Experiences with Wastes and Composts in Nursery Substrates

During the past 20 years, the Ornamental Nursery Research Program at the former Horticultural Research Institute of Ontario (now part of the University of Guelph) has been conducting applied research dealing with environmentally friendly and sustainable nursery production practices with emphasis on container production. The use of farm, industrial, and consumer waste by-products as amendments in nursery substrates has been a major focus. The program has evaluated hundreds of potting mixes derived from individual or combined, raw or composted waste by-products including spent mushroom compost, turkey litter compost, paper mill sludge, municipal waste compost, corrugated cardboard, apple pomace, wood chips from pallets, pulverized glass, and various types of tree barks. With few exceptions, all the above waste by-products tested under our cultural conditions provided acceptable to excellent container-growing media, often in amounts exceeding 50% and sometimes up to 100% by volume in No. 2 containers (6 L), even despite initially elevated and potentially toxic contents of soluble salts expressed in terms of electrical conductivity measured up to 8.9 dS·m -1 in 1 substrate: 2 water (by volume) extracts in many of the substrates. A key to these successful results is that salts leach quickly from the containers to benign levels (∼1.0 dS·m -1 ) with normal irrigation practices. High initial pH in most waste-derived substrates (up to 8.9) has had little or no discernible effect on growth of a wide assortment of deciduous nursery species. By-products such as paper mill sludge and municipal waste compost with soluble salts contents typically ranging from 0.8 to 2.0 dS·m -1 , also provide acceptable rooting media provided salts are leached before use to values ≤0.2 dS·m -1 . The porosity and aeration characteristics of waste-derived substrates tend to be comparable to, or better than, those of bark.