THE NATURE OF SOIL 27 



etc., and mixed with the remains of plants and 

 animals. The value of decayed vegetation, or 

 humus, in a soil is so great, and the farm practices 

 resting upon this fact are so important, that this 

 matter is treated in a separate chapter (XII) . 



The mineral contents of a soil depend upon the 

 kind of rock from which it has been made. These 

 rocks are of many kinds; the nature of a soil may 

 often be determined by seeing specimens of the 

 rocks it contains, provided the soil is south of the 

 region that was covered by the great soil mixers, the 

 glaciers. There is no mineral in any soil that 

 cannot be found in the rock from which it came; 

 there is no mineral in any plant that is not in 

 the soil from which it sprang. 



Soil is being made from many kinds of rocks, 

 principally quartz, feldspar, mica, apatite, zeo- 

 lites, nornblende; and various combinations of 

 these, as granite, which is made of quartz, feldspar, 

 and mica. Quartz and feldspar form the largest 

 proportion of most soils. The chief constituent of 

 all soils that have been made from rocks is silica 

 (pure sand), which is the principal ingredient of 

 quartz. This is because silica is the hardest kind 

 of rock material and hence it is not dissolved and 

 lost as rapidly. 



The rocks of the earth, and the soils made from 

 them, contain from 65 to 70 so-called "elements/* 

 the simple ingredients; as iron, carbon, oxygen. 

 These elements, however, unite with one another to 

 make innumerable "compounds," or combinations 

 of several elements. This might be illustrated by 

 saying that eggs, salt and milk are the elements 

 or ingredients of a compound omelet. It is the 

 great number and the intricacy of these compounds 

 that make geology and chemistry so complex. 



