Finnish Forest Research Institute  Metla

Press release 30.11.2006

Combined effect of wood ash and nitrogen application on upland forest soils

On upland forest soils, the nutrient that most commonly limits growth is nitrogen. The positive effect of nitrogen fertilization lasts for less than ten years. In field experiments carried out on a dry upland Scots pine stand at Muhos (64o16’N, 26o04’E) , fertilization was tested both with nitrogen alone, and with nitrogen and wood ash combined. The effect on growth obtained with the latter treatment continued even after the immediate effect of nitrogen had ended (see, the Figures). At the moment, however, it is unclear whether the increase of growth after 20 years was due to the combined effect of nitrogen and wood ash or the soil-improving effect caused by ash in the course of time.

The growth-improving effect of wood ash has been known since the times of slash-burning farming methods. Large amounts of wood ash are accumulated in wood-burning energy plants. Wood ash is interesting because it contains several nutrients and reduces soil acidity. Spreading wood ash on peatlands and upland forest soils would help to substitute for the nutrient loss due to harvesting and leaching, and prevent soil acidification. It would comply with the principles of ecologically sustainable use of forests, with wood ash being recycled to reduce waste problems.

Wood ash fertilization clearly improves the growth of trees on peatlands that are short of, especially, potassium. Upland forest soils, however, mainly lack nitrogen, which is not present in wood ash. On upland forest soils, wood ash has not increased the availability of nitrogen, either. Hence, wood ash has not increased growth on nitrogen-poor upland forest soils. To induce growth increment in upland forests, fertilization seems to need not only wood ash but also nitrogen.

The study conducted by the Finnish Forest Research Institute (Metla) investigated how combined nitrogen and wood ash application affects the chemical properties of the humus layer and mineral soil, and the growth response on a dry forest site (Empetrum nigrum-Calluna vulgaris type). At the establishment phase, the stand age was 60 years and stem volume was 51 m3 per hectare. The treatments were unfertilized control, nitrogen fertilization with urea and nitrogen with three different doses (1, 2.5 and 5 t ha -1) of wood ash. The nitrogen dose was the same (185 kg ha -1) in all the fertilizer treatments. Urea was applied in autumn 1978 and wood ash in spring 1979.

Fertilization with wood ash reduced acidity of the humus layer. Even after 23 years, with wood ash treatments of 2.5 and 5 tons the soil pH values were respectively 0.3 and 0.9 units higher than on the control. Below the humus layer, in mineral soil the effect of wood ash on soil acidity was less significant. After 23 years, acidity of mineral soil treated with the largest wood ash dose had reduced by 0.5 units. Nitrogen fertilization did not affect soil acidity. The effect of the highest wood ash dose was detectable in the amounts of calcium and magnesium in the humus layer and mineral soil even after 23 years.

The stand was thinned before the treatments. This can be seen as improved volume growth on the second five-year period, when trees had seized the newly available space for their growth. Fertilization with nitrogen typically increased the growth clearly for less than ten years. Combined application of wood ash (2.5 t ha -1 in the figures) and nitrogen increased growth for at least ten more years. See, the Figures.

Publications: Saarsalmi, A., Kukkola, M., Moilanen, M. & Arola, M. Long-term effects of ash and N fertilization on stand growth, tree nutrient status and soil chemistry in a Scots pine stand. Forest Ecology and Management 235 (1-3): 116-128.

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