Sustainability of soil organic matter at organic mixed vegetable farms in Michigan, USA

Abstract

Soil samples at thirty (30) USDA organic-certified mixed vegetable farms in Michigan, USA were obtained and analyzed for their percentage of soil organic matter (SOM). The intent was to characterize the changes in SOM content, specifically the overall change since organic certification, the patterns of increase or decline, and whether certain agricultural practices are associated with levels of SOM. A short survey of farmers provided supplemental information about each sampling site, including tillage practices, pesticide use, and cover crops. The results indicate a significant majority (24/30) of organic farms showed increases in the percentage of organic matter content, with one farm exhibiting no change and five farms decreasing. These results were achieved in a variety of soil textures, climatic conditions, and over durations from 1–17 years of organic certification. Organic farms had more than twice the percentage of soil organic matter as the ten (10) conventional farms used as a benchmark. In addition, the soil at the sampled organic farms exhibited high positive correlations between SOM and cation exchange capacity, and between pH and cation exchange capacity. Tillage mean depth at those farms increasing their percentages of organic matter was 70% lower than those farms with decreasing percentages of SOM, which suggests low-till or no till practices may help preserve organic matter. In combination with other management methods, such as cover crop usage and soil amendments, a key indicator of soil health was improved and sustained—a primary objective of organic farming.