554 STATE BOARD OF AGRICULTURE. 



namely, the effect of lemperatiirc upon llu; atmospheric jtressure, and 

 its inflneDce in turn upon ihe aeratiou of soils. 



The diurnal variation of the atmospheric pressure may be classified 

 thus: the chief maximum usually occurs at about 10 in <lie morning, the 

 chief minimum at 4 in the afternoon, a secondary maximum at 10 in tiie 

 evening, and a secondary minimum at 4 in the morning. The pressure is 

 thus sui)ject to double oscillation in the course of a day. The amplitude 

 or amount of daily variation is always small, never amounting to more 

 than 0.2 of an inch, and in many places being often less than a tenth 

 of this. 



Now, temperature undoubtedly plays a large part in causing this 

 diurnal variation but the exact way in which it operates to produce the 

 result cannot be satisfactorily stated. Theoretically, however, the maxi- 

 mum would be expected at the time of least temperature, that is, at 

 about sunrise, and the minimum at the highest temperature, that is, 

 from 1 to about 3 o'clock in the afternoon. At these respective 

 periods the surface soil also attains its maximum and minimum tem- 

 perature. At these maximum and minimum temperatures there occurs 

 the greatest amount of gas exhaled or inhaled. Hence, atmospheric pres- 

 sure, as influenced by temperature, tends to aid in the ventilation of soil, 

 but in a very small degree, if any, since its diurnal variation is so in- 

 significant. 



SUMMARY. 



In this bulletin there are presented the results of an investigation upon 

 the influence of temperature on certain physical processes in the soil, 

 such as the capillary movement and retention of moisture, gravitational 

 flow of water, thermo-osmose, gas flow, aeration of soils, etc. It was point- 

 ed out at the outset that the influence of temperature upon these physical 

 processes in the soil lacked experimental study and consequently our 

 knowledge consisted almost entirely from a priori deductions from the 

 physical laws of surface tension of liquids, viscosity of liquids and gases, 

 and expansion of gases, as affected by temperature. The general prob- 

 lem was subjected to an experimental study and the results obtained 

 may be summarized as follows: 



The investigation upon the effect of temperature on the movement of 

 water vapor and capillary moisture in soils shows that: 



(1) When one-half of a column of soil of uniform moisture content is 

 maintained at 20° and 40° C. and the other half at 0° C. for eight hours, 

 the percentage of water moved from the warm to the cold soil increases 

 in all the different types of soil with rise in moisture content until a cer- 

 tain water content is reached and then it decreases with further increase 

 in moisture content. The results then plot into a parabola. The per- 

 cent of moisture at which the maximum thermal translocation of 

 water occurs is different for the various classes of soil, but the percentage 

 of the maximum thermal translocation of water is about the same for 

 all classes of soil for any one of the temperature amplitudes. The percent- 

 age of moisture at which this maximum thermal translocation occurs is 

 designated as fhciinal critical moisture content. 



These results are contrary to what might be expected from the laws 



