Effect of Douglas‐Fir Sawdust Mulches and Incorporations on Soil Microbial Activities and Plant Growth

Abstract

AbstractSawdust and other wood wastes applied to soil may retard plant growth by inducing nitrogen deficiency but these effects with Douglas‐fir are less pronounced than with many other residues of wide C:N ratio. While Douglas‐fir wood is extremely low in nitrogen it is low also in available carbon. The predominating lignin‐cellulose complex is resistant and only slowly metabolized, enabling a minimum amount of nitrogen, repartitioned among successive crops of microbes, to suffice for long‐continued decomposition.Sawdust showed no direct toxicity and had little effect on soil pH. Resins and other extractives were readily decomposed. Apparent decomposition percentages based on 50 days CO2 production without added available nitrogen in soil respiration experiments were: for dextrose 60, wheat straw 48, red alder 40, ponderosa pine 33, western red cedar 33, Douglas‐fir 30, pitch 30, western hemlock 27, bark 26, and lignin 6%, respectively. Although added nitrogen fertilizer hastened decomposition of organic matter having wide C:N ratio, it commonly depressed cumulative CO2 evolution in soil respiration.Greenhouse and field studies indicate that to avoid induced nitrogen deficiency 5 to 10 pounds of nitrogen per ton of sawdust are generally sufficient during the first year; nitrogen additions the following 2 or 3 years may be successively halved. Douglas‐fir sawdust without added nitrogen often causes no retardation and in some cases significant increases of certain crops such as sunflowers and strawberries. Corn appeared much more sensitive to nitrogen deficiency. All wood wastes and cereal straws at 100 tons per acre have been found to depress plant growth unless nitrogen is added.