112 PLANT BIOLOGY 



the soil differs in composition from the air above the surface. As a 

 rule, the soil air contains less oxygen and more carbonic acid (CC>2), 

 ammonia, and vapor of water. The increased amount of carbonic 

 acid and ammonia have their origin in the organic matter or humus. 

 A soil is not in the best condition for the production of crops unless 

 there is within its depths a free circulation of air. This is true 

 because oxygen in the soil is as essential for the life of the plant as 

 it is for the animal. . . . 



"When the soil is full of water to within a few inches of the sur- 

 face, there can be no circulation of air among its particles. Ade- 

 quate ventilation can be provided for such a soil only by drainage. 

 Drainage ventilates the soil by lowering the ground water three or 

 four feet, and thus makes it possible for the roots of plants to pene- 

 trate soil more deeply. In time these roots die and decay and 

 afford passageways throughout the soil for the ready movement 

 of the air." 1 



124. Relation of soil to heat. The influence of the tempera- 

 ture of the soil on crop production is a factor of considerable 

 importance. The life processes of a plant are practically suspended 

 below a certain minimum temperature, which is about 40 degrees 

 Fahrenheit for most cultivated crops. Above this temperature 

 all the vital activities, as germination and growth, increase until 

 the optimum is reached. Above this point these life processes de- 

 crease in activity until the point is reached when they cease. The 

 soil is a great factory that has its production vastly increased as 

 the temperature rises. . . . The minimum temperature at which 

 corn germinates and also the minimum for its growth is 48 or 

 49 F. Its optimum is about 93 F. . . . 



"The sources of the heat of the soil are the internal heat of the 

 earth, the sun, and decaying vegetable matter. It is difficult to 

 estimate to just what extent the internal heat of the earth, which 

 itself is very great, affects the temperature near the surface of the 

 earth. However, the amount of heat from this source is insignificant, 

 is a constant factor, and is entirely beyond the control of man. 



1 Bailey's "Cyclopedia of Agriculture," Vol. IX, "Farms," p. 357. 



