284 SOILS 



until water is applied to them. In many cases the 

 amount of water in the soil measures its producing 

 power more than the amount of plant food in it. 

 Furthermore, the tons of plant food in a soil are as 

 valueless as sand unless the soil has the power to 

 move water rapidly to meet the needs of the crop. 



Under-drainage may make an unproductive, yet 

 rich, soil productive. Plowing under green- 

 manures may effect a similar improvement. These 

 methods are discussed in detail in subsequent 

 chapters. The point to be emphasised is that 

 although most farm soils are very rich in plant food, 

 usually but a small percentage of this can be used 

 by crops, and that tillage, drainage, a rotation of 

 crops and the addition of humus are methods of 

 increasing the usefulness of native plant food. 



SOILS EXHAUSTED OF PLANT FOOD 



We ordinarily think that a soil becomes ex- 

 hausted of plant food chiefly by continuous cropping. 

 We see the yields from a soil that produced sixty 

 bushels of corn per acre fifty years ago, when it was 

 virgin, gradually dwindle to twenty-five bushels, 

 with prospects of going lower still. On the face of 

 it, this is due to the exhaustion of the plant food in 

 the soil by the corn crop. But is it ? The drain of 

 crops upon the soil's store of plant food is really 

 so slight, when compared witn the total amount 

 of plant food in the soil, that it is scarcely worth 

 mentioning as a cause of the increasing unpro- 

 ductiveness of that soil. A crop of cotton of one 

 bale per acre, which is twice the average yield, 

 makes a draft upon the soil of 28 Ibs. of nitrogen, 

 9 Ibs. of phosphoric acid, and 13 Ibs. of potash each 

 year per acre. A crop of 50 bushels corn per acre 



