Logs to ashes

I recently attended a bonfire party. There were various kinds of wood being put on the fire, including treated timber, such as old furniture and fences, and recently cut stumps and branches. Nearly all of it burned away, but at the end there was still some ash. Which bits of the wood don’t completely burn, and why is there a residue left? ■  Like many plants, trees make sugars, cellulose and other organic molecules using carbon from the carbon dioxide in the air. They join it with hydrogen obtained by splitting water from their roots. The oxygen in the water is discarded into the atmosphere as part of this system of photosynthesis. All the other elements they need, like nitrogen, phosphorus and metals such as potassium, manganese, iron and zinc are obtained with the water from the soil. When wood is burned, oxygen rejoins with the carbon and hydrogen from the organic compounds, releasing stored energy. Oxygen also joins with  the trace elements, forming  metal oxides and phosphates.  It is these compounds that make up the solid ash, which is an excellent fertiliser, giving back nearly all the minerals originally taken from the soil. Unfortunately, the nitrogen returns to the atmosphere. This explains why slashing and burning trees and other plants to create fields produces good yields for only a year or two before the land becomes nitrogen deficient. Keith Ross Villembits, France ■  There are 17 chemical elements considered to be essential for  the growth of most plants.  These are carbon, hydrogen, oxygen and nitrogen in large proportions; medium proportions of phosphorus, potassium, calcium, sulphur and magnesium; and tiny amounts of boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. These elements are the constituents of the wood that don’t burn – the wood ash. Different plant tissues contain these elements in proportion from 0.2 to 4 per cent, by dry weight. Therefore, the amount  of ash left will depend on the  tree species and which part of the tree has been burned – bark, trunk, branch, root or leaves. In addition, burn temperature can significantly affect the quantity and composition of the pile of residue, which is frequently less than the figures above due to flying ash. The result is that most wood  ash contains a high percentage  of potassium and is recycled by gardeners who call it “potash”. Peter Gosling Farnham, Surrey, UK