SummaryIncubation of soil with14C‐rye straw for 448 days resulted in the evolution of about 50 per cent of the carbon of the substrate as CO2The two main sugars of the straw, glucose and xylose, were degraded to approximately the same extent (70 per cent). The same results were obtained whether the soil was derived from granitic or basic igneous parent material.There was very little transformation of the substrate to galactose, mannose, arabinose, rhamnose, or fucose, and a much slower rate of degradation than with soil incubated with14C‐glucose over a similar period.Hydrolysis of the soil samples by a preliminary treatment with 5 N H2SO4, before treatment with 24 N H2SO4, followed by heating with N H2SO4did not release significantly greater amounts of sugar than treatment with 24 N H2SO4and N H2SO4alone. Separate analysis of the hydrolysates showed that 90 per cent of each of galactose, mannose, arabinose, xylose, rhamnose, or fucose had been extracted by 5 N H2SO4, but only 50 per cent of the glucose.Fractionation of the straw‐soil mixture after 224 days incubation showed that the specific activity of the glucose was higher in the humin fraction than in the fulvic acid, as would be expected if the remaining14C were still in the form of unchanged plant material.This evidence that plant polysaccharide persists in soil could explain the presence of much of the xylose in the soil organic m