248 AGRICULTURE 



air, is an excellent form, and may be applied at the rate 

 of two or more tons to the acre. (3) Ground or finely 

 crushed limestone direct from the quarries is widely used 

 in regions where it is easily obtainable. From one to two 

 tons to the acre will usually correct the acidity. 



The form of lime to be used will depend chiefly on which 

 is most easily available and cheapest. The amount required 

 is determined by the degree of acidity in the soil. 



6. Crop Rotation and Fertility 



The rotation of crops can not properly be said to in- 

 crease the fertility of the soil. For every crop removes 

 from the soil some quantity of each of the elements required 

 for plant growth. Certain advantages come from rotation, 

 however, which at least save the rapid soil exhaustion re- 

 sulting from growing one crop continuously. Rotation also 

 brings increased yields. In this sense a proper rotation 

 may have the same effect as the application of a fertilizer, 

 though it can never serve as a substitute. 



What is meant by rotation of crops. By rotation of 

 crops is meant a regular order follozved for a period of years, 

 and alternating on different fields. If this order is hit-and- 

 miss, or the result of whim or chance, it can not be called a 

 rotation. Rotations may be planned on a two-year, three- 

 year, four-year, or any other cycle. 



What is accomplished by rotation. Rotation of 

 crops improves the physical condition of the soil. Grasses 

 and legumes have a larger supply of roots than most cereals, 

 and so increase the organic matter. Different crops send 

 their roots to different depths, and so use new portions of 

 the soil. The cultivation of inter-tilled crops clears the soil 

 of weeds, and opens it up to air and moisture. 



Rotation also aids in destroying insects and other ene- 



