Durability of wood exposed in ground – Comparative field trials with different soil substrates

• The durability of five different wood species was tested in different soil substrates.
• Graveyard tests were performed with different field soil substrates, sand, gravel and concrete.
• Decay rates differed significantly in dependence of the substrate and the specimen dimension.
• The effect of adding turf, fertilizer and bark mulch to the field soil was negligibly small.
• In general decay proceeded slowest in concrete followed by the non-field soil containing substrates sand and gravel.

The durability of wood in ground contact is influenced by various factors such as substrate quality, climate and micro flora of the soil, which should consequently be considered for service life prediction of wooden components used in ground. In particular the impact of horticultural soil management and melioration measures demand consideration. Therefore this study aimed on comparing the impact of different soil substrates on the performance, service life, and finally durability of different European grown wood species.Specimens of Scots pine sapwood (Pinus sylvestris L.), Norway spruce (Picea abies Karst.), Douglas fir (Pseudotsuga menziesii Franco), European beech (Fagus sylvatica L.), and English oak (Quercus robur L.) were exposed to six different soil substrates: field soil, fertilized field soil, a turf – field soil mix, bark mulch covered field soil, silica sand, and gravel. Furthermore specimens were partly embedded in concrete. Mini stake specimens (8 × 20 × 200 mm³) and standard stake specimens (25 × 50 × 500 mm³) were exposed in parallel at the field test site in Hannover-Herrenhausen, Germany, to allow also for identifying effects of the specimen size and volume.After 4.5 years of exposure the decay rates differed significantly between field soil containing substrates and those containing no natural soil substance. In general decay proceeded slowest in concrete followed by sand and gravel. The effect of adding turf, fertilizer and bark mulch to the field soil was negligibly small. Concrete embedding as protective measure performed well during the first 2–3 years, but showed increasing decay rates afterwards. In comparison, the decay process was often similar between substrates, once decay has started. In all soil substrates decay was dominated by soft rot followed by white rot.The average lifetime of the mini stake specimens was remarkably shorter compared to the standard specimens indicating a potential to shorten test durations. However, the use of mini stakes involved practical problems and led to some extent to an underestimation of wood durability. The meaning of different soils and specimen types, independent of other site influences, for service life prediction and durability assessment of wood is discussed and the need for further studies on this topic is highlighted.