68 PLANT PRODUCTS 



of many materials. The grains of the soil are wetted by 

 the soil water. The soil water adheres as a thin film to the 

 grains, and when the grains are close enough together, the 

 films unite, so that water can pass from the surface of one 

 grain to the surface of the next, until equilibrium is reached. 

 As a consequence of this fact, water will move through the 

 soil by means of the films adhering to the surface of the soil 

 grains. This action is often called capillary attraction, 

 because it is more conveniently measured in tubes, but the 

 problem is one of surfaces, and not tubes. When rain falls 

 on the soil the water sinks downwards, partly because of 

 the action of gravity, and partly because capillary equilibrium 

 has been upset. When evaporation takes place from the 

 surface, so that the films of moisture adhering to the soil 

 grains become thin, then equilibrium is again established 

 by water moving up from those grains which are more 

 completely wetted. The rate of movement will be dependent 

 not merely upon the motive power supplied by the difference 

 of degrees of wetness in one part of the soil and another, 

 and the motive power of gravity, but also upon the resistance 

 due to the varying viscosity of the soil water, and the 

 magnitude of the interstices between the soil grains. 



It is a common observation that drains will run for a 

 long time after rain has fallen. The resistance to the 

 passage of water is large in proportion to the small motive 

 forces, therefore velocity is low. As gravity is all the time 

 acting upon any such water in the soil, the height to which 

 water will rise by capillary action reaches a practical, if 

 not an absolute, limit. It is for this reason that a mulch 

 on the surface of the ground is so often valuable in conserving 

 water. The water must rise through the soil quicker than 

 evaporation can take place, otherwise the growing plant gets 

 a very poor share of the water. The mulch allows water to 

 reach a fair degree of concentration at the point where the 

 plant roots are working. The height to which water will rise 

 by capillary action in heavy soils composed of small 

 particles is greater than in light soils composed of coarse 

 particles, but soils of a coarse character will oppose much 



