78 FUNDAMENTALS OF AGRICULTURE. 



2. All plants do not require the same amounts of 

 the different elements of plant food. Potatoes and to- 

 bacco use large quantities of potash, while grain crops 

 draw heavily on the supply of nitrogen and phosphoric 

 acid. As these elements become available slowly and 

 are likely to be washed out of the soil by rains, a 

 series of crops tends to utilize them more completely 

 without loss. 



COTTON GROWN IN ROTATION. 



3. Some plants feed deeply in the soil while others 

 feed near the surface. Oats and rye are shallow 

 rooted plants while the roots of corn and alfalfa pene- 

 trate to considerable depths. By alternating these 

 crops the reserve supply of plant food in the subsoil 

 may be utilized. The openings left by the decay of 

 roots in the subsoil aid also in draining and aerating 

 the soil. 



4. A cultivated crop every three or four years tends 

 to keep down weeds. Corn, potatoes, tobacco and 



