Effect of Compost Amendment on Soil Properties in Low and High Organic Matter Soils.

We conducted an interdisciplinary field experiment designed to observe the effect of compost and biocontrol agents on potato diseases, insect populations, soil quality, and yield at two sites: Aroostook Farm (AF, a conventionally managed research site) in Presque Isle, Maine and Wood Prairie (WP, an organically managed commercial farm), in Bridgewater, Maine.  Conifer-based (Hemlock bark) compost amendment (Cobscook blend, Coast of Maine Organic Products, Inc.) was added by hand at an approximate rate of 19 Mg/ha. Each treatment was replicated four times.  Nutrient levels were measured using a standard soil test before planting; moisture, soluble C, soluble N, and water stable aggregates (WSA, > 1, 0.25, and 0.053mm) were measured several times during the growing season.  Soil quality was very different between the two sites: specifically total soil C was 1.9% at AF and 5.0% at WP.  The WP farm included potato only one year in four and grew green manure crops.  In contrast the AF site included potato in alternate years and no green manure crops.  In 2007 WP had higher levels of WSA at each of the three sampling times than AF; the largest differences between soils were in the larger macroaggregates (> 1mm).  Compost amendment did not significantly affect aggregation at either site.  Throughout the 2007 growing season soil moisture was significantly higher at WP than AF; compost amendment did not affect soil moisture at either site.  Soluble inorganic N levels were higher at AF than WP in late June due to the use of inorganic N fertilizer at AF.  At other sampling dates, N levels were similar at the two sites and unaffected by compost use.   Compost amended soils had higher levels of soluble C than nonamended soils at the last two sampling dates suggesting compost decomposition later in the season.