66 THE STORY OF THE PLANTS. 



he cultivates. These raw materials are called 

 manures ; they are mostly rich^in. nitrate, pi ^ aa^- 

 phosghatesj and as they are usually the only 

 tEings^ directly supplied to plants by human 

 agency the carbonic acid and water being 

 supplied by wind and rain in the ordinary 

 course of nature they help to strengthen the 

 popular misapprehension that plants grow 

 directly out of the soil. Manures consist chiefly 

 of compounds of nitrogen, phosphorus, and pot- 

 ash. These are the things of which the plants 

 take most from the soil ; and when the crops 

 are cut down and carried away, it becomes 

 necessary to restore them. This is generally 

 done by means of farmyard manure, bones, or 

 guano. Most manures are really the remains 

 or droppings of animals ; so that when we lay 

 them on the soil, we are merely returning to it 

 in another form what the animal took from it 

 when he eat the plants up. 



All plants, however, do not equally exhaust 

 the soil of all necessary materials. Some require 

 one sort of food, and others another. That is 

 why farmers have recourse to what is called 

 rotation of crops, so as to follow up one sort of 

 plant in a field by another, whose needs are 

 different. Thus corn is alternated with swedes 

 or turnips. Virgin soil will produce crops for 

 several seasons together without the need for 

 manuring ; but when many crops have been cut 

 from it in succession, the earth gets exhausted 

 of nitrates and phosphates, and then it becomes 

 necessary to manure and to rotate the crops in the 

 ordinary manner. 



