500 FOREST LlTTEIl. 



is incomplete. Souv humus is formed in swamps nnd feus and 

 has an acid reaction owing to the absence of alkaUs or bases. 

 It also occurs in poor sandy districts, where it passes into the 

 form of dry humus. Sour humus and the consequent acidity 

 of the soil in which the roots of trees grow, are the greatest pos- 

 sible hindrances to the growth of most European woody. species. 

 The beech is most sensitive to the action of sour humus ; 

 oaks, sycamore and other maples, Scotch pine and spruce with- 

 stand it better; alder, birch, poplars, and willows are not 

 affected. It is well known that rhododendrons, azaleas and 

 camellias thrive on sour humus. 



(b) Dry humus. — Dry humus is the product of a decomposi- 

 tion, in which plenty of air and heat are the chief factors, 

 but moisture is limited. Whilst in sour humus all the inter- 

 stices of the soil are tilled with water and mild humus forms a 

 loose moist mass, dry humus is rich in ash and carbon and when 

 quite dry is a mere dust. It is formed wherever there is free 

 access of air and heat, and insufficient moisture ; as on dry 

 moorlands, southerly slopes, blanks in forests, clear-cut felling- 

 areas, over-thinned old woods, especially on poor sandy soils and 

 above hot calcareous rocks. 



This kind of humus [resulting from the decomposition of 

 heather, grasses, and Iceland moss {Cladonia rangiferina)] is 

 not beneficial to vegetation, as the light dusty soil is easily 

 blown away and has besides very little nutritive properties ; 

 being rich in carbon, after it has lost nearly all its moisture 

 and oxygen its further decomposition is difficult, it adds little to 

 the chemical ingredients of soils, and supplies very little carbon 

 dioxide to the subsoil. Being deficient in alkalis it is sour, and 

 is the chief cause of the formation of moor-pan ; heather-soil 

 is its commonest form. 



(c) Mild humus. — Mild humus, known also as forest humus or 

 leaf-soil, results from the chemical change in forest litter to 

 which air has free access, and when sufficient heat and moisture 

 are present to hasten the process. Free organic acids are not 

 then formed, but are combined with alkalis, the decomposition 

 liberating only carbon dioxide and water. The beneficial cflects 

 of humus on the fertility of the soil can then act unimpaired, 

 a mild humus which is very useful in the production of wood 

 resulting from the decomposition of the soil-covering in forests. 



