Effect of Compost Rate On Soil Organic Matter Content.

Sequestration of carbon in soils is a major environmental benefit of composting.  Incorporation of organic materials in soil reduces release of carbon dioxide to the atmosphere and improves soil quality. In 2007, organic vegetable plots were established on the Water’s Organic Farm at Tennessee Tech University in Cookeville, TN.  Using a random block design, four compost treatments (0, 22, 66, and 110 lbs  per plot) were applied to thirty six plots each measuring 1.2 m X7.6 m.   Squash was grown in year one, followed by onions, cabbage, and lettuce in subsequent years. Cover crops were utilized for soil management between cropping seasons.  Annual soil samples were extracted  at depths of 0-6 inches and 7-12 inches.  Carbon was determined by ignition.  Before initiating treatments, percent carbon at the 0-6 inches and 7-12 inch depths averaged 2.70 and 1.37, respectively.   In 2010, the average percent carbon at 0-6 inches for treatments 0, 22, 66, and 110 lbs compost averaged 5.09, 5.48, 5.91, and 6.17 %, respectively.  The average percent carbon at 7-12 inches for treatments 0, 22, 66, and 110 lbs compost averaged 4.02, 4.94, 4.28, and 4.16 %, respectively.  At the 0-6 inch depth, there were significant differences (P<.05) between the 0 lb rate and the 66 and 110 lb rates.  At the 7 – 12 inch depth, the 0 lb rate was significantly different from the 22, 66, and 110 lb rates of added compost. No significant different was indicated when comparing the 22, 66, and 110 lb rates.  Organic carbon was significantly increased in all plots from 2007 to 2010. Higher levels of organic carbon occurred in the upper six inches with the 66 and 110 lb compost rates, but at the 7-12 inch depth, the amount of organic carbon was similar for all three compost rates.