98 A YEAR IN AGRICULTURE 



the water-holding capacity by preventing a larger amount 

 from running off. The farmer may decrease the soil water 

 by drainage, but at the same time increase the available 

 water for his crop. The best control of soil water is seen 

 in systems of irrigation and dry farming. 



Soil air. A considerable amount of air is found in the 

 interstices of all good, live soil. This air functions in sup- 

 plying to the soil the elements of oxygen, nitrogen, and car- 

 bonic acid gas. It is not definitely known what the whole 

 work of oxygen in the soil is, but we know that the roots of 

 most economic plants must have oxygen about them or, 

 like animals, they will die for want of this element. The 

 bacteria of the soil, necessary in decomposing organic mat- 

 ter and in extracting nitrogen from the air for the legumes, 

 must have an oxygen supply or else their processes do not 

 continue. Carbonic acid gas functions in disintegrating the 

 mineral elements for plant-food. The farmer may control 

 to a certain extent the air of the soil. The incorporation of 

 organic matter, the proper tillage, drainage, etc., open up 

 the soil, making avenues for the movements of air which 

 function as described above. 



Soil temperature. It is a well known fact that the tem- 

 perature of the soil must be congenial to the growth of plants 

 adapted to that soil. The soil must have sufficient heat for 

 the germination of the seed, for the growth of the roots, and 

 for the activities of the soil bacteria. The temperature of 

 the soil is affected by its color, slope, water content, evapora- 

 tion, and organic matter. The way in which these things 

 affect the soil temperature is evident. Dark colored soils are 



