ROTATION OF CROPS 277 



be used as cattle food, because the price received for farm 

 fertility as meat is usually greater than the price received 

 for it as grain. Besides, when cattle are raised, much of 

 the farm's fertility can be restored, as manure. 



Dairying is profitable, both because dairy products bring a high 

 price, and also because they do not remove much fertility. If we sell 

 butter, for example, we are selling chiefly fat, composed of carbon, 

 hydrogen, and oxygen (cf. 224) ; that is to say, we are selling a product 

 made out of water and the carbon dioxide of the air (cf. 310). 



301. Rotation of Crops. Men once tried to restore the 

 fertility of a field by plowing it, and then letting it lie 

 unused, or " fallow," for a year or two. But this is expen- 

 sive, for the field is bringing in no return. Later they 

 learned that a more thorough tillage (cf. 296) would 

 bring better crops. A very important advance was made 

 when farmers decided to rotate, or change, their crops. 

 Thus, instead of growing corn on a given field year after 

 year, the farmer can grow corn, grain, and hay in succes- 

 sion, coming back to corn every third year. 



The rotation of crops depends on the fact that different 

 crops remove different amounts of soil elements. Grains, 

 such as wheat, take up much phosphorus; grass takes up 

 little phosphorus. Corn is exhausting to the soil, for it 

 removes much of all three of the elements nitrogen, 

 phosphorus, and potassium. If the crop is changed at 

 intervals, the field that has lost a great deal of an element 

 has time to recover, and to get enough of that element into 

 a form in which the plant can use it. 



Rotation has other good results. The root systems of plants are 

 different (cf. 316); hence one plant often breaks up the ground for 

 another. A long growing of the same plant in a field frequently 



