CHAPTER V. 



FERTILIZERS. 



Fertile Soil The fertility of soil depends upon its 

 ability to supply plants with all the elements of food 

 which they require. No one of the elementary sub- 

 stances which have been enumerated as always found 

 in the composition of a plant can be dispensed with. 



As an animal cannot live or thrive without a proper 

 supply of the ordinary elements of food, so a plant re- 

 quires a regular supply of these various elements from 

 the soil. A plant poorly supplied with potash or nitro- 

 gen, for instance, would produce only a sickly growth, 

 and if entirely deprived of these, or of any other essen- 

 tial element, would die. 



Fertile soil, therefore, must contain not only large 

 quantities of plant food, but sufficient quantities of every 

 kind of food which plants obtain from the soil to supply 

 the wants of the crop. 



So, too, soil must not only contain these elements, but 

 they must be in a form in which plants can make use of 

 them. An acre of soil may contain many tons of nitro- 

 gen or phosphoric acid, and yet may be totally unfit 

 to produce a crop, because these cannot be converted 

 into suitable forms fast enough to supply the amount of 

 food required. 



The following list gives the percentage of different sub- 

 stances which may exist in ordinary dry, fertile soils. 



(77) 



