44 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



plants by cementing the plant in a bell jar, so that the stem and 

 leaves were in the outer air, while the roots were within the 

 vessel and exposed to any gases that might be placed therein. 

 The horse chestnut died in 7 to 8 days when its roots were placed 

 in carbonic acid gas, in from 13 to 14 days in nitrogen, or hydro- 

 gen, while the plant remained healthy to the end of the experi- 

 ment (21 days) when the roots were in contact with air. The 

 experiment shows that the roots of this plant require oxygen, 

 though it lived for some time without oxygen. Other experi- 

 ments show that roots absorb oxygen and give off carbon dioxide. 

 The roots of some plants, which prefer heavy, wet soils, probably 

 do not require oxygen. The difference in the root requirements 

 of plants for oxygen is probably one factor in their adaptation to 

 various types of soil. 



Nitrogen. The nitrogen of the air, which makes up four-fifths 

 of its volume, is in the free state, and enters into combination 

 only with difficulty. So far as animals and the majority of plants 

 are concerned, the nitrogen of the air serves only as a dilutant 

 for the oxygen, which would have too energetic an oxidizing 

 action if in the pure state. 



To be of value to animals or to most cultivated plants, nitrogen 

 must be in combination. This store of combined nitrogen is com- 

 paratively small. Plants and the bodies of animals contain some 

 combined nitrogen ; there is some in the soil, coal contains a small 

 percentage, and there are some deposits of nitrate of soda. 

 Combined nitrogen is lost when organic matter is burned, any 

 nitrogen present being evolved in the free state. Explosives are 

 rich in nitrogen, which is set free when they are used. In certain 

 processes of decay, free nitrogen is evolved. 



The supply of combined nitrogen in the soil is comparatively 

 small, and it is constantly drawn upon by crops. Under our 

 present system of agriculture, the stores of nitrogen in the soil 

 are exploited and depleted. A considerable quantity of nitrogen 

 is washed from the soil by water. Maintaining the fertility of 

 the soil is largely a question of maintaining its store of combined 

 nitrogen. Fortunately, we have now obtained the means for 



