24 MANUFACTURE OF FERTILIZING MATERIALS 



minerals containing aluminum, true clay is formed, 

 which is of the greatest importance in the con- 

 stitution of the soil. The compounds of aluminum 

 are not important as plant food except when they 

 contain potash but they form valuable constituents 

 of the soils, furnishing a large part of its bulk, and 

 modifying in the most profound degree its physical 

 properties. It is the custom of some to use the 

 word clay to designate the fine particles of soil 

 which have, in general, the same relations to 

 moisture and tilth as the particles of weathered 

 feldspar, etc. In a strict chemical sense, however, 

 the term clay is applied only to the silicates of 

 alumina and the silica found therewith formed. 

 The fertility of a soil is indirectly dependent on 

 the quantity of clay which it contains, its relations 

 to moisture and amenability to culture being 

 largely conditioned upon its clay content. The 

 determination of the percentage of clay in soils 

 is an operation of the highest utility in forming an 

 opinion of the value of a soil based on physical 

 data alone. 



Calcium is one of the commonest and most 

 important elements of the earth's crust, of which 

 it has been estimated to compose about one- 

 sixteenth. It does not occur free in nature, but 

 its most common condition is in combination with 

 carbon dioxide, forming the mineral calcite, 

 marble, and the very abundant limestone rocks. 

 In this form it is slightly soluble in water, 

 especially when containing carbon dioxide, and 



