HOW PLANTS DRINK. 6 1 



farmer recognises quite equally the importance of 

 water. But he never recognises the true impor- 

 tance of carbonic acid. That is why most people 

 wrongly imagine that plants grow out of the soil, 

 not out of the air. Still, when we burn them, the 

 truth becomes clear. The portion of the plants 

 derived from air and water goes off again into 

 the air in the act of burning : so too does the 

 nitrogen : the remaining portion derived direct 

 from the soil is only the insignificant residue re- 

 turned to the soil as ash when we burn the 

 plant up. 



Nevertheless, the farmer often needs to sup- 

 ply certain raw materials to the soil for the plants 

 he cultivates. These raw materials are called 

 manures ; they are mostly rich in nitrates and 

 phosphates ; and as they are usually the only 

 things directly supplied to plants by human 

 agency — the carbonic acid and water being sup- 

 plied by wind and rain in the ordinary course of 

 nature — they help to strengthen the popular mis- 

 apprehension that plants grow directly out of the 

 soil. Manures consist chiefly of compounds of 

 nitrogen, phosphorus, and potash. These are the 

 things of which the plants take most from the 

 soil ; and when the crops are cut down and car- 

 ried 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 ate the plants up. 



All plants, however, do not equally exhaust 

 the soil of all necessary materials. Some require 



