MODEL TESTS OF VAPOUR MOVEMENT IN SOIL 73 



Kausch (1955). The basal soil columns hardly lost water, whereas the soil 

 in the apical tubes was much desiccated, by the plants, and on the sixth day 

 permanent wilting percentage was reached. 



Within the test sets, however, the soil of the lower tubes lost so much of 

 its water content, due to the temperature gradient, that on the first day of 

 the test the moisture in the upper container increased, and -in spite of 

 transpiration from the plants- was kept nearly on the same level during the 

 following days, decreasing but slightly. In this case the loss from transpira- 

 tion was nearly completely reimbursed by the water supply from below. 



(b) Dry Soil in the Apical Tubes 



The apical containers were filled with soil that had been previously dried 

 out to an approximate moisture content of 4%. The basal tubes held soil of 

 either the same low water content (4°o) or a moist packing of nearly 11% 

 water (Fig. 4). 



The other environmental factors, in particular the temperature of air and 

 water bath, were the same as in the preceding section. 



The basal soil columns within the control sets did not lose water, as in 

 the preceding cases, and the graphs of soil moisture loss in the apical tubes 

 are similar to one another. The small difference in water loss may be 

 attributed to slightly different transpiration from the control plants. All 

 control plants wilted on the sixth day of the experiment. 



The test plants in the sets containing dry soil in top and bottom behaved 

 in the same way. The moderate temperature gradient of io°C on average 

 could not force sufficient water vapour from the lower containers which 

 lost but very little of their moisture into the apical tubes. Test plants wilted 

 there on the same day as did the controls after having exhausted their soil 

 columns. 



When sufficient water was available, however, within the lower soil 

 columns, vapour could move upward in such quantities that in the upper 

 containers the water loss from transpiration could be reduced considerably 

 at the expense of a corresponding marked loss of moisture from the basal 

 tubes. Consequently the test plants did not wilt in these cases. 



(c) Extreme Temperature Gradients (30°C) 



For these tests soil of the same moisture was used as described in section (b), 

 but the temperature gradient was raised to the extreme limit of 30°C, and 

 even more, by an ice- wrapping appHed to the top of the apical tubes while 

 the bottom of the basal tubes stood in the hot-water bath. This time the 

 control containers, too, were heated by the water bath, thus being subject 



