542 



STATE BOARD OP AGRICULTURH. 



meter for a gradient of 1 mm. per centimeter would be 0.077 -f- 0.13 or 

 0.59 cc. By applying this form of computation to the foregoing experi- 

 mental data the following figures are obtained. 



TABLE 8.— TRANSPIRATION CONSTANT OF SOILS AT DIFFERENT TEMPERATURES. 



Soil. 



Quartz sand . . . 

 Sandy loam . . . 

 Clyde silt loam 



Clay 



Peat 



0° C. 



.590 

 ..596 

 .090 

 .084 

 .073 



10° C. 



20° C. 



.567 

 .497 

 .064 

 .066 

 .059 



.497 

 .361 

 . 053 

 .055 

 .049 



30° C. 



40° C. 



50° C. 



.452 

 .275 

 .046 

 .038 

 .084 



.403 

 .221 

 .034 

 .032 

 .038 



.351 

 .093 

 .024 

 .030 

 .025 



As would be expected from the experimental results presented in table 

 7, the transpiration constant is greatest in soils with least colloidal 

 matter, and decreases regularly, and in many cases very rapidly, with rise 

 in temperature. 



EFFECT OF TEMPERATURE CHANGES ON THE AERATION OF 



SOILS. 



Unquestionably the aeration of soils is one of the most important prob- 

 lems in soil physics; yet strange to say, it has never been studied ex- 

 perimentally from the temperature point of view. This lack of experi- 

 mental study is mainly due to the prevalent notion that temperature 

 changes occasion very little, if any, influence on the process of soil area- 

 tion, because of the small coefficient of gas expansion which amounts 

 only to 1/273 per degree Centigrade. It is reasoned that since the aver- 

 age diurnal fluctuation of temperature to the depth of 12 inches is not 

 very marked, the volume of gas diurnally exchanged in the upper one 

 cubic foot of soil, is small. Thus King^- has calculated that the soil 

 ventilation due to diurnal changes in soil temperature will range from 

 up to possibly 20 cubic inches per square foot. Ramann-^, in discussing 

 the subject, makes the following statement : "The influence of temperature 

 fluctuations on the volume of the soil air is not appreciable, and occurs 

 only in the upper layers of the soil. Since the coefficient of expansion of 

 gas is only 1/273 per degree Centigrade, and inasmuch as the tempera- 

 ture fluctuations to the depths of from 4 to 8 inches are small, the 

 diurnal exchange of gas is consequently small." 



The problem was subjected to an experimental study and the results 

 obtained show the remarkable fact that temperature has a tremendous 

 influence upon the aeration of soils, and that this influence is due to two 

 other factors besides the mere expansion of gases. 



The general method of investigating the problem consisted in heating 

 a definite volume of dry or moist soil to different temperatures and meas- 

 uring (1) the volume of gas expelled, and (2) the pressure exerted. The 

 volume of gas was measured over water in a gas burette, Avhich is shown 

 on figure 12, and the pressure was measured under mercury on the mano- 

 meter shown also on the same figure. The soil was contained in a brass 

 tube 8 inches long, and IY2 inches in diameter, closed at one end with a 

 solid rubber stopper, and the other, with a single holed rubber stopper, 



22Physlcs of Agriculture, p. 208 (1901). 

 23Bodenkunde. p. 386 (1911). 



