MODEL TESTS OF VAPOUR MOVEMENT IN SOIL 79 



Inverse temperature gradients achieved by cooling the basal tubes con- 

 cluded the evidence. The upper soil columns now lost water, in a propor- 

 tional ratio to their initial moisture content, to the lower containers, and 

 the test plants wilted several days earlier than the control plants. 



More difficult is the problem how far these model tests enable conclusions 

 to be drawn about field conditions. 



First of all the range of water volume moved by the temperature 

 gradients will be considered. Leaving aside the extreme data there was an 

 average quantity of o- 4-2-0 g of water translocated per area of 15-3 cm^ 

 every day. This would correspond to 0-3-1 -3 mm precipitation per day. 

 This evidence is confirmed by investigations of Trenel (1954, 1955) who 

 measured a water gain of at least 40 mm per year due to condensation in the 

 soil at 15 cm depth near Berlin. Trenel presumed that there are approxi- 

 mately 50 days in the year having suitable temperature relations to make an 

 extensive vapour condensation in soil possible. According to a personal 

 communication by Prof. Dr. Trenel, investigations recently made under 

 subtropical conditions in Viemam, revealed a gain of 90 mm condensed 

 water within one week at the same depth in the soil. In this case, however, 

 vapour condensation from the atmosphere should be included. 



The absolute levels of temperature used in our tests come near to field 

 conditions. 



The temperature gradient of io°C along a distance of 16 cm is doubtless 

 somewhat higher than the average field values in our Central European 

 climate at the same depth of soil. In the loamy sand soil of our field station 

 at Potsdam we found an average temperature gradient of 5-8°C between 

 2 and 20 cm depth on bright days in summer. Similar values are reported 

 by the most authors (Kraus, 191 1 ; Heilig, 1930; Volk, 193 1 ; Miiller-Stoll, 

 1935; Walter, 1951; Geiger, 1950). 



There is considerable evidence, however, of higher temperature gradients 

 even in Central Europe. Miiller-Stoll (1935, p. 186 f.) ascertained tempera- 

 ture gradients exceeding i8°C between 5 and 1 5 cm depth in a silt loam soil 

 of xerotherm grass slopes in the Kraichgau (S.W. Germany) on bright days. 

 Under similar conditions Heilig (1930) found temperature gradients 

 exceeding even 30°C between the ground surface and a depth of 20 cm 

 in the Kaiserstuhl (S.W. Germany). Kraus (1911, p. 132) measured 

 temperature gradients up to I2°C between 2 and 12 cm depth in a naked 

 garden soil at Wiirzburg (Germany) during summer. Similar temperature 

 differences may be read from the tables and graphs pubhshed by Geiger 

 (1950). 



The experimental conditions of our model tests thus come near to the 



