THE BIOLOGICAL CONDITIONS IN THE SOIL 221 



subsoil ever comes back again sufficiently quickly to be an 

 important factor in plant growth.^ 



The facts have been studied by two sets of investigators. 

 Plant physiologists and ecologists have divided up the water 

 into three kinds : — 



1. The unavailable or useless water, being all below a 

 certain minimum amount which the plant cannot absorb ; 



2. The available water, being the portion absorbable by 

 the plant, but not including the excess water; 



3. The excess water, being the amount which causes 

 water-logging of the soil and exclusion of air, with conse- 

 quent injury to the plant. 



The unavailable water is marked off from the available 

 supply by the point at which wilting just begins.^ The per- 

 centage of moisture in the soil at this point, calculated on the 

 dry weight, is called the wilting coefficient : it has been in- 

 vestigated by Heinrich (129) and by Briggs (55^, c). The 

 experimental method consists in growing plants in large 

 pots of soil, which is protected from evaporation and not 

 supplied with water ; then when the plants wilt and die, de- 

 termining the amount of water left in the soil. 



It must be admitted that the wilting point is not a definite 

 constant. For any given soil and plant it varies in a regular 

 and predictable way with the evaporating power of the air : ^ 

 wilting occurs as soon as the rate of loss of water by trans- 

 piration from the leaves exceeds the rate of entry through 

 the roots. The loss is determined by atmospheric conditions 

 and the entry by resistance of the soil to the passage of 

 water : the wilting coefficient, therefore, includes not only the 

 water which is too firmly held by the soil to pass into the 



^ For a review of the literature see F. J. Alway and G. R. McDole, Journ. 

 Agric. Research, 1917, 9, 27 ; and for a calculation of the theoretical maximum 

 height of rise see B. A. Keen, yourn. Agric. Set., 1919, 9, 396. Leather 

 (167a) obtained but little evidence of movement at Pusa. 



'■* I.e. permanent wilting, when the leaves do not recover, even in a saturated 

 atmosphere, without the addition of water to the soil. 



'^ J. S. Caldwell, The Relation of Environment Conditions to the Phenomenon 

 0/ Permanent Wilting {Physiol. Res., 1913, I, 1-36). 



