Sawdust, Bark and Other Wood Wastes for Soil Conditioning and Mulching

Adding relatively inert or slowly decomposable wood wastes to soil provides increased moisture retention, greater aeration, and better tilth. Mulches retard erosion, hinder weed emergence, and retain warmth. Residues for soil improvement should be evaluated for particle size and carbon-to-nitrogen ratio and nitrogen availability. When organic matter has a nitrogen content of about 1 percent or less, all the nitrogen is consumed by microorganisms in utilizing the carbon, and an extended nitrogen starvation may result; this condition can be corrected by the addition of nitrogenous fertilizers. Plant residues of mixed composition are acted upon in the soil by various species of microbes in well-defined stages of decomposition. Although the value of organic soil additives is ultimately determined by field tests, laboratory analyses, particularly of carbon and nitrogen transformation, are useful in predicting effects. In such tests, comparative rates of decomposition are usually determined by soil respiration techniques. Data shows the relative decomposability of sawdust and other organic materials, such as alfalfa and pea vines, compared to dextrose. Greenhouse pot experiments have been used to study the influence of incorporated sawdust on the growth of sunflowers and barley on a silty clay loam soil. Treating of wood wastes to increase nitrogen content is justified when intense use makes the process profitable. Tests on ammoniated Douglas-fir bark indicate that it can be used on the soil without inducing nitrogen starvation.

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