Tom Corcoran, of Wanaka, asks :-

What is in wood ash?

John Walker, a scientist at the University of Canterbury's School of Forestry, responded.

The ash content of trees seldom exceeds 0.5 per cent of the dry weight of wood. This is very low compared to coal. However, mineral contents as high as 5 per cent have been reported in some trees, mostly from the tropics.

The major mineral elements are the alkaline earths (calcium, magnesium and potassium) plus sodium combined in the form of carbonates, phosphates, silicates, sulphates and as salts of organic acids. On burning these can be reduced to oxides. Like us, trees also require smaller amounts of essential elements to thrive (boron, manganese, iron, copper, zinc). While some people take calcium and selenium tablets to ensure good health, some foresters swear that trees on some NZ soils need boron - and many plantations are top-dressed with boron to improve both growth and wood quality.

Some species, especially in the tropics, have a high silicon content (greater than 2 per cent). This results in the rapid dulling of woodworking tools and this is partly responsible for the lack of interest in many tropical tree species and accounts for the appalling logging waste as fewer than one tree in 20-100 is considered valuable. The slash and burn of shifting cultivators has a perverse logic if most trees have no commercial value - the ash at least provides a temporary green flash until the rains wash the topsoil away. Wood ash has valuable properties as a fertilizer, due to its high content of potash.

Finally, a note of caution: wood with a greenish tinge has probably been treated with copper-chrome-arsenate and should not be burnt as that will release these elements. In an open fire some of the arsenic volatilises so you will be breathing small amounts of toxic gas as you lounge about watching TV, while the heavy metal elements will eat into and corrode the grate itself.