I4 AGRICULTURE. [CH. 



in composition. There is in fact a range of 20 per 

 cent, for each of these groups. If, for example, a 

 soil contains 10 per cent, of sand and the rest is 

 clay, we should call it a clay soil. If, again, a 

 soil contained 40 per cent, of sand and the remainder 

 is clay, it would be a loam. This is a classification 

 which will be sufficient for our purposes, and may 

 conveniently replace the complicated systems which 

 are supposed to be necessary when treating the 

 subject more in detail. 



21. In addition to this physical analysis of the soil 

 we have to consider the several ingredients of which 

 it is composed, and these are determined by chemi- 

 cal analysis. Chemistry reveals to us the fact that 

 soils contain a large number of different substances, and 

 that the proportion in which they exist is very variable. 

 It is desirable that these substances should be familiar 

 to the mind, and their general influences clearly 

 understood. They are briefly referred to here without 

 going into those fuller details which may be found in 

 any textbook on chemistry. 1 



22. The soil consists of two distinct classes of 

 bodies, viz., those which are mineral or inorganic 

 matters, and those which are organic substances. 

 When a soil is exposed to the action of fire these two 

 groups are separated, the organic matter is burnt off 

 and dispersed in a gaseous form, but the inorganic 

 matter remains. 



23. The Inorganic matter found in soils may 

 be briefly noticed here. Silica or silicic acid first 

 claims our attention. This body forms a very large 

 proportion of sandstone, and it exists abundantly in 

 granite and other crystalline rocks. When combined 

 with alkalies or with an alkaline earth, it forms 

 silicates, a series of bodies of the utmost importance in 



1 Johnston's "Catechism of Agricultural Chemistry," by 

 Voelcker, followed by Roscoe's "Lessons in Chemistry," may 

 be named as presenting valuable elementary instruction, 



