xii INTRODUCTION. 



circumstances and conditions. Apart altogether from 

 the influence exerted by climate, latitude, altitude, 

 and exposure, it may be said to be dependent 011 

 properties of a physical, chemical, and biological 

 nature. 



The first class of properties consist of the absorptive 

 and retentive powers of the soil for water, gases, and 

 heat. These properties depend on the proportion 

 in which the so-called proximate constituents of a 

 soil are present such as gravel, sand, clay, humus, 

 and lime as well as on the size of the soil-particles, 

 and on their colour. The chemical composition of the 

 soil furnishes, however, the most important source of 

 fertility. As the plant has to derive a portion of its 

 food from the soil, the possession by the latter of the 

 ingredients constituting this food is a fundamental 

 condition of plant growth. A very small portion of 

 the soil is directly concerned in promoting growth. 

 Some of the necessary ingredients are apt to be lack- 

 ing in sufficient amount, and it is in making good this 

 want that the chief function of manures consists. The 

 substances in which most soils are generally found to 

 be deficient are nitrogen, phosphoric acid, and potash. 

 Manures, therefore, are chiefly applied to make good 

 this deficiency. While, however, this is so, manures, it 

 must not be lost sight of, perform other and important 



