MODEL TESTS OF VAPOUR MOVEMENT IN SOIL 69 



Experiments were performed during June and July, 1957, in a glass- 

 covered green-house. 



III. EXPERIMENTAL EVIDENCE 



I. Model Tests on Vapour Movement in Naked Soil due to Tempera- 

 ture Gradients 



A series of introducing tests was made without hving plants in order to 

 check the range of water transfer through the soil columns used for these 

 experiments. 



For the purposes of these tests both open ends of the test set were closed 

 by rubber plugs ; thermometers were inserted through the apical stopper 

 (Fig. lb). Experiments with these closed systems were run over 12 hours. 

 Loss or gain of moisture in both glass containers of each set were registered 

 by weighing every 30 minutes. 



Results differ according to soil moisture gradients. Three typical varia- 

 tions will be described here, see Fig. 2. 



(a) Equal and Sufficient Moisture in both Containers 



The temperature of water bath was maintained constantly at 45°C from 

 30 minutes after the beginning of the test. The temperature of air varied 

 due to insolation being increased by the accumulation of heat below the 

 glass roof of the greenhouse. 



Temperatures within the soil columns changed but shghtly keeping in 

 the basal tubes close to the water temperature, and about io°C lower in the 

 upper containers thus exceeding the air temperature by far, except in the 

 forenoon. 



Due to the lack of a uniform temperature in the atmosphere, the 

 temperature gradient between the lower and the upper containers could 

 not be kept exactly constant, but a difference of io-i2°C on average was 

 maintained fairly steadily during the entire period of the experiments. 



Within the apical soil columns moisture content increased in a nearly 

 linear relation from ii-o up to 12-4%. The corresponding loss of water 

 could be measured from the basal tubes. 



(b) Different Soil Moisture in both Containers 



Fig. 2b shows similar relations to Fig. 2a, but resulted from a different 

 initial situation. At the beginning of the test the apical tube held dry soil 

 (2-8% moisture content). From the moist soil (ii'0% water content) in the 



