408 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



The only material of importance to be added to the list of constituents 

 already referred to is organic matter, which is present in all soils as the 

 result of the growth and death of plants. It is through the accumulation 

 of vegetable matter in soils that they become rich in nitrogen in a state of 

 combination available as plant food. But it must be remembered that the 

 air is the ultimate source from which soils have derived their organic mat- 

 ter and nitrogen, and the process of slow combustion known as decay, 

 steadily returns these materials back again to the air, or converts them 

 into water and nitric acid. 



A broad distinction is therefore drawn between the fixed or " inorganic " 

 constituents of soils, and the organic substances which are in a perpetual 

 state of change. Leaving the further consideration of humus, we must 

 direct our attention to the mineral constituents of soils, which are also the 

 constituents of the ash of plants. 



The following substances occur in all soils, and in the ashes of all cul- 

 tivated plants, and are also found to exist in the crystalline rocks. 



VOLCANIC ROCK, SOIL. ASH. 



CONSTITUENTS. CONSTITUENTS. CONSTITUENTS. 



Potash. Potash. Potash. 



Soda. Soda. Soda. 



Magnesia. Magnesia. Magnesia. 



Lime. Lime. Lime. 



Phosphorus. Phosphorus pendoxide. Phosphorus pentoxide. 



Sulphur. Sulphur teroxide. Sulphur teroxide. 



Silica. Silica. Silica. 



Chlorine. Chlorine. 



Ferrous oxide. Ferric oxide. Iron. 



Alumina. Alumina. 



Fluorine and manganese are also found to occur in many soils, in very 

 small quantities. 



The fertility of a soil must depend, among other things, upon the pre- 

 sence of every constituent required by plants, and so completely is this the 

 case that the absence of even, what might appear, the least important sub- 

 stance, renders a soil unable to ripen crops. 



In the above list those constituents which occur in least quantities in 

 the soil are generally those which the ash contains in largest proportion, 

 and are therefore most required by plants, and it is for this reason that 

 potash and phosphorus pentoxide are highly valued as applications for 

 land. 



The constituents of soils occur in various conditions. It is not enough 

 to know that a soil contains all the requisite materials for supplying ash 



