84 AGRICULTURE. 



of that taken up by repeated crops, particularly 

 of grain, is in some way returned to the soil. 

 This plant-food (phosphate) also will be further 

 discussed under "Fertilizers." 



(2) POTASSIUM. Pure potassium is a silvery 

 white metal, but it does not exist in nature un- 

 combined with other elements. Potassium com- 

 pounds are important ingredients in the forma- 

 tion of starch in the leaves and the transference 

 of starch to the fruit. Since starch is so impor- 

 tant in the formation of wood, it follows that 

 the salts of potassium are essential to the devel- 

 opment of the firm, woody tissue of the stems. 

 Potassium forms the base of the acids of fruits 

 and over half the ash of fruits. It is particularly 

 necessary to fruit and root crops. It is also 

 found in the juices of plants which are somewhat 

 acid, where it neutralizes a part of such acids 

 as, citric, tartaric, and oxalic by forming the 

 salts of these acids. Potassium forms a large 

 per cent, of the wood of fruit-trees. 



C FERTILITY OF THE SOIL. 



A fertile soil "contains all the material req- 

 uisite for the nutrition of plants in the required 

 quantity and in the proper form." That is, all 

 the materials for the nutrition of plants not de- 

 rived from the air are contained in a fertile 

 soil. 



One must know whether the food elements 



