MODEL TESTS ON THE ECOLOGICAL EFFECT 



OF VAPOUR MOVEMENT AND CONDENSATION 



IN SOIL DUE TO TEMPERATURE GRADIENTS 



W.R. Muller-Stoll &G.Lerch 

 Botanisches Institut, Padagog. Hochschule, Potsdam, E. Germany 



I. THE PROBLEM 



It is well known that the surface of naked and sparsely overgrown soils is 

 subject to extreme temperature fluctuations between day and night, in 

 particular during bright weather. 



Most of the solar energy directly reaching the earth is absorbed by the 

 uppermost shallow layers of soil, and, therefore, the soil surface grows 

 hotter than the adjoining parts of soil and air. 



At night the inverse procedure prevails. A great deal of the accumulated 

 heat is re-radiated leaving the ground surface much cooler than the regions 

 above and below it. In this way a temperature gradient results between the 

 soil surface and the adjoining layers of air and soil. This gradient is most 

 distinct under a clear, cloudless sky when great differences in temperature 

 commonly arise between the soil surface and just a few inches above or 

 below it (see Geiger, 1950). 



Due to this temperature gradient water vapour moves from points of 

 higher energy level to those of lower free energy, i.e. from warm moist 

 parts into cold dry layers where condensation follows above ground, well 

 known as dew. 



This, too, happens underground. By day the deeper parts are cooler than 

 the surface layers, and vapour moves downward. At night, on the con- 

 trary, vapour tends to move upward condensing to hquid water near the 

 ground surface. 



It has never been denied that water vapour does move in such ways. 

 There are very different opinions, however, on the question whether an 

 ecological importance should be attributed to that vapour movement 

 caused by temperature gradients, i.e. whether the water volume trans- 

 ported in this way might be sufficient to affect noticeably the water turnover 

 of plants. 



At present the majority of authors in soil science tend to deny this. Their 

 reasoning was outlined and summarised recently by Veihmeyer, 1956 

 (citing further hterature), and formerly by Walter (1951)- In brief, these 

 reasons^are based upon two points : 



