312 



SOIL PHYSICS AND MANAGEMENT 



prevailing westerlies every three or four days, corresponding to the 

 movement of " highs " and " lows." These variations amount to 

 an average of about one-half inch in the height of the mercury. At 

 the Illinois Station the average weekly change for five years has 

 been 0.45 inch. The minimum during this time was 0.20 inch, 

 while the maximum was 1.45 inches. According to Boyle's law, a de- 

 crease in pressure increases the volume of a gas, while an increase in 

 pressure diminishes it and in proportion to the increase or decrease. 

 A difference of 0.5 inch in pressure is equivalent to 1/60 of an 

 atmosphere. If a cubic foot of soil with 50 per cent of pore space 

 has one-half of this occupied by air, it will contain 432 cubic inches 

 of air. An increase in pressure of 1/60 of an atmosphere will 

 force seven cubic inches of air into the soil. With a corresponding 

 decrease in pressure the soil air expands, forcing out the same 

 amount. 



(d) Temperature Changes. When gases are heated they ex- 

 pand, and when cooled they contract. The amount of expansion or 

 contraction is a definite quantity. Air changes in volume 1/491 

 for each change of one degree Fahrenheit, or 1/273 for each degree 

 Centigrade. If a cubic foot of soil contains 432 cubic inches of 

 air, a change of one degree will result in a change of approximately 

 one cubic inch in volume. During the growing season the average 

 daily range for soils to a depth of four inches is about twelve degrees, 

 as shown in the table below. 



Range of Temperature of Plowed and Unplowed Land at Different Depths 

 (Degrees Fahrenheit) Average 1912-1915 2 



* Average of 2 years. 



This would give a change in volume of about 12 cubic inches in 

 a cubic foot of soil, and this amount would be expelled during the 

 day and taken in at night. The aeration brought about by changes 

 in pressure and temperature produces almost a complete change of 

 the air in the surface few inches of soil each week. 



