The influence of different soil substrates on the service life of Scots pine sapwood and oak heartwood in ground contact

The durability of wood in ground contact is affected by numerous influences, such as substrate quality, climate and microflora of the soil, which need to be considered for the prediction of service life of wooden components to be used in the ground. In this study the influence of different soil substrates on the service life of Scots pine sapwood (Pinus sylvestris L.) and oak heartwood (Quercus petraea Liebl.) was investigated. Mini-stakes were exposed in six different soils at the test site of the Federal Research Centre for Forestry and Forest Products (BFH) in Hamburg, Germany. The effect of partly embedding the test samples in concrete or polyethylene films was also examined. After 3 years of exposure the decay rates for both wood species differed significantly between the soil substrates. Compost soil and fertilized test field soil induced the highest decay activity, whereas in pure sand the lowest decay rates were observed. Surprisingly, exposure in gravel also led to higher decay ratings than sand. Protective measures by means of concrete embedding and polyethylene films performed well during the first 2 years of exposure, but showed increasing decay rates in the third year. The overall decay rating for all soil types was higher for oak heartwood than for pine sapwood. The meaning of different soils, independent of other site influences, for service life prediction of wood is discussed, and the need for further studies on this topic is highlighted.