ABSORPTION OF WATER 



145 



surrounding soil particles. The smaller these particles are and the 

 more colloidal substances are present in the soil, the greater is the 

 amount of water retained and unavailable to the root cells. This 

 competition for water by the plant has been formulated by Sachs. 

 His diagram showing the absorbing root hair among the water- 

 retaining soil particles is presented here (Fig. 59). 



To determine the amount of water in the soil that is unavailable 

 to the plant, the following procedure may be used. When the plant 

 under investigation has developed sufficiently, watering of the soil 

 is stopped and the pot is kept in a shaded place until the plant 

 begins to wilt. Wilting indicates that the water supply from the 



Fig. 59. — A root hair in the soil, absorbing water held by soil particles. 



soil has ceased. At the moment of permanent wilting the amount 

 of water that still remains in the soil may be measured by taking 

 a sample of the soil and drying it at 100° C. This is the reserve 

 unavailable to the plant. 



The most detailed investigation of the amount of unavailable 

 water in different soils was carried out by Briggs and Shantz. 

 They investigated numerous samples of various soils and estab- 

 lished a simple relationship between the amount of unavailable 

 water which they called the wilting coefficient and the other water- 

 holding properties of the soil, for instance, hygroscopic capacity 

 and the total water-holding capacity, as well as its mechanical 

 composition. From their data, these authors were able to make 

 the following formulae for the calculation of the wilting coefficient q, 



