264 NITROGEN AND ITS RELATION TO PLANTS 



Commercial fertilizers generally contain nitrogen, phospho- 

 rus, and potash in amounts varying with the requirements of 

 the soil. Wheat requires a large amount of phosphorus and 

 quickly exhausts the ground of that food 

 stuff; a field which has supported a crop 

 of wheat is particularly poor in phosphorus, 

 and a satisfactory fertilizer for that land 

 would necessarily contain a large percent- 

 age of phosphorus. The fertilizer to be 

 used in a soil depends upon the character 

 of the soil and upon the crops previously 

 grown on it. 



The quantity of fertilizer needed by the 

 farmers of the world is enormous, and 

 the problem of securing the necessary sub- 

 stances in quantities sufficient to satisfy 

 the demand bids fair to be serious. But 

 modern chemistry is at work on the prob- 

 lem, and already it is possible to make 

 some nitrogen compounds on a commercial 

 scale. When nitrogen gas is in contact 

 with heated calcium carbide, a reaction takes place which 

 results in the formation of calcium nitride, a compound suit- 

 able for enriching the soil. There are other commercial 

 methods for obtaining nitrogen compounds which are suit- 

 able for absorption by plant roots. 



Phosphorus is obtained from bone ash and from phosphate 

 rock which is widely distributed over the surface of the earth. 

 Bone ash and thousands of tons of phosphate rock are treated 

 with sulphuric acid to form a phosphorus compound which is 

 soluble in soil water and which, when added to soil, will be 

 usable by the plants growing there. 



The other important ingredient of most fertilizers is potash. 



FIG. 163. Water cul- 

 tures of buckwheat : 

 I, with all the food 

 elements ; 2, without 

 potash ; 3, without 

 nitrates. 



