410 SMITH'S INTERMEDIATE CHEMISTRY 



permanently a certain part of the supply. Thus, in the case 

 of nitrogen, an average crop of maize or corn (45 bushels) removes 

 63 pounds per acre, a crop of cabbage (15 tons) removes 100 pounds 

 per acre, clover hay (2 tons) 82 pounds, and wheat (15 bushels) 

 31 pounds. When the supply becomes reduced, the crops be- 

 come poor. Moreover, the necessary elements must be present 

 in soluble form, or they cannot enter into the plant system. 



Felspar KAlSi 3 O 8 is a common constituent of many rocks, such 

 as granite (p. 4). When such rock material, contained in the soil, 

 is decomposed by weathering, through the action of carbonic acid 

 from the atmosphere, the felspar gives clay HAlSi0 4 and soluble 

 compounds of potassium. There are immense quantities of 

 felspar available, but the process of weathering is very slow, and 

 in many agricultural regions the soil is therefore deficient in sol- 

 uble salts of potassium. 



It is just as necessary to feed crops as to feed cattle, and equally 

 foolish to starve either of them. Fertilizers are used to make 

 good the original, or acquired deficiences of the soil in the most 

 important elements, nitrogen, phosphorus, potassium and calcium. 

 It is absolutely necessary, in addition, to keep up a sufficient 

 supply of fresh organic material in the soil, or the use of fertiliz-^ 

 ers may result in more harm than benefit. 



The value of the systematic use of fertilizers is indicated by 

 comparison of the average crop of wheat per acre in different 

 countries. The average of ten successive years is : Denmark 40 

 bushels, Great Britain 33, Germany 29, United States 14. 



/I 

 c^VjL ^ 



Nitrogen^- The nitrogen is supplied as sodium nitrate or 

 guano (p. ^69), calcium nitrate (p. 313), ammonium sulphate 

 (p. 299), calcium cyanamide (p. 392), manure or the offal (" tank- 

 age ") and ground bones from slaughter houses. 



Over every acre of the earth's surface there are 34,000 tons of 

 '/ cP" free atmospheric nitrogen. Plants in general are incapable of 

 drawing upon this immense store for the nitrogen necessary for 



