THE WATER OF SOILS. 



211 



portion of the soil by cold rain or irrigation may tend to raise 

 additional supplies from below. At the change of seasons we 

 not uncommonly find, in digging tree holes or wells, a wet 

 streak at from 9 to 18 inches below the surface, caused evi- 

 dently by the condensation of subsoil moisture, at the limit of 

 a cold zone resulting from the penetration of unseasonable tem- 

 perature ("cold snap") from above. Such movements of 

 soil-moisture by means of evaporation and recondensation 

 within the soil can of course take place even when the mini- 

 mum of liquid absorption has been reached and direct capil- 

 lary movement has ceased. It is, as it were, dew within the 

 soil. 



Proportion of Moisture Available to Growing Plants. Not 

 all the capillary moisture contained in soils is available to 

 plants, as can readily be seen from the fact that many plants, 

 especially when growing in pots, begin to wilt while the soil 

 still appears visibly moist. The limit of wilting differs greatly 

 in different plants, and in the open ground it is difficult to as- 

 certain that limit, because the deeper roots continue to supply 

 moisture from moister substrata. Hence potted plants wilt 

 while the soil appears much moister than when the same grow 

 in the field. King l has determined the amounts of moisture 

 down to 43 inches in a Wisconsin soil in which clover and corn 

 were at the wilting point, as in the following condensed table : 



It is plainly show r n here that the roots of clover and corn 

 were unable to utilize the higher moisture-content of the sub- 

 soil-clay to the same extent as the smaller amounts present in 

 the surface foot, and in the sandy substrata. Evidently the 

 moisture in the clay soil was more tenaciously retained. 



1 Physics of Agriculture, p. 135. 



