Assessment of Compost Maturity Indices for a High Salt/High pH Compost.

     The use of indices to determine the maturity of a compost are important because  an immature compost can adversely affect plant growth if used as a soil amendment due to the presence of undecomposed, phytotoxic compounds as well as high microbial activity which can scavenge available nutrients.  Different indices of compost maturity were assessed for their accuracy when using a compost with high salt and high pH characteristics.  The traditional and best tests, plant bioassays, provided results that were highly variable with time and did not approach any equilibrium for the duration of this study.  These results were probably due more to the high salinity and pH than the overall compost maturity.  Chemical indices, specifically those related to the optical density and aromaticity (%), proved best for this situation.  The optical density, based on E4/E6 values from an extract containing humic substances had a non-linear correlation with time (R² = 0.94) and reached an equilibrium at a maximum value of 9.8 by day 120.  The aromaticity (%) measure, based on peak area ratios from ¹³C CPMAS NMR spectral data, also exhibited a non-linear correlation with time (R² = 0.91) and reached an equilibrium at a maximum value of 18.7 by day 93.  The two chemical indices were also strongly correlated with one another (R² = 0.84).  This data indicates that optical density and aromaticity are the best measures to use when assessing the maturity of a high salt high pH compost.