CCREF Research Reports
Rick Carr, Master of Science
Compost Production Specialist
Rodale Institute
Kutztown, Pennsylvania
Composted substrates provide a diversity of nutrients for plant growth and development. Compost producers can create feedstock recipes that will provide predictable results in compost nutrient content over time, which is attractive to end-users such as landscapers and the agricultural community. However, many composting operations use a variety of feedstocks to generate compost and the nutrient profile may not be as predictable. For instance, certain feedstocks are able to influence the nutrient profile of cured compost, and while part of the goal is organics recycling, some end-users prefer consistent nutrients when amending with compost. Over the last five years, Rodale Institute has been monitoring the nutrient content of finished compost to gain a better understanding of the chemical variability in a large-scale windrow composting operation. Windrows were prepared with a variety of agricultural and municipal compost feedstocks and percent content of each feedstock was measured. Time and temperature was documented in accordance with the USDA National Organic Program. Moisture content, bulk density, volume reduction, weed seed germination and amendment rate bioassays were several other metrics used to further characterize the composting process and finished material. Compost ready for land or greenhouse application was sampled and submitted to Penn State’s Agricultural Analytical Services Lab for compost nutrient analysis. A total of 36 different windrows were sample to date, and the results were analyzed for linear correlations among (1) feedstock content, (2) age of compost at the time of testing, and (3) various chemical properties.Average chemical/biochemical properties were as follows: pH 7.7 (± 0.4), soluble salts 4.6 (± 2.1) mmhos/cm, solids 47.7% (± 8.0), moisture 52.3% (± 8.0), and organic matter 22.8% (± 5.2). Average macronutrient content varied less than most micronutrients that were measured. Linear correlations existed when the percent mixture of leaves:grass was tested. Total nitrogen(N), phosphorus(P), and potassium (K) increased with increasing grass, as well as soluble salt content. When percent grass content in a windrow was held constant and leaves:wood chips was tested there was little to no correlation in total N and P, percent carbon, and organic matter; however, total K and soluble salts tended to increase with increasing wood chip content. These results and several others including micronutrient dynamics in changing compost recipes will be discussed so that compost producers may have a better understanding of the relationship between feedstock recipes and subsequent nutrient content in finished compost.