The UD IPM program has an extension project looking at the effect of compost on soil health. While this project is focusing on vegetable crops, some initial observations can equally apply to evaluating compost for ornamental and turf usage. The following is more information.
We are evaluating 3 different composts, an agricultural grade compost made specifically for improvement of field soils, a yard waste based compost, and spent mushroom soil compost. These were applied at a ½ inch rate and 2 inch rate and then mixed into the soil. Plant performance and soil quality measurements are being evaluated and compared to fumigated soil and untreated controls
The Ag grade compost was hard to apply due to the high moisture content; whereas the yard waste compost spread easily (well screened and appropriate moisture). Plant performance in the Ag grade compost has been excellent at both rates; the yard waste based compost has shown signs of nitrogen deficiency especially at high rates; and the spent mushroom soil based compost has showed signs of salt injury in the high rate treatment but good growth in the low rate treatment. These observations illustrate the need to evaluate compost sources before application. Some critical factors to consider are compost maturity, C:N ratio, salt content (EC), nutrient composition, pH, partical size and uniformity, moisture content and weight. In these studies the ag grade compost was fully mature, had moderate levels of nutrients, and a low salt content. The yard waste based compost was not mature (thus the N deficiency), had low nutrient content, and low salts. The spent mushroom soil compost was not well screened, had high nutrient content, but also had high salt levels.
Gordon Johnson, Extension Agriculture & Horticulture Agent, UD, Kent County