510 "FIELD CROPS 



required, which in itself tends to place the work of the farm 

 on a stable and systematic basis. 



668. Rotations Do Not Conserve Fertility. Many people 

 hold that rotations conserve soil fertility. While crop yields 

 will decrease much more slowly where several crops are 

 grown in a rotation than where any one is grown continu- 

 ously, crop rotation is just as certain to exhaust the supply 

 of available fertility eventually, if no fertilizers are used, as 

 is a single cropping system. The various crop plants all use 



Fig. 151. Good plowing is essential to the production of good crops. 



the same elements of plant food, though some draw more 

 heavily on one and some on another. The three which are 

 most largely used and which are most likely to become 

 depleted are nitrogen, phosphorus, and potassium. The 

 legumes take the nitrogen from the air and store it in the 

 soil in a form available for other plants, so that if a legu- 

 minous crop is grown as often as once in three years there is 



