Il8 SOILS AND FERTILIZERS 



- Nitrogen is demanded by all crops ; it forms the chief 

 building material for the proteids of plants. In the ab- 

 sence of sufficient nitrogen, the rich green color is not 

 developed ; the foliage is of a yellowish tinge. Nitro- 

 gen is one of the constituents of chlorophyll, the green 

 coloring matter of plants; hence when there is a lack of 

 nitrogen only a limited amount of chlorophyll can be 

 produced. Plants with large, well-developed leaves of 

 a rich green color are not suffering for this element. 

 Nitrogenous fertilizers have a tendency to produce a 

 luxurious growth of foliage, deep green in color. 



ATMOSPHERIC NITROGEN AS A SOURCE OF PLANT 



FOOD 



126. Early Views. In addition to carbon, hydrogen, 

 and oxygen, which form the organic compounds of 

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

 present century that plants also contain nitrogen. The 

 sources of carbon, hydrogen, and oxygen for crop pur- 

 poses were much easier to determine and understand 

 than the sources of nitrogen. Priestley, the discoverer 

 of oxygen, believed that the free nitrogen of the air 

 was a factor in supplying plant food. De Saussure ar- 

 rived at just the opposite conclusion. Neither of these 

 assumptions was convincing because methods of chem- 

 ical analysis had not yet been sufficiently perfected to 

 solve the question. 39 



