PERMANENT AGRICULTURE 331 



becomes a matter of good farming to restore, to the land 

 in some way that which is removed from it by the crops 

 grown in it. 



We may, it is true, increase our crop yields by the selec- 

 tion of better seed, by the selection of superior varieties, 

 by improved methods of cultivation, or otherwise ; but a 

 permanent agriculture, upon which the future prosperity 

 of the nation depends, rests finally upon the maintenance 

 of fertility of the soil. 



238. The Result of selling Crops from the Land. — The 



fertility of the soil is very largely, though not altogether, 

 a matter of the presence in it of those elements which 

 plants must have in order to make healthy growth, — par- 

 ticularly nitrogen, phosphorus and potassium. (Section 7.) 



Let us suppose that an acre of ground produces a crop 

 of fifty bushels of ear corn. This amount of corn contains 

 approximately fifty pounds of nitrogen, nine pounds of 

 phosphorus and fifteen pounds of potassium ; and if the 

 crop is sold, these three elements will be lost from the farm 

 in the amounts named. 



The small grains do not remove so much fertihty from 

 the soil as does corn, since the amount of food material 

 which they produce is actually much less. Thus a crop 

 of twenty bushels of wheat per acre takes twenty-four 

 pounds of nitrogen and about five pounds each of phos- 

 phorus and potassium from the land, while a fifty-bushel 

 crop of oats removes from the soil approximately thirty- 

 two pounds of nitrogen, six pounds of phosphorus and 

 eight pounds of potassium. 



Timothy may remove even more of the elements of 

 fertihty from the soil than do the small grains. A crop 

 yielding one and one half tons of hay per acre, takes 

 about thirty-seven pounds of nitrogen, seven pounds of 



