94 THE TEXTURE OF THE SOIL [chap. 



loose mass. But soils give a very different set of 

 measurements, the rate of evaporation slowing down 

 much more rapidly as the water content of the soil 

 is reduced. If the soil is first ignited, thus destroying 

 the colloids, it then behaves like sand or glass. The 

 soil colloids, therefore, may be regarded as exercising 

 a specific attraction for water, weak as regards a large 

 quantity of water, but becoming stronger and stronger 

 as the proportion of water in the "gel" is reduced. 



If the rate of evaporation of water from a soil is 

 plotted, the curve is a smooth one, showing no breaks 

 or discontinuities which would indicate the removal at 

 that point of all the water in a particular state. The 

 points determined, the maximum, minimum or optimum 

 water content or the hygroscopic moisture, do not stand 

 for portions of the soil water that are held in a different 

 fashion, but merely conditions of equilibrium between 

 the moist soil and the external system with which it 

 was in contact. 



The roots of plants, for example, have a certain 

 attractive power for water ; as long as this is greater 

 than the soil's the roots will continue to draw water 

 from the soil. But as the amount of water in the soil 

 is reduced, there comes a point when the attraction of 

 the soil balances that of the root, the roots cease to 

 draw any more water from the soil and the plant begins 

 to wilt. The proportion of water then held has been 

 called the wilting coefficient of the soil. But again, it 

 represents an equilibrium point, and not a change of 

 condition, when the free water has been removed and 

 only bound or combined water remains. 



All the same, only the water in the soil in excess of 

 the amount indicated by the wilting coefficient is 

 available for the crop. Sachs was the first to show 

 by experiments in pots that tobacco plants will begin 



