26 LOCALITY AND FOREST VEGETATION. 



estimating the qiialitj^of a soil l)y its geological origin only has 

 almost invariably failed, since tiie same rock may produce 

 soils of varying chemical composition and physical properties. 



;}. ('i»)ip(>Hiti()ii III' Soil. 



Soil consists generally of the following substances : — 



(a.) Mineral matter, taken from the decomposed rock, or 



carried to the area by water and air currents. 

 (b.) Organic matter, being remnants of plants and animals, 

 (c.) AVater, partly liquid, partly in the shape of vapour. 

 (d.) Gases, such as air, carbon dioxide and ammonia. 



A. Mineral Subsf/mces in Ihc Soil. 



. The mineral substances form, in the majority of cases, the 

 greater part of the soil ; they may be arranged into the following 

 four groups : — 



{a.) Earths. 



{},.) Salts. 



(c.) Acids. 



(</.) Metals. 



a. I'Miilis. 



Silica, alumina, lime, and, next to these, magnesia are the 

 earths which occur most frequently in soils. 



Silica is represented chiefly in sand. 



Silica combined with alumina forms daii. 



Clay with sand forms loain. 



Lime appears principally as calcium carbonate in calcareous 

 soils, also as calcium sulphate in gypsum. 



Magnesia is most frequent in dolomite, though smaller 

 quantities are found in most other soils. 



//. Salts. 



The most important salts are : — 

 Potassium carbonate (potash). 

 Sodium car))onate (soda). 



