The Accumulation of Soil Organic Matter from Wood Chips

A study was made in cylinders to measure the effect of incorporating 10 tons of pine and oak chips per acre to a Merrimac sandy loam. The influence of such factors as level of nitrate nitrogen, soil reaction, and size of wood chips, were investigated. Using nitrate nitrogen depression as a measure of rate of decomposition, it was found that oak chips decomposed more rapidly than pine chips. This was supported by annual sieving experiments. These measurements showed significantly less oak chips remaining at the end of 12 and 24 months than pine chips. Although 55 to 85% of the wood chips decomposed, organic matter accumulation proved to be meager. No significant differences due to treatments were found. There was some indication that soils at pH 5.0–5.5 accumulated somewhat greater amounts of organic matter than those at pH 6.0–6.5. The relationship between carbon, as determined by the chromic acid titration method, and total carbon, did not change appreciably after 2 years of wood chips decomposition. There was no significant difference in the amount of chromic acid oxidizable carbon due to treatment. The effect of various treatments upon total soil nitrogen was small. No constant relationship could be found, in spite of wide differences in the amount of soluble nitrogen added to the soils. Although the carbon-nitrogen ratios of the soils generally widened, increases were small. Soil reaction did not change appreciably due to additions of wood chips. Ammonia accumulations were negligible. After 2 years of decomposition, 150 pounds of nitrogen gave highly significant increases in the yields of beets and spinach. Wood chip residues did not depress plant growth. Small chips resulted in significantly higher yields than treatments containing large chips. An auxiliary study showed that 100 and 150 pounds of nitrate nitrogen added to beets and spinach, respectively, overcame any depressing effect of additions of 10 tons of small pine chips. With low available nitrogen, wood chips depressed the growth of beets and spinach. This was not true after 12 months of decomposition.