Mulching with compost improves growth of blue spruce in Christmas tree plantations

Compost derived from source-separated organic household waste was used in a blue spruce (Picea pungens var. glauca) Christmas tree plantation as a mulch in tree rows in a 3–year trial. The amounts, which were applied at the start of the experiment, were 29 t ha–1 (treatment 1) and 58 t ha–1 (treatment 2; dry matter), corresponding to 3 and 6 combined annual compost applications, respectively. The compost mulch reduced the shoot growth of the 6-year-old blue spruce trees in the first year after compost application by 6% and 8% in treatment 1 and 2, respectively. In the second year, growth was enhanced for 8% and 5% in treatment 1 and 2, respectively. The growth enhancement was well proportioned and characterized by increases in both shoot growth and the number of lateral buds. In the third year, shoot growth was no longer significantly different from the untreated control. The soil contents of plant-available nitrogen and potassium, pH and electrical conductivity increased significantly through the compost application. The slightly increased water content of the soil positively influenced tree growth at this dry and sandy site.