PRINCIPLES OF ROTATIONS. 99 



uniformity of composition, and consequently require for 

 their growth the same food materials from the soil. Plants, 

 again, belonging to a different order the Leguminosce, for 

 instance though possessing the same uniformity among 

 themselves, differ more or less widely in their composition, 

 and thus may be able to obtain a supply of food from a 

 soil which has been exhausted by the growth of previous 

 straw crops. There are other considerations, too, of an 

 economic nature, which support the policy of a rotation of 

 crops : the labour of the farm is more equally and bene- 

 ficially distributed, and the productive returns rendered 

 more regular and secure. Experience, therefore, has long 

 ago pointed out to us the advantage of changing our crops 

 on the same ground as frequently as we can; and we see 

 that everywhere, far and wide throughout the country, 

 the land is farmed upon some sort of a systematic rotation, 

 which does, or is intended, to secure the crops against the 

 evils consequent upon the continuous cultivation of the 

 same plants on the same ground. Besides the chemical 

 reasons in favour of a distinct system of rotation in our 

 crops, there are others of a physiological nature, with 

 which we are less perfectly acquainted, but which, no 

 doubt, have an equal bearing upon their vigour and 

 healthy development. Both of these are naturally met 

 by arranging that the crops following each other in the 

 rotation shall differ, not only in the "order" to which 

 they belong, but also in their habits of growth, as widely 

 as possible. The food requirements are then less likely to 

 be interfered with, and the other conditions of growth 

 which we know to be necessary to them are more fully 

 secured. 



In the Norfolk or four-course rotation, clover as a rule 

 follows barley and precedes wheat. In the five-years, six- 

 years, or longer rotations, it is met with following either 

 barley, oats, or wheat. In these rotations it is kept down 



