48 PLANT GROWTH 



soil, followed by careful experiments with various methods 

 used to build better soils. Constructive work may be done 

 each year, when a well-managed soil should continue to get 

 more productive. 



By irrigation and sprinkling to improve the water con- 

 tent of soil the growth of plants is probably improved most 

 because growth is restricted more by unfavorable soil water 

 content than by any other single factor. Soil water is often 

 divided into three types: capillary, gravitational, and hygro- 

 scopic. 



Capillary water is that water which is held to the soil 

 particles by cohesion or molecular attraction. The excess of 

 water that passes through the soil after a long rainy period 

 is called gravitational water because the pull of gravity over- 

 comes the force with which the water is held to the soil. The 

 capillary water is in thin films covering every soil particle to 

 a varying thickness depending on the moisture content of the 

 soil. A sandy soil may hold as much as 20 per cent of its 

 weight as capillary water which would be available to grow- 

 ing plants, but a fine soil with a high colloidal and humus 

 content may have a capacity five times as great as the sand. 

 As the plants take water from the soil, or as it evaporates, 

 the films become thinner. The force with which the water 

 adheres to the soil is inversely proportional to the thickness 

 of the film, which may become so thin that the plant absorbs 

 it more slowly than it loses water by transpiration; therefore, 

 it may wilt during the day but recover at night when the 

 plant loses water very slowly. Finally, the reduction of 

 water in the soil increases the force with which it adheres to 

 the soil particles, until the force holding it to the soil is equal 

 to the force with which it enters the plant and absorption 

 ceases. To go back to the theory of diffusion, the concen- 

 tration of the cell sap is not great enough to cause the water 

 to diffuse from the soil particles into the root hair. This bal- 

 ance of forces is reached while the films are thick enough to 



