ATMOSPHERIC NITROGEN 99 



large, well-developed leaves of a rich green color are 

 not suffering for nitrogen. Nitrogenous fertilizers 

 have a tendency to produce a luxurous growth of 

 foliage, deep green in color. 

 ATMOSPHERIC NITROGEN AS A SOURCE OF PLANT FOOD 



115. Early Views. In addition to carbon, hydro- 

 gen, and oxygen, which form the organic compounds 

 of plants, it was known as early as the beginning of 

 the present century that plants also contained nitrogen. 

 The sources of the carbon, hydrogen, and oxygen for 

 crop purposes were much easier to determine and 

 understand than the sources of nitrogen. Priestley, 

 the discoverer of oxygen, believed that the free nitro- 

 gen of the air was a factor in supplying plant food. 

 De Saussure arrived at just the opposite conclusion. 

 Neither of these assumptions were convincing because 

 methods of chemical analysis had not yet been suffi- 

 ciently perfected to solve the question. 39 



116. Boussingault's Experiments. Boussingault 

 was the first to make a careful study of the subject. 

 In a prepared soil, free from nitrogen, and containing 

 all of the other elements necessary for plant growth, 

 he grew clover, wheat, and peas, carefully determining 

 the nitrogen in the seed. The plants were allowed 

 free access to the air, being simply protected from 

 dust, and were watered with distilled water. But 

 little growth was made. At the end of two months 

 the plants were submitted to chemical analysis, and 

 the amount of nitrogen present was determined. 



His first results are given in the following table : 4 



