This paper describes studies into the effect of soil factors and water stress on the growth and biomass production of willow (Salix viminalis L.) on a clay landfill cap soil. Individual plants were grown in lysimeters containing Oxford clay and using different soil amendments, bulk densities and watering regimes. Three years from planting, stem biomass production in well-watered plants was least (0.28 kg plant-1) with high bulk density soil (1480 kg m-3) and no nutritional amendment but was increased 10-fold (2.53 kg plant-1) by reducing soil bulk density (1200 kg m-3) and adding thermally dried sewage sludge and fertilisers (N, P and K). This was low, however, compared with production on a sandy loam soil (6.23 kg plant-1) with the same amendments and a similar bulk density. These large treatment differences were also reflected in number of stems plant-1, stem basal area plant-1 and plant leaf area. Plants growing on the Oxford clay had higher root:stem ratios than those on the sandy loam. Water stress reduced stem biomass production by 26 - 37% and caused higher root:stem ratios. Foliar and soil analyses and pot trials indicated that the effects of the soil amendments could be attributed to the addition of N and P which are low in Oxford clay. The research suggests that reasonable biomass production from willow SRC on Oxford clay landfill caps will be dependent on the application of nutritional amendment to the soil at these sites
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