Willows (Salix) are the focus of investigation as renewable feedstock for bioenergy in the Northeastern region of the US. The potential increase of acreage for willow biofuel production requires new information about how yield is influenced by variances in soil factors. The objectives of this experiment were to compare yields across several different soil series and to identify factors that may influence plant productivity. Four cultivars of shrub willow were planted in 2011 at five sites in Connecticut. Biomass yields were measured at the end of the fourth growing season. The best conditions for willow growth were recorded on Watchaug soil series, where the highest yield values for 'SX67' and 'SX61' (17.3 and 14.4 Mg ha(-1) yr(-1), respectively) are setting benchmarks for the potential of willow yield in Connecticut. These two willows were the most productive cultivars across all sites. 'Allegany' and 'SV1' performed better on sites with specific soil characteristics, clearly demonstrating greater cultivar x site interactions. Important soil characteristics that correlated with willow growth were soil organic matter, pH, as well as phosphorus, iron, magnesium and aluminum availability. All cultivars responded similarly to aluminum toxicity: the yields increased when exchangeable soil aluminum levels were below 200 mg kg(-1). The yields also increased for most cultivars when the phosphorus levels were high. The yields consistently reached a maximum when the phosphorus to aluminum ratio was greater than 0.05. This study showed that soil factors correlated with willow yields and revealed that soil amendments could be exploited for yield gains.