COMPOSITION AKD FERTILITY OF THE SOIL. 19 



Potash, an element of felspar, is present in large quan- 

 tity in soils derived from the primitive and igneous rocks. 

 It varies in different soils from the merest trace to one or 

 two per cent. Sandy and peaty soils and marls are in 

 general deficient in this alkali. Soils rich in alumina are, 

 with some exceptions, generally rich in potash. It exists 

 in the soil in combination with silica, forming a substance 

 which is to some extent soluble in water. Soda is a less 

 important constituent in soil than potash, and, unless 

 near the coast, is present in even smaller quantity. 



Of the other ingredients which have been named, ferric 

 oxide, invariably found in soils, is sometimes found in 

 the subsoil in injurious forms. The ferric orper-oxide, 

 better known as the red rust of iron, is its most favorable 

 condition in the soil. In its less perfectly oxidized form, 

 soluble in vegetable acids, it exists in undrained veg- 

 etable soils, and, on drainage, entering the pipes with 

 the w^ater W' hich is being drawn off, it forms, on per-oxi- 

 dation in the presence of the air, an insoluble ferric de- 

 posit, which is sometimes in quantity sufficient to choke 

 the pipes. Magnesia is found in all fertile soils, in pro- 

 portions, however, often amounting to a mere trace. 

 Sulphuric acid and chlorine occur very sparingly in most 

 soils. 



Classifications of Soils. — In talking of soils, a precise 

 nomenclature should be adhered to in preference to local 

 terms. Otherwise men in different districts will often fail 

 to understand what particular kind of soil is alluded to. 

 The most common classification of soils is based on their 

 composition; and the names applied to them take after 

 their ^predominant ingredients. Thus where sand, clay, 

 lime, or organic matter predominates in a soil, it is sandy, 

 clayey, calcareous, or vegetable, as the case may be. A 

 mixture of sand and clay is called loam. If it is needful 

 to be more specific, loams, etc., are designated by the pre- 



