Resistance of Sawdusts, Peats, and Bark to Decomposition in the Presence of Soil and Nutrient Solution

The ability of sawdusts, peats, and bark to withstand decomposition when used as soil substitutes in greenhouse culture was determined for differing particle sizes, nutrient additions, and admixtures with soil by means of incubation in the laboratory over a 36‐month period. Particle size was not an important factor but the presence of soil greatly increased the rate of decomposition of bark, sawdusts, and sphagnum peats, which shows the necessity of avoiding contamination of these media by soil. The additions of a nutrient solution containing fairly low levels of N, P, and K increased the rate of decomposition greatly in red alder ( Alnus rubra Bong.) sawdust, moderately in Douglas‐fir ( Pseudotsuga menziesii (Mirb.) Franco) sawdust, but by only a small amount in Douglas‐fir bark, and decreased the rate of decomposition in the peats. The combination of soil and nutrient solution was particularly detrimental to alder and Douglas‐fir sawdusts but had less effect on Douglasfir bark. The addition of sand to Douglas‐fir and alder sawdusts did not increase their decomposition rates either with or without nutrient solution and its inclusion in a growing medium may be useful in facilitating the establishment of new transplants.