THE SOIL 9 



in but small quantities. Yet it would seem that some of the 

 latter are very important to the plant. For all practical 

 purposes, those which concern planters and agriculturists are 

 but few: silica (sand) and silicates, sulphates and phosphates 

 of alumina, iron, lime, magnesia, potash and soda in a com- 

 pound form, together with carbonate of lime, the carbon of 

 organic origin. Nitrates, except where they occur in extensive 

 deposits, as in Chili, enter little into the composition of the 

 crust, and in the soil are continuously being used up and re- 

 placed. From these elements trees and other plants derive 

 the supplies of nitrogen, sulphur, phosphorus, potash, iron and 

 other elements which they require in addition to the carbon 

 obtained by the leaves from the carbonic acid gas of the 

 atmosphere. 



The layer of soil from which plants can obtain their sup- 

 plies is very limited, being only a few feet, and sometimes, 

 in extreme cases, even only a few inches in depth. It is com- 

 posed of particles or grains of silica and the other minerals 

 above noted, in all sorts of irregular shapes and sizes. Mixed 

 up with them are decayed vegetable and animal remains. This 

 mass of humus and soil represents a sort of capital fund, the 

 savings, as it were, of past ages, which living trees and plants 

 draw upon for what they require during their lives. When they 

 die and decay their own dead bodies are added to replenish 

 the stock. 



As there can only be a limited amount of the necessary 

 mineral salts in the small particles of the soil, the question may 

 arise in some minds : How is it that during centuries past these 

 stores have not been all rendered and used up? Certainly 

 one does not lose sight of the fact that the fall of the leaves and 

 the decayed bodies of the trees themselves, when they have 

 died, have during those long periods re-enriched and ever 

 added to the humus and salts of the soil. There are also the 

 food supplies liberated by weathering of rocks. But yet this 

 does not entirely explain matters. It is largely due rather, as 

 already stated, to the fact that there is a constant showering 

 down of fresh particles of dust on the surface of the soil. In this 

 way fresh particles containing mineral salts are always being 

 added to the surface-soil. The atmosphere is always full of 

 such dust, the causes of which have been discussed. Were it 



