THE SOIL AND THE PLANT 37 



Now if a farmer plant the same crop in the same field each 

 year, that crop soon uses up all of the available plant food 

 that it likes. Hence the soil can no longer properly nour- 

 ish the crop that has been year by year robbing it. If that 

 crop is to be successfully grown again upon the land, the 

 exhausted element must be restored. 



This can be done in two ways: first, by finding out 

 what element has been exhausted and then by restoring 

 this element either by means of commercial fertilizers or 

 manure; second, by planting on the land crops that feed 

 on different food and that will allow or assist kind Mother 

 Nature "to repair her waste places." An illustration may 

 help you to remember this fact. An element called nitro- 

 gen is one of the commonest plant foods. Nitrogen may 

 almost be called plant bread. The wheat crop uses up 

 a good deal of nitrogen. Suppose a field were planted 

 in wheat year after year. Most of the available nitrogen 

 would be taken out of the soil after a while, and a new 

 wheat crop, if planted on the field, would not get enough 

 of its proper food to yield a paying harvest. This same 

 land, however, that could not grow wheat could produce 

 other crops that do not require so much nitrogen. For 

 example, it could grow cowpeas. Cowpeas, aided by their 

 root tubercles, are able to gather a great part of the 

 nitrogen needed for their growth from the air. Thus a 

 good crop of peas can be obtained even if there is little 

 available nitrogen in the soil. On the other hand, wheat 

 and corn and cotton cannot utilize the free nitrogen of 

 the air, and they suffer if there is an insufficient quantity 

 present in the soil. Hence the necessity of growing 

 legumes to supply the deficiency. 



