The rate at which organic matter added to soils decomposes under temperate conditions seems to have been little studied. It depends, as Russell (1950) remarks, on the soil aeration, the calcium supply, and the temperature, and tends, in an arable soil, to an equilibrium value depending on the crop rotation practised for a given soil under given climatic conditions. And while this equilibrium value is lowest for a rotation containing a high proportion of wide-spaced intertilled crops or cultivated fallows, it increases with crops of small grains or grass. The results at Rothamsted, Broadbalk Field, as judged by the nitrogen content, show that this equilibrium value with no organic additions has remained almost constant from 1865 to 1945 and similarly, at a higher level, where 14 tons of farmyard manure are applied annually, has remained constant from 1914 to 1945.