104 SOILS AND FERTILIZERS 



of chlorophyl, the green coloring-matter of plants, 

 hence with a lack of nitrogen only a limited amount 

 of chlorophyl can be produced. Plants with large, 

 well-developed leaves of a rich, green color are not 

 suffering for nitrogen. Nitrogenous fertilizers have 

 a tendency to produce a luxurious growth of foliage 

 deep green in color. » 



ATMOSPHERIC NITROGEN AS A SOURCE OF PLANT FOOD 



no. Early Views. — In addition to the carbon, hy- 

 drogen, and oxygen, which form the organic com- 

 pounds of plants, nitrogen also, at the beginning of 

 the present century, was known to be present. The 

 sources of carbon, hydrogen, and oxygen, for crop 

 purposes, were much easier to determine and under- 

 stand than the sources of nitrogen. Priestley, the dis- 

 coverer of oxygen, believed that the free nitrogen of 

 the air was a factor in supplying plant food. De Saus- 

 sure arrived at just the opposite conclusion. The facts 

 which led to these beliefs were not convincing because 

 the methods of chemical analysis had not yet been 

 sufficiently perfected to solve the question. 38 



in. Boussingault's First Experiments. — Bonssin- 

 gault was the first to make a careful study of the sub- 

 ject. In a prepared soil, free from nitrogen, and con- 

 taining 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 pro- 



