Assessment of the Reliability of a Commercial Maturity Test Kit for Composted Manures

Stability significantly affects the potential for beneficial utilization of composts but is difficult to measure by farmers and compost producers. A simple four hour test (the Solvita® maturity test) that measures CO2 evolution and ammonia emission from compost samples was compared to a traditional three-day, 25°C, CO2 evolution rate procedure and to measurements of ammoniacal nitrogen concentrations in manure composts to assess the reliability of this test. Three composts — 1) Dairy manure amended with wheat straw, 2) The same dairy manure but amended with sawdust and 3) Swine manure amended with sawdust and ground wood pallets — were composted in windrows for 120 days. Samples were removed weekly to biweekly. CO2 evolution rates of the three composts decreased from initial means (n=6) of 3.41, 3.42 and 9.35 to 0.63, 0.76 and 0.31 mg CO2-C g−1 VS day−1, for the dairy manure-straw, dairy manure-sawdust, hog manure composts, respectively. The corresponding mean Solvita CO2 test values for these composts increased from 3.4, 3.0 and 3.2 to 6.8, 6.5 and 7.0, respectively. Correlation analysis between CO2 evolution rates and Solvita CO2 test values gave linear correlation coefficients (r) of −0.82, −0.78, and −0.87 for the straw-amended dairy manure, the sawdust-amended dairy and the hog manure composts, respectively. The Solvita NH3 test gave highly significant correlations (p<0.0001) with ammoniacal-N concentrations (correlation coefficients (r) = −0.43, −0.64 and −0.65, respectively). The Solvita® maturity index, a combination of Solvita CO2 and NH3 values, correlated significantly with both CO2 evolution rate and ammoniacal-N concentrations. However, the Solvita CO2 index alone was the best predictor of compost CO2 evolution rate or stability. The Solvita Maturity test, which combines the Solvita CO2 and NH3 tests, provided useful information about the potential for the development of a toxic response in plants due to excessive concentrations of ammoniacal-N present in some stable compost samples that would not have been detected if the CO2 stability test were used by itself. We conclude that the Solvita maturity test provided a simple, inexpensive relative test of compost stability and NH3 emission for diverse samples of composted manures. Even so, it did not accurately predict their CO2 evolution rates measured by respirometry nor their ammoniacal-N concentrations. The test would be most useful for on-farm applications.