FORCES HOLDING WATER IN SOIL 6i 



described, is a measure of the soil forces resisting the with- 

 drawal of water, being just greater than these. They have 

 not made many determinations, but they found, for example, 

 that the suction force of a bean rose from I'l to 2*i atmo- 

 spheres as the soil dried out, the latter figure being reached 

 when wilting set in. 



Such widely different methods as those of ShuU and 

 Ursprung, give concordant results. The plant wilts when 

 the forces retaining water in the soil are quite low, amounting 

 only to I or 2 atmospheres. With appreciably less water in 

 the soil than the wilting coefficient, Xanthium seeds with- 

 draw nearly as much water as from saturated soil or pure 

 water, and this from very different types of soil, coarse 

 sand only excepted. As the moisture content falls the 

 retaining forces increase very rapidly, until in air-dry soil 

 they reach about looo atmospheres. It is, therefore, not an 

 increase in the retaining forces which is responsible for 

 wilting ; film water is still present, and the force necessary 

 to withdraw it is well within that at the disposal of ordinary 

 plants. The reserve, however, cannot be large, for a further 

 reduction of 3 or 4 per cent, enormously increases the 

 holding forces — imbibitional forces begin to dominate. 



The water films at the wilting point must therefore be 

 veiy thin, and this indicates that the important factor is the 

 rate of water transfer in the soil. Sachs recognised that 

 when a root hair removes water from a particular small 

 area, the equilibrium of surface forces is disturbed and a 

 flow of water towards the root hair results. The flow is 

 ready when abundant water is present, but when the films 

 become thin the resistance offered to flow by surface tension 

 and friction increases, and with very fine films the flow is 

 very much impeded. The point, probably not well defined, 

 at which this takes place has been called the lento-capillary 

 pointy and it must lie just above the wilting coefficient. 

 When it is reached a sufficiently rapid absorption of water 

 is impossible and shortly after wilting occurs. Only at 

 water-contents well below the wilting coefficient does the 

 plant require to exercise its full potential suction force to 



