118 PLANT FOOD 



lighted is phosphoric acid. As a plant food phosphoric 

 acid is obtained from manure, from phosphate rock, from 

 slag and manure, and from bones and other waste ma- 

 terials of meat-packing houses. Phosphate rock is some- 

 times treated chemically to reduce it to phosphoric acid 

 before it is used on the land ; but sometimes it is merely 

 crushed and then applied directly to the soil. In this 

 second case it is converted into plant food very slowly. 

 Animals separate their food roughly into bones and 

 manure. Bones are rich and manure poor in phosphoric 

 acid. But manure contains potash while bones do not. 

 Hence when we return bones and manure to the soil, we 

 give back to it in a measure what the animals remove 

 from it. 



Phosphoric acid aids in the transfer of protein (see 

 Chapter XXVI) in the plant, and is especially necessary 

 for the formation of seeds. 



77. Pure potassium is a silvery white metal, tinted 

 with a pale blue color. It ignites when thrown on water, 

 burning with a violet flame. Like phosphorus, it is a 

 deadly poison in its pure state, but it is seldom or never 

 found in that state in nature. 



With oxygen it forms a compound called potash, the 

 form in which we know it best. One pound of potassium 

 forms one and one fourth pounds of potash. Most of our 

 potash comes from Germany. The crude rock is there 

 mined under such names as kainit, camallit, sylvinit, 

 hartsalz, and steinsalz. These rocks run about twelve 

 per cent potash. Hardwood ashes contain from two to 

 twelve per cent potash, ordinarily averaging about six 

 per cent. 



The function of this plant food is twofold: it is 

 needed in the transfer of carbohydrates such as sugars, 

 and for the growth of seeds. 



