82 SOILS 



the land? And then, again, another question: Is the 

 potential plant food in sufficient store in the soil to meet 

 all plant demands on it? We have discussed, already, 

 the forms of plant food, and we shall not, at this time, 

 consider the many ways open to us for rendering these 

 forms available. 



Let us, however, just as nearly as we are able, deter- 





A CROP THAT GETS NITROGEN FROM THE AIR 



mine the quantities of food demanded of the soil by some 

 continuous system of farming. 



For the purpose of illustration, let us assume that a 

 young man at the age of 21 years secures a farm which 

 he is to manage during his life for 50 years, we will 

 say. He plans a system of plant rotation as follows : 

 (i) corn, (2) wheat, (3) clover, (4) clover and timothy, 

 and (5) timothy: a five-year cycle. During the 50 years 

 each crop will be removed 10 times. Our problem is this : 

 In case each of these crops is removed 10 times and noth- 

 ing returned to the land, how many pounds of nitrogen, 

 phosphorus and potassium will be demanded of the soil, 

 provided the yields are : corn 50 bushels, wheat 25 bush- 

 els, clover 2 tons, clover and timothy i l / 2 tons, and timo- 

 thy i ton during each five-year cycle? 



On the bases of average composition we get the follow- 

 ing: 



