NITROGEN OBTAINED FROM THE AIR 129 



species of plants live in the water, while others live either para- 

 siticallj or saprophytically on plants and animals. The soil in 

 which plants live must have a suitable texture and must fur- 

 nish certain elements which constitute most of the crude food 

 materials of the plants. Since our agricultural crop plants are 

 dependent upon the soil and since the animals are dependent 

 upon the plants, it is evident that all animal life is dependent 

 upon the soil. If we are to understand plant growth we must 

 know something about the soil in which the plants grow. 



The various soils are made up of numerous small particles 

 of disintegrated rocks of various kinds, in which is usually 

 mixed more or less decaying or decayed organic materials. The 

 disintegration of the rocks is, due to the freezing of the water 

 which penetrates the minute crevices, to the grinding action of 

 moving masses of ice, to tlie mechanical and solvent action of 

 water and to many other minor influences. Soils are designated 

 as gravel, sand, sandy loam, loam, clay, etc., dependent upon the 

 coarseness or the fineness of the particles of which they are com- 

 posed. The variation in the texture of the soil results in varia- 

 tion in its power to retain water. 



The most important elements in the soil which either serve 

 as food or influence the growth of plants are phosphorus, potas- 

 sium, calcium, magnesium, sulfur, iron, sodium, chlorine, sili- 

 con, manganese, aluminum and nitrogen. All of these except the 

 last can be found in the ash of the plant and of course must have 

 been taken from the soil by the plant. These elements may be 

 in the soil in proportions too great or too small for the needs of 

 certain plants and satisfactory for others, or may be present, 

 but in such a form as to be unavailable for the plant. Since the 

 difl"erent species of plants are unlike in their requirements we 

 find them more or less in groups dependent to some extent upon 

 the character of the soil in which they grow. 



Nitrogen Obtained from the Air. — Probably the most im- 



