SOIL EXHAUSTION AND ROTATION IN CROPS. 77 



which we properly regard as settled. I shall endeavor then, 

 as far as the time admits, to put before you some of those con- 

 siderations whicli seem adapted to furnish guiding ideas in 

 respect to my subject. 



By Exhaustion of Soil is properly understood, not a com- 

 plete depi'ivation of producing power, but simply a reduction 

 of this power below a profitable point. This is indeed a some- 

 what indefinite definition, because the point of profit is not 

 easy to decide upon, but it is sufficient for our purpose. 



What does exhaustion consist in ? It consists either in the 

 removal of certain materials from the soil, materials which 

 serve to feed the crop and become a part of it, and which, by 

 continually taking off harvest after harvest, become diminished 

 in quantity, so that after a certain time there is not enough 

 left in the soil to produce a fair crop, or else it means that the 

 materials which may still exist in the soil no longer occur in 

 that condition in which the crop can make use of them. We 

 may have a soil containing potash in large quantities, many 

 hundred pounds, or tons even, in an acre, taken to the depth 

 of two or three feet ; but if this potash exist there exclusively 

 as an ingredient of some mineral which is acted upon so slowly 

 by the natural process of solutitDU that there is no available 

 potash, as we say, nothing which the crop can get hold of, such 

 a soil would be unproductive. Again, we may have a soil 

 which contains but a thousandth part as much potash, but 

 which is fertile from the simple fact that the alkali occurs 

 there in sucli a state as to become available as rapidly as the 

 crop requires it. 



To cure exhaustion, we must either restore the nutritive 

 matters which have been removed from the soil, or we must 

 change the state of those which still exist there so that they 

 may become available. 



Chemical science has established the fact that every crop 

 requires a variety of materials to support it. I have here a 

 number of printed sheets, containing a table of the average 

 quantities of the chief ingredients of our ordinary cultivated 

 crops, of which I would like every gentleman present to have 

 a copy. 



