212 



SOIL PHYSICS AND MANAGEMENT 



in fine-grained soils, as shown in the next table, and would be suffi- 

 cient to sustain a column of water of the height given in the third 

 column. 



The Capillary Lift of Soil Constituents " 



Osmosis in Soils. Lynde and Dupre 14 have demonstrated 

 that soils containing fine particles act as semi-permeable membranes, 

 probably producing only a fractional part of the pressure of a mem- 

 brane. Movement of this kind takes place when a difference in 

 concentration of solutions exists in adjacent soil masses. The direc- 

 tion of movement is toward the point or zone of greatest concentra- 

 tion. The osmosis is increased by a higher temperature, so that the 

 movement is greater in summer than winter. 



King in has found that manure incorporated with soil caused a 

 rise of water into the upper three feet of soil, due to a stronger solu- 

 tion and greater osmotic pressure. Fertilizers when applied to a 

 soil dissolve and cause a greater concentration of the soil solution 

 as well as a greater surface tension, with the result that water is 

 drawn to the surface. It is probably true that tillage and the appli- 

 cation of lime, both of which may aid bacterial action in developing 

 plant food and thus producing stronger soil solutions, may promote 

 better surface moisture conditions. 



Use of Capillary Water. Capillary water is the form used by 

 plants in their growth. Even in the most severe drouths plants 

 cannot extract all of the film moisture. The common crops may 

 use some gravitational water, but only to a very slight extent. Rice 

 and cranberries are naturally adapted to growth in a very wet or 

 even saturated soil. The amount of water in a soil for best growth 

 varies within rather wide limits, but our common crops make best 

 growth when soils contain from 40 to 60 per cent of their total 

 moisture capacity. This is the optimum water content. 



Wilting Coefficient. 16 The moisture content of the soil at 



