SOIL CONDITIONS AND WATER SUPPLY 57 



TABLE XIII 

 Water Relations of Moorland Soils and Plants 



The most extensive work is that of Briggs and Shantz 

 (19 12, a and b), who have determined the wihing coeffi- 

 cients of many species in a variety of soils, and have studied 

 their relation to the soil constitution. They found that for 

 any given soil the wilting coefficient was practically a constant 

 and did not vary for different plants. Calling the average 

 coefficient of about 100 species studied unity, the extreme 

 variants were Japanese rice with 0*9, and Colocasia with 

 i"i3. Succulents like Echeveria, xerophytes like Artemisia, 

 and mesophytes like the tomato and the clover, all reduce 

 the soil moisture to practically the same point when wilting 

 sets in. This result is, of course, contrary to experience, 

 which recognises that some plants can exist in much more 

 arid conditions than others, and does not agree with the 

 results of Crump quoted in Table XIII. It is explained 

 by the method employed by Briggs. His plants were 

 allowed to exhaust the soil slowly under shelters or glass 

 where evaporation was relatively low. These conditions 

 throw the weight of influence on the set of factors acting in 

 the soil, and are therefore calculated to make the best use of 

 the wilting coefficient as an indicator of soil conditions. 

 They do not, however, express the capabilities of the plant 

 in natural conditions. This may be illustrated by further 

 reference to Crump's work (19 14). With experiments 

 conducted in the open he found, for example, that in 

 peaty soils one-quarter of the water was non-available for 



