2 SOILS AND MANUKES 



general, it consists of a heterogeneous mixture of loose 

 mineral and organic matter, but the bulk of it is not plant 

 food at all. On the contrary, it consists of material which, 

 by reason either of its physical condition or its chemical 

 nature, is incapable of yielding any nourishment to plants 

 growing in it. Natural soils, of course, always contain 

 some plant food mixed with the great mass of inert matter 

 and, for the most part, derived from it by physical, 

 chemical and biological processes. The distinction between 

 these different processes is somewhat arbitrary, but it is 

 a convenient one for purposes of reference and explana- 

 tion, and it has become customary to speak of the 

 physical, chemical and biological properties of the soil. 



It is obvious that the insoluble and chemically inert 

 matters, of which the soil is mainly composed, must exer- 

 cise a preponderating influence upon its physical proper- 

 ties. These substances are, therefore, often called the 

 physical or mechanical constituents. The soluble and more 

 active substances, salts, etc., which are in a condition to 

 be taken up by plants, are rarely present in quantity suffi- 

 cient to appreciably affect the physical properties, and 

 are, by contrast, often referred to as the chemical con- 

 stituents. It was at one time generally supposed that the 

 productiveness of the soil depends mainly, if not entirely, 

 upon the physical properties. In later times the import- 

 ance of the chemical constituents became more manifest, 

 and perhaps undue emphasis was placed upon it by scien- 

 tific writers. Until within comparatively recent years 

 nothing was known of the biological properties of soils, 

 and that aspect of the subject was entirely ignored. With 

 increase of knowledge these things are now seen in more 

 just proportion, and it is recognised that a high degree 

 of productiveness can only be attained when all the con- 

 ditions are complied with. 



A casual inspection of soils reveals marked differences 



