IV EDITOES PREFACE. 



we can conceive that those portions of the soil traversed 

 by the numerous root ramifications will be more or less 

 exhausted of food elements, whilst the immediate neigh- 

 boiQ'ing portions are still rich in them. If, therefore, a 

 succeeding crop is to grow equally well on all parts 

 of a field, there must be a thorough mixing of the 

 exhausted and of the unexhausted portions of soil. This 

 is effected by mechanical means, by manures, or by 

 certain chemical compounds. Hence the necessity of 

 becoming acquainted with the nature and properties of 

 the soil and subsoil. The second chapter is devoted to 

 this subject. 



The soil consists of arable surface soil and subsoil. 

 In the former is accumulated the nutriment of plants 

 chiefly cultivated for the food of man. This accumula- 

 tion is effected by the absorptive power of the arable 

 soil for mineral matters, by which soluble salts are 

 removed from solution, and even chemical decompo- 

 sition of the most stable compounds is brought about, 

 and the bases or acids are retained by the soil in a firm 

 state of combination. It is the presence of food in the 

 soil in this state of j^^iysical combination which is alone 

 available for the nutrition of plants. On the abundant 

 or scanty supply of food in this state depends the fertility 

 or sterility of a soil. In fertile soils food is present 

 also in another form, in which it is not immediately 

 available for the nutrition of plants. It exists as 

 chemical compounds which are not soluble in water, 

 or acids until rendered so by the action of powerful 

 chemical afjents, or to a much smaller extent bv the 



