146 TEXTBOOK OF PLANT PHYSIOLOGY 



or the unavailable moisture reserve of the soil, in percentages of its 



dry weight. 



per cent of hygroscopic water 



q = 



1 0.68 



per cent of water at full saturation —21 

 2.9 



According to the data of mechanical analysis, the unavailable 

 reserve may be calculated as follows: 



q = per cent sand X 0.01 — per cent silt X 0.12 



— per cent clay X 0.57 



Thus, different soils show a very different wilting coefficient. 

 For coarse sand, this coefficient is 1 per cent; for fine sand, 2 to 3; 

 for loam, 5 to 10; for heavy clay soil, 14 to 16 per cent. Regarding 

 the adaptability of different plants, it has been found that with 

 sufficiently slow wilting, all of them leave in the soil a perfectly 

 equal amount of water, though the osmotic suction tension of their 

 root systems may be very different. This fact may be explained 

 as follows: When the capillary water, which is readily available 

 to all plants, is exhausted, the water-retaining forces of the soil 

 rapidly increase to several atmospheres. As the suction tension 

 of the plants usually does not exceed a few atmospheres, differences 

 in this regard between various plants play almost no role. More- 

 over, the water loses its capacity of moving through the soil; 

 hence, the soil particles which are not in direct contact with the root 

 hairs no longer give up their moisture to them, however great the 

 suction tension of the root hairs may be. 



The data by Briggs and Shantz are in perfect agreement with 

 those of Bogdanov, obtained at a much earlier date by studying 

 the relation of germinating seeds to soil water. Bogdanov has 

 found that, independently of their nature and properties, the seeds 

 of various plants can germinate only when the water content of 

 the soil is no less than twice its hygroscopic capacity. His method 

 of calculating the unavailable water has been made use of in 

 agronomic practice. In calculating the amount of soil water 

 available to the plant, usually twice the amount of hygroscopic 

 water is subtracted from the total amount present. Under condi- 

 tions of rapid evaporation, the amount of unavailable water is 



