ROTATION OF CROPS. 86 



farmer, and not the plaster, that has so greatly injured 

 his land. The rule becomes clear and imperative, that 

 every one who uses such special manures to make 

 good a special deficiency, should at the same time 

 keep up the general stock by a liberal use of ordinary 

 manure. 



SECTION IV. ON THE PRINCIPLES OF ROTATION IN CROPPING. 



Nearly all of the foregoing statements in this chap- 

 ter, and in the preceding one, have borne more or less 

 distinctly upon the theories or facts connected with 

 the rotation of crops. It may be well to make a few 

 direct applications of the knowledge we have now 

 gained, with this particular subject in view. 



All good farmers know that the most exhausting 

 system that can be devised, is to cultivate the same 

 crop on the same soil, year after year. When a longer 

 or shorter period has elapsed, as the land may have 

 been at the commencement richer or poorer, the yield 

 begins to decrease; an increase may be obtained again 

 by the free use of manures, but the quantity necessary 

 is so large, and requires to be so often renewed, that 

 it is in most situations more profitable to change the 

 crops, or alternate them. 



From such practical observations, have arisen the 

 various systems of rotation that are in vogue in dif- 

 ferent districts. Table III. shows how practical ex- 

 perience has, in this case, hit upon the very course 

 which science would have recommended. It has been 

 shown by that table, and attention has been called to 

 the fact, that there are several distinct classes of crops, 

 when we consider them with regard to the composition 

 of their ash. The classes are those which are found 

 to bear a part in every good rotation, that is, grain 

 crops, root crops, and grass crops, or the same three 

 classes that were distinguished from each other in the 

 early part of this chapter. 

 8 



