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Some of the effects of afforestation on poor soils, typical of those at present undergoing large-scale tree planting were examined. The majority of the research work was carried out in an experimental plantation in N.W. England, with supplementary work in the surrounding commercial plantations of Sitka spruce. The quantitative, qualitative and dynamic nature of the soil organic matter was assessed. Quantitative studies revealed that there was no evidence for a build up of organic material in the soil, and an equilibrium was established in the forest floor after 10 years. Marked fluctuations in total carbon content were observed in both the forest floor and soil during a seasonal sampling programme. The qualitative and dynamic nature of the soil organic matter was studied using techniques which identified long term changes, ie 14¬C enrichment studies, gel filtration and NMR spectroscopy. The radiocarbon studies revealed that these afforested soils were characterised with carbon incorporated into the soil at the time of planting. The resistance of some components of the litter to decomposition seemed to be the rate determining factor in the slow incorporation of 'recent* material. Readily decomposable material, such as fine roots, appeared to be respired rather than added to the soil organic matter pool. Gel filtration showed that there were slight differences in the molecular weight distribution of soil extracts from under different tree stands, and that these could be related to the rate of soil organic matter turnover. The use of NMR spectroscopy was not successful in this study. The reasons for this are not clear, but may be related to the use of sodium borate as the soil extractant. There was strong evidence that changes in the qualitative nature of the soil organic matter had occurred, which were related to the rate of soil organic matter turnover under different tree stands, and the length of time under afforestation.