THE PRODUCTIVITY OF WOODLAND SOIL 507 



constituents are left in the soil in a form easily available as 

 plant-food. Hence, under conditions favourable to complete 

 decomposition, there is no danger of good leaf-mould being 

 present in too large an amount. It is only when conditions 

 are unfavourable to decomposition that such a danger exists, 

 and that the presence of raw humus may be unfavourable to 

 the growth of timber-crops. 



About thirty or forty years ago Ebermayer's investigations 

 showed the favourable influence which well-decomposed leaf- 

 mould exerts as regards soil-moisture and soil-temperature, 

 while more recent researches have been directed to ascertain 

 its influence on the porosity or the cohesiveness of the soil- 

 particles. Raw (non-decomposing) humus has been found to 

 be always injurious in its action, while fine (decomposed) leaf- 

 mould is always beneficial ; and the finest mould is formed 

 by beech-foliage, while a soil-covering of moss not underlain 

 by raw humus is favourable both to porosity and to soil- 

 moisture. But in Beech-woods from which the dead foliage 

 is annually removed the porosity diminishes and the soil 

 becomes hardened and less productive ; and the researches 

 of Prussian experimenters showed a loss in twenty-five years 

 of 25 per cent, of the basal increment of the stems while the 

 dead foliage was being annually removed from Beech-woods 

 seventy to one hundred years old. 



In a light, porous soil good leaf-mould tends both to retain 

 the soil-moisture holding plant-food in solution, and to produce 

 a larger supply of mineral food held in solution ; but when 

 large masses of raw humus are present in the woods, there 

 is always a tendency towards the formation of a subsoil-pan 

 or hard, impermeable layer, which acts unfavourably, as the 

 acid solutions thus formed (especially under heather and on 

 water-logged spots covered with moss) are injurious to the 

 growth of timber-crops by bringing into solution compounds 

 of iron that are both toxic in their effect and also tend to the 

 formation of moorpan. The thicker the layer of raw humus, 

 the greater is the production of these injurious acids (humic, 

 ulmic, geic, etc.), which have themselves a strong affinity for 

 the ammonia that might otherwise become utilised in the 

 nourishment of the tree-crops. 



The injurious result of removing dead foliage from wood- 

 lands is of course greater and more rapidly apparent on poor 



