THE BIOLOGICAL CONDITIONS IN THE SOIL 225 



vapour from solutions of known osmotic pressure and their 

 absorption of water vapour when suspended over dilute 

 solutions of sulphuric acid. Fig. 22 shows the force with 

 which the water is held in a heavy loam where the wilting 

 coefficient is high : between 80 per cent, and 20 per cent, of 

 moisture the hold is but slight, then it increases rapidly and 

 continuously till at 5 per cent, of moisture it amounts to I ooo 

 atmospheres. But there was no break at the wilting coefficient 



80 



60 



40 



bJ 



2O- 



20O 



IOOO 



ATMOSPHERES. 



FIG. 22. Magnitude of force with which water is held by soil, showing 

 change with decreasing water content. (C. A. Shull, 264.) 



(4 atmospheres) : indeed, seeds absorbed nearly as much water 

 from soil at this point as they did from pure water. 



With sand the hold is slight almost all the way: only 

 near the end does it become great. 



Bouyoucos (49^) has made a division by a somewhat 

 different method. He measured the depression of the freezing- 

 point of the soil water as the soil became successively dryer 

 and drew the qualitative conclusion that in soil some of the 



15 



