fiOS STATE BOARD OF AGRICULTUUE. 



of tbo cold soil with the high moisture content is less than that of the 

 soil with low moisture content, the greater case with Avliich the warm 

 soil with high water content i)arls with moisture enables the reduced 

 affective pull to draw more water from the warm to Ihe cold side. As the 

 moisture content of the soil is continually increased its attractive ])ower 

 is satisfied and the curvature of the ca])i]Iar.v films decreased, correspond- 

 ingi}*; the total effective pull of the cold cohunn of soil is continually de- 

 creased but the ease with which the warm column of soil gives up mois- 

 ture is also continually increased, so that the thermal translocation of 

 water is constantly increased Avitli rise in moisture content. Finally, a 

 degree of moisture content is reached in which the effective pull of the 

 cold column of soil is able to extract the greatest amount of water from 

 the warm column of soil. This degiiee of water content is the thermal 

 critical moisture conicnt. At this point the attractive power of the soil 

 for water is considerably satisfied but far from being entirely appeased; 

 the total effective pull of the cold column of soil is also consideraldy less 

 than that of the preceding columns of soil, but the Avarm column yields 

 water to this pull with such ease that there occurs a maximum thermal 

 water translocation. Inasmuch as the. water attractive power is dif- 

 ferent for the various kinds of soils, this thermal critical moisture con- 

 tent is necessarih' different. After this thermal critical moisture 

 content is reached the effective ])ull of the cold column of soil is further 

 decreased with continued increase of moisture content; and although the 

 willingness of the warm column of soil to ])art more readily with mois- 

 ture is also increased, yet the ])ull of the cold column of soil is not suffi-- 

 ciently strong to draw it, and consequently the thermal movement of 

 water commences to decrease; and continues to diminish very regularly 

 and gradually with continued increase in moisture content. When the 

 highest percentage of water is reached the warm soil is very willing to 

 part with a very large amount of water but since the etfective pull of the 

 cold soil is reduced almost to minimum, only a small amount of moisture 

 is drawn from the former to the Ititter. 



The degree of moisture of the different soils could not be further in- 

 c-reased on account of the difficulty of sifting them, and consequently it 

 cannot be stated with certainty whether the thermal movement of water 

 would become zero at a still higher moisture content. From the 

 theoretical point of view, however, it should not become zero because the 

 pull due to the surface tension of water alone is not affected by increase 

 of moisture content, but remains constant. The portion of pulling force 

 which is decreased constantly with rise in moisture content is that per- 

 taining to the attractive power of soil for water, and to the curvature 

 of the capillary film. At or near the point of saturation the pulling 

 power due to these two factors is probably zero; at this point the soil 

 may be considered to be passive. Any thermal movement of water that 

 takes place at or near the point of saturation is to be attributed to the 

 surface tension of the soil water. If this assumption is correct, and if 

 the percentage of moisture moved at the highest moisture contents em- 

 ployed, is to be considered as a measure of the amount of thermal trans- 

 location due to surface tension of water alone, it will be found that the 

 quantity due to this force, is very small indeed. As will be seen from 

 the experimental data, the percentage of moisture moved at both ampli- 

 tudes of temperature is reduced to an insignificant value at the highest 

 moisture contents. 



