Impact of storage time on the composition of a finished compost product: a case study

Several studies have provided information concerning the loss of nitrogen, phosphorous, potassium, carbon, and organic matter from manures and plant residues during active composting. However, very little information was found to provide insight into the changes in compost composition as the product ages during curing and storage in uncovered windrows. The objective of this study was to observe changes in compost composition after it was removed from a compost-ing shed and was stored in large un-covered windrows at a compost production site that used turkey poult litter (manure and wood shavings) as the primary ingredient. Compost samples and production records were obtained for 7 windrows and it was determined that the active composting time under the shed (AC) averaged 99 days and the time allowed for curing and storage in the uncovered windrows (CS) ranged from 29 to 583 days. As a result, the total compost age (CA = AC + CS) at the time of sampling ranged from 131 to 674 days. The quantities measured were moisture, pH, bulk density, electrical conductivity (EC), carbon (C). organic matter (OM), total ammoniacal N (TAN = NH4+-N + NH3-N), nitrate-N, organic-N, total-N (TN), P2O5, K2O, Ca, Mg, S, Zn, Cu, Mn, Fe, Na, and Al. The TN content was not found to change significantly while being stored in the outside windrows. However, the TAN content decreased significantly with storage time while nitrate-N and organic-N concentrations increased. The results showed evidence of nitrification of ammonium-N and a build-up of organic-N during storage. Storage time did not significantly impact concentrations of P2O5, K2O, Al, Na, and all minor plant nutrients measured. The pH fell from 8.9 at a compost age of 131 days to a mean of 6.8 by day 363 providing evidence of formation of organic acids during storage. Significant decreases during storage were observed for C, OM, and C:N. The rate of organic matter loss during storage was-0.623 g OM/kg DM/day and carbon was lost at the rate of-0.447 g C/kg DM/day. The electrical conductivity was not correlated with storage time and the mean was 3.80 & PLUSMN; 3.25 mmhos/cm. The bulk density increased significantly during curing and storage (R2 = 0.693) and was believed to be the result of compression settling.