COMPOSITION AKD FERTILITY OF THE SOIL. 17 



cumulates and the climate is cool ; so t-hat if we exclude 

 living roots and root fibres, there is, even in the oldest 

 pasture-land, no great quantity of organic matter, not- 

 withstanding the continued decay of the roots and under- 

 ground stems of plants and the occasional addition of 

 manure. 



The Inorganic or illineral portion of the soil consists 

 of the same substances as the inorganic part of plants, 

 with the addition of alumina. The mineral constituents 

 of soils include the following substances: — 



Silica. Potash. 



Alumina. Soda. '• 



Calcic carbonate. Ferric oxide. 



Phosphoric acid. Magnesia. 



Sulphuric acid. Chlorine. 



These constituents exist in very different proportions 

 in different soils. The first three — sand, clay, lime — 

 represent more than ninety per cent, of the substance of 

 most soils; and, as one or other of them prevails, the soil 

 is characterized as calcareous, clayey, or sandy. The most 

 active constituents of the soil, however, phosphoric acid 

 and the alkalies, occur in very small quantities, as do the 

 other and less important constituents — magnesia, chlo- 

 rine, and sulphuric acid. 



Silica exists in very different proportions in different 

 soils, but chiefly in an insoluble form, and that most 

 largely in the poorest sands, fertile soils alone containing 

 it in a soluble form. Sandy soils contain eighty per 

 cent, and upwards of silica : even stiff clay soils from 

 sixty to seventy per cent., and calcareous or lime soils and 

 marls from twenty to thirty per cent. In sandy soil there 

 is an abundance of silica, but it is not available. In clay 

 it is also abundant, but it is the quantity of it which is 

 soluble that determines its value as contributing to the 

 food or life of the plant. 



It is in the form of soluble silicates that silica does its 



