WATER RELATIONS OF PLANTS 



289 



the more colloidal substances there 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 plants has been 

 pictured by Sachs in the form of a diagram, showing the absorbing 

 root hair among the water-retaining soil particles (Fig. 88). 



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 



Fig. 88. — A root hair in the soil, absorbing water held by soil particles. 



from the soil has ceased. At the moment of permanent wilting, 

 the amount of water that still remains in the soil may be meas- 

 ured by taking a sample of the soil and drying it at 100°C. This 

 is the water reserve unavailable to the plant, called ''unavailable 



moisture." 



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" of the soil, 

 and the other water-holding properties of the soil, e.g., the 

 hygroscopic coefficient and the total water-holding capacity, 

 as well as its mechanical composition. From their data, these 

 authors proposed the following formulas for the calculation of 

 the wilting coefficient q, or the unavailable moisture reserve of 

 the soil, in percentages of its dry weight: 



