134 AGRICULTURE FOR COMMON SCHOOLS 



phosphoric acid as wheat, and uses more potash. A rotation 

 then keeps the plant food in the soil in balance. 



Secondly, plants have different root systems, and so draw 

 their food from the soil differently. Oats and wheat have 

 shallow root systems and get most of their food near the sur- 

 face. Corn has a large root system and draws from the soil 

 near the surface as well as rather deep down in the soil. 

 Clover and alfalfa have long tap roots and get their food 

 mostly from the deeper layers of soil. By using plants having 

 such differences in their root systems a larger volume of soil 

 is made to furnish food for plants. 



Thirdly, a proper rotation always has a legume crop in it. 

 As we have already learned, the legumes leave in the soil 

 more nitrogen than they take out, that is, they gather nitro- 

 gen. All other crops, however, use up nitrogen and do not 

 gather any. So it can be seen that, with a proper rotation, 

 the nitrogen supply in the soil can be kept up. 



In the fourth place, a rotation enables the farmer to hold in 

 check to some degree injurious insects, troublesome weeds, 

 and some plant diseases. Few of these pests can be com- 

 pletely controlled by rotation systems, but nearly all can be 

 made less harmful. For example, the corn root louse is not 

 often troublesome where a rotation is used ; the weed known 

 as ragweed is held in check; and a disease of potatoes known 

 as potato scab is rarely injurious where potatoes are raised 

 on different fields each year. 



Besides all the above advantages the rotation method en- 

 ables the farmer to raise live stock and feed his crops at 

 home. This gives larger quantities of manure for the land, 

 and consequently lessens the need for commercial fertilizers. 

 Furthermore, the rotation system gives work throughout 



