Compost trials on horticultural tree nursery fields: application rates, tillage techniques and varieties over three years

The town of La Conner, Washington composts the biosolids and septage from the town, local municipalities and septage haulers with sawdust and locally collected landscape trimmings to produce a high quality agricultural soil amendment. In 2009, the town began working with a local tree nursery to investigate optimum application rates and techniques for Skagit Valley agriculture. This paper summarizes a series of field trials of the compost on over 100 horticultural tree varieties on 4 fields with varying rates over 4 years in silt loam soils in the Skagit Valley. In collaboration with Urban Forestry Nursery (UFN), strips of compost approximately 6 m wide were applied across two fields in which 1,966 trees of 63 varieties were planted. The compost strips were applied 3 months apart. Three fields were added in years following with lower application rates and different tillage practices. Tree vitality and soil parameters in and out of the compost strips were measured yearly 1 to 4 times and results analyzed to develop recommendations for future field applications. Soil samples composited from in and out of the compost strips were analyzed for EPA 503 metals, organic matter, pH, EC, CEC, moisture, and nutrient content. Tree responses to heavy compost applications were found to be highly species dependent. Four species reacted most negatively to the deepest application rate the first year, while 5 others appeared noticeably healthier in the compost strips. Negative impacts decreased with time following application. By the 3^rd year following application, even the most sensitive species, that survived, appeared larger and had darker green leaves in the compost areas. Trees planted across the lowest application rate strips were not visibly impacted. Soil responses depended upon application rates and included lowering of pH and bulk density, and increased CEC, OM, nutrients and micro-nutrients, and zinc.