for S. alba) indicating that compost stimulated the root elongation for both species. GI values were higher than 80% so that the compost was mature on the first date and it was not phytotoxic for any species on any date, except for S. alba with dilution 1:5 on dates 1 and 3, for which it was moderately phytotoxic.