Abstract:
Spent mushroom substrate (SMS) has the potential to produce compost with different additives. However, some
substances and by-products produced during the process can be toxic to seed germination and plant growth.
Evaluation of the phytotoxicity is important for guaranteeing compost quality. Therefore, this study was
conducted to evaluate the phytotoxicity of spent mushroom substrate compost (SMSC) using a seed germination
bioassay. Compost aqueous extracts (CAEs) were prepared from SMSC produced by amending different
additives (green leaves and urea) with and without an inoculum. Dilution series of 25%, 50%, and 100% were
tested against seed germination of radish (Raphanus sativus L). Germination index (GI) in all 25% CEAs of this
study were phytotoxic free and they indicated the availability of phytonutrients and/or phyto-stimulants except
the non-inoculated SMSC without amendments. Similarly, all 50% CAEs were phytotoxic free except Tithonia
amended inoculated SMSC with less phytotoxicity. Of 100% CAEs, non-inoculated Pueraria amended SMSC was
highly phytotoxic, non-inoculated Tithonia and urea amended SMSC were phytotoxic free, and the rest were less
phytotoxic. Seed germination of R. sativus was not inhibited by any of the CAEs of Tithonia amended SMSC
indicating the higher usability as an amendment in SMS. The water extractable NH4
+
concentration was
significantly higher (p<0.05) in the inoculated SMSC amended with urea and it was significantly lower (p<0.05) in
the non-inoculated SMSC amended with Tithonia and inoculated SMSC without amendments. Further, the
inoculated SMSC extracts showed higher performance in GI than non-inoculated SMSC. GI showed a higher
negative significant correlation (r=-0.841, p<0.01; r=-0.778, p<0.01) with EC in both inoculated and non inoculated compost samples, respectively. Moreover, the EC was significantly correlated with all extractable
tested ions except Cu2+ and Ni2+ in inoculated treatments. pH values in all aqueous extracts were recorded in the
range of 6.27 to 8.08. Extractable elements Cu2+ (r=0.645, p=0.01) and Zn2+ (r=0.577, p=0.05) had positive
correlation with pH. SMS can be recycled as compost integrating nitrogenous additives and inoculum to use safely
as a soil amendment
