18 COMPOSITION OF THE SOIL. 



1. Like the plant, it consists of an organic and a mineral 

 part, of which the former burns away when the soil is heated to 

 redness in the open air. They differ in this that while in the 

 plant the combustible part forms by far the largest proportion 

 of the whole, the contrary is the case in the soil. The plant 

 contains from 80 to 99 per cent of organic matter, the soil from 

 3 to 10 per cent only. In peaty soils, alone, it sometimes 

 amounts to 60 or 80 per cent. 



The organic part of the soil is derived, for the most part, from 

 the remains of vegetables and animals which have been naturally 

 or artificially buried in it. It consists, like that of the soil, of 

 carbon, hydrogen, oxygen, and nitrogen the proportion of the 

 last of these, however, being usually smaller than is contained 

 in the organic part of living vegetables. 



The mineral part of the soil contains all those mineral or 

 inorganic substances which are found in plants, besides a 

 variable proportion of many others, which are not necessary to 

 vegetable growth. Here again, however, there is a great 

 difference between the plant and the soil. Those mineral sub- 

 stances which are most abundant in the soil are usually least so 

 in the plant ; while of those which are essential to a luxuriant 

 vegetation sometimes mere traces only can be detected in the 

 soil. Yet there is a purpose in this. Were they too abundant 

 in the soil, the plant would be liable to absorb them in too large 

 proportion, and thus to become unhealthy. This scarcity of 

 soluble mineral matter, however, is one of the reasons why 

 manuring with mineral substances has so often been found to 

 produce good effects. 



Silica and alumina are most abundant in the soil. The for- 

 mer, when in the state of sand, imparts openness, and what is 

 called lightness, to the soil ; the latter makes it stiff, tenacious, 

 and heavy qualities which the practical man knows to be indis- 

 pensable to the successful culture of some of his most valuable 

 crops. 



Potash, soda, lime, magnesia, and the sulphuric and phos- 

 phoric acids, are usually present in the soil to the extent of a 

 small percentage only. The oxide of iron is not unfrequently 

 found in too large quantity. It has a tendency to collect in the 

 under soil, and thus to prove injurious to the roots of plants. 



