216 EXPERIMENT STATION" EECORD, 



ranging from very low to very high, are reported, the purpose being to study the 

 movement of moisture (1) from a warm to a cold column of soil of uniform 

 moisture content, (2) from a moist and warm column to a dry and cold column 

 of soil with and without an air space between the two columns, and (3) from a 

 moist and cold column to a dry and warm column of soil. Only two temperature 

 amplitudes were employed, to 20° C. and to 40°. 



It was found that when one-half of a column of soil of uniform moisture 

 content was maintained at 20 and 40° and the other half at 0° for eight hours 

 the percentage of water moved from the warm to the cold soil increased in all 

 the different types of soil with a rise in moisture content until a certain water 

 content was reached, and then decreased with further increase in moisture 

 content. The percentage of moisture at which the maximum thermal transloca- 

 tion of water occurred was different for the different classes of soil, but the per- 

 centage of the maximum thermal translocation of water was about the same for 

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

 of moisture at which this maximum thermal translocation occurretl is designated 

 as the " thermal critical moisture content." 



These results led to the conclusion that the capillary movement of water in 

 moist soils is not controlled entirely by the curvature of the capillary films, as 

 is generally believed, but also by the unsatisfied attractive forces of the soil for 

 water, 



"When a moist column of soil was kept at 20 and 40° and a dry column of soil at 

 0° for eight hours and the two columns were separated by an air space, the per- 

 centage of moisture distilled over from the moist and warm colunm to the dry 

 and cold column of soil was very insignificant for both amplitudes of tempera- 

 ture and was about the same for all moisture contents. 



These results led to the conclusiou (1) that the amount of water lost from the 

 soil by water vapor is very small, (2) that there is no rising of vapor during 

 the night from the warmer soil below to the cold soil above, and (3) that the 

 water of the dew is not derived from the soil vapor, as is commonly believed. 



When a moist column of soil was in contact with a dry column of soil and the 

 former was kept at 20 and 40° and the latter at 0° for eight hours the amount of 

 moisture moved from the moist and warm soil to the dry and cold soil increased 

 with temperature and Mith moisture content. But wlien the moist column of 

 soil was maintained at 0° and the dry column of soil at 20 and 40° for the same 

 number of hours there was very little, if any, movement of water from the former 

 to the latter. 



These results led to the conclusion that temperature has a very marked influ- 

 ence on the conservation of moisture by mulches. 



Effect of temperature on some of the most important physical processes in 

 soils, G. J. BouYOUCOS {Michigan Sta. Tech. Bui. 22 (1915), pp. 6S. figs. 34). — 

 This bulletin reports the studies notetl above, and in addition reports exi»eri- 

 ments on thermo-osmose in soils ; the effect of temperature on the percolation 

 of water, the retention of water, and the rate of fiow of air in soils ; and the 

 effect of temperature changes on the aeration of soils. 



No thermo-osmotic phenomena were observed in soils. It was found that in 

 the case of sandy loam, silt loam, clay loam, clay, and muck the rate of water 

 percolation increased with rise in temperature up to about 30° C. and then de- 

 creased with further rise in temperature. In the case of sand, however, the 

 rate of percolation increased with a constant rise of temi>eratiire. The water- 

 holding capacity of soils and the rate of flow of air through soils decreased with 

 rise in temperature. Temperature changes were found to have a marke<l in- 

 fluence on soil aeration. " This influence is not due merely to the expansion of 



