9 ^ Studies in Forestry [CHAP. v. 



of attack to water, mosses, herbage, and other disintegrating 

 influences. 



Vegetation plays no unimportant part in the formation of 

 earth, not only by helping to fissure and cleave the soil and 

 subsoil, so as to enable water to effect an entrance, but also by 

 reason of the decomposition of the organic debris of foliage, 

 dead wood, &c., and the formation of humus or mould, which 

 takes place under the combined action of oxygen, moisture, 

 and a minimum warmth of 52 Fahr. When the humifica- 

 tion takes place only under partial exposure to the atmosphere, 

 humic and similar acids are formed, and saprophytic fungi 

 aid in the work of decomposition, by attacking the albuminoid 

 substances first of all. These acids (humic, ulmic, geic, &c.) 

 have a strong affinity for ammonia (NH 3 ), which is itself 

 essential to the nourishment of trees, and which cannot be 

 assimilated by the foliage from the free nitrogen of the air, 

 although, as Hellriegel, Frank and others have shown, it can be 

 obtained by the Leguminosae from the air circulating within 

 the soil by means of symbiosis with a fungus (Bacillus radicicola] 

 found in the nodules on the roots (see page 74). 



Humus condenses gases and atmospheric moisture, and 

 possesses a certain amount of warmth, partly owing to reten- 

 tion of atmospheric warmth received from sunshine, and partly 

 to the generation of heat by the chemical process of decomposi- 

 tion. Thus, whilst mechanically making clayey soils less stiff, 

 and sandy soils more binding, it also warms the former, and 

 makes the latter less liable to be affected by the changes in the 

 atmospheric temperature. 



II. The Classification of Soils. 



Any classification of soils according to their geognostic or 

 geological origin would be very misleading, for (i) the same 

 kind of rock does not always yield similar soil, (2) the quali- 

 ties of the soil depend on the extent to which decomposition 



