116 FIRST PRINCIPLES OF AGRICULTURE. 



a means of maintaining fertility was apparent in early 

 times, when the manures were confined to the natural 

 wastes of the farm, and when the growth of livestock 

 and production of dairy products were industries of but 

 little importance. It is less apparent now, when the 

 materials furnishing available plant-food elements or arti- 

 ficial manures are so abundant and cheap. Formerly, 

 the proportion of active soil constituents was almost en- 

 tirely dependent upon the natural forces that were 

 brought to bear upon the dormant constituents to con- 

 vert them into activity; under the conditions that exist 

 now, it is frequently more economical to purchase the 

 active constituents and apply them to the soil; in other 

 words, to supplement natural forces by artificial. 



Bare Fallow. In the older systems of rotation, it 

 was customary to allow the land to lie bare, or "fallow," 

 once in two or three years, in order that the natural 

 agencies, sun, air, and water, might have free access, cause 

 a more rapid decay of the soil particles, and make it more 

 fertile, a practice extending the period of profitable crop- 

 ping without manure. Sometimes the fields were left 

 entirely to themselves, while in others they were fre- 

 quently plowed or stirred in order to hasten the decay. 



Fallow Crops. Following this method came "crop- 

 fallowing," which is still practised; that is, instead of 

 allowing the land to remain idle after a grain crop has 

 been removed, a cultivable crop, as turnips or roots, is 

 planted, or a catch crop, as clover, is seeded, the culti- 

 vation of the one assisting in the decay of vegetable and 

 mineral matter, thus improving for a subsequent grain 

 crop ; while the other, because of its different method of 



