MAINTAINING SOIL FERTILITY 301 



likely to exhaust it sooner of the available plant food 

 that corn needs than if clover, wheat and potatoes 

 are grown in a rotation with corn. The producing 

 power of a soil is measured by the amount of the 

 essential plant food it contains which is least 

 abundant; if it contains 20,000 Ibs. of potash and 

 only 2,000 Ibs. of phosphoric acid, it can produce no 

 larger crops than the supply of available phosphoric 

 acid is sufficient to nourish. A rotation of crops, 

 if it is well planned, does not subject the soil to a 

 continuous drain of plant foods in the same pro- 

 portions; it changes the proportions and so makes 

 the plant food in the soil go farther. 



The Different Rooting Habits of Crops. Another 

 reason why a rotation of crops is easier on a soil 

 than single-crop farming results from the different 

 rooting habits of plants. Timothy or blue grass, 

 for example, are shallow-rooting; they draw most 

 of their nourishment from the upper six inches of 

 soil. Clover and alfalfa are deep-rooting; their 

 long tap roots penetrate many feet deep in all 

 ordinary soils, gathering a large amount of food 

 below the depth to which the roots of timothy and 

 blue grass penetrate. The roots of corn forage 

 deeper than the roots of oats. Mangels, sugar- 

 beets and parsnips root deeper than round turnips 

 and table beets, and so on with other farm crops. 



The relation of this fact to soil fertility is the 

 advantage to the soil of having crops grown upon it 

 that root at different depths. A soil may be almost 

 exhausted of available plant food for shallow-rooting 

 crops yet contain much for deep-rooting crops. 

 The fertility of the soil is thus conserved by ro- 

 tating crops that not only differ in their demands 

 upon the soil but also in the relative area of soil 

 that they place under tribute. 



