8o W.R.MULLER-STOLL AND G.LERCH 



temperature relations on dry locations, and even the extreme temperature 

 gradients used in these tests do not appear too monstrous if temperatures 

 of the ground surface are included in the considerations. 



Similar temperature differences result within a short distance in the upper 

 layers of soil due to nightly radiation, in particular under a clear sky, see 

 Geiger, 1950; Lebedeff (cited by Trenel, 1949, p. 145) ; Walter, 195 1. Then 

 water vapour moves upward from the warm deep layers of soil to its 

 cooler surface. 



The real distances through which soil moisture is moved in the field due 

 to temperature gradients may be estimated to be even much greater than 

 the length of the small apparatus used in our tests. This means also a greater 

 temperature gradient between the soil surface and the deeper layers of 

 soil where considerable resources of water are available. 



Of the two possible directions of water translocation in soil due to 

 temperature gradients -by day downward from surface to depth, at night 

 upward from depth to surface -the latter one must be given the greater 

 practical importance. At night the ascending soil moisture may be able to 

 increase the water content of the upper layers in soil. This procedure is 

 supported by condensation of dew from the atmosphere. 



In humid cHmates the temperature gradients in soil are much lower, of 

 course, but under such conditions water movement in this way will not 

 be of any practical importance, anyway, to the plants. High atmospheric 

 precipitation combined with low evaporation rates involve a sufficient 

 abundance of capillary water in the upper layers of soil, and roots are not 

 dependent on the meagre quantities of vapour condensation. Thus, humid 

 soil conditions, as a matter of course, should not be taken into the practical 

 considerations of this problem at all (Trenel, 1954, 1955 ; Trenel et al, 1958). 



Under dry conditions, however, if the capillary water resources have 

 been used up to a great extent, the vapour movement and condensation 

 due to the much stronger temperature gradients may certainly be of 

 practical importance in the water supply of plants, provided that there is a 

 distinct effective radiation surface, i.e. that the plant cover is not too dense 

 and too high. Such conditions may be found even in Central Europe, i.e. 

 on warm slopes of dry land weakly overgrown with natural sand or steppe 

 plant communities. 



By day the steady strong insolation must cause the soil moisture in the 

 upper layers not only to evaporate into the dry air but also partly to move 

 down into cooler layers of soil. At night it will return to the upper layers 

 cooled down by nightly radiation and will be used there by the roots. 



In this connection one more result of our tests should be interesting : when 



