CH. ij What is Plant Food ^ 11 



What is plant food ? 



In a general way the grower knows that he feeds his 

 plants when he gives them stable manure, Uquid manure, 

 soot, bone meal and other substances. The Ust of plant 

 foods is very large, indeed probably larger than that of 

 animal foods. When, however, these foods are examined 

 by the chemist they are found to owe their value to 

 the presence of five substances : nitrogen, phosphorus, 

 potassium, calcium, and magnesium. These there- 

 fore represent the essential constituents of the foods 

 supphed. Closer investigation has shown that in ad- 

 dition sulphur, iron, and probably small quantities of 

 other substances are needed. These eight or nine 

 elements are commonly spoken of as the nutritive 

 elements. They can only be utilised when they are 

 combined in some way, and as a rule it is in the form 

 of soluble salts that they are actually taken up by the 

 plant. Thus the nitrogen is commonly taken in the form 

 of nitrates or ammonium salts ; phosphorus in the form 

 of phosphates; potassium, calcium, and magnesium in 

 the salts of these metals. All these substances occur in 

 the soil, and they are often spoken of as plant food. 



It must be admitted that the term is not entirely 

 above criticism, because we do not know that all the 

 substances which enable a plant to grow bigger are really 

 foods in the ordinary sense of the term. Indeed there is 

 physiological evidence to show that they are simply the 

 raw materials out of which the food is made by the 

 plant for its own use. Further, by far the greater part 

 of the material of the plant is derived from water, carbon 

 dioxide, and oxygen, substances which come from the 



