THE SOIL AND THE PLANT 35 



in so doing they add nitrogen to the soil. You now see 

 the importance of growing such crops as peas and clover 

 on your land, for by their active aid you can constantly 

 add plant food to the soil. Now this much needed nitrogen 

 is the most costly part of the fertilizers that farmers buy 

 every year. If every farmer, then, would grow these 

 tubercle-bearing crops, he would rapidly add to the rich- 

 ness of his land and at the same time he would also escape 

 the necessity of buying so much expensive fertilizer. 



EXPERIMENT 



Take a spade or shovel and dig carefully around the roots of a 

 cowpea and a clover plant; loosen the earth thoroughly and then pull 

 them up, being careful not to break off any of the roots. Now wash 

 the roots, and after they become dry count the nodules, or tubercles, 

 on the roots. Observe the difference in size. How are they arranged ? 

 Do all leguminous plants have equal numbers of nodules? How do 

 these nodules help the farmer ? 



SECTION XI THE ROTATION OF CROPS 



I am sure you know what is meant by rotation, for your 

 teacher has explained to you already how the earth rotates, 

 or turns, on its axis and revolves around the sun. When 

 we speak of crop rotation, we mean not only that the same 

 crop should not be planted on the same land for two suc- 

 cessive years but that crops should follow one another in 

 a regular order. 



Many farmers do not follow a system of farming that 

 involves a change of crops. In some parts of the country 

 the same fields are put to corn or wheat or cotton year 



