64 The Plant World. 



will lose to the stronger. * There is thus a tendency for the 

 equalization of salt content in different parts of the solution, as 

 there was for equalization of water content in different parts 

 of the soil. This may have great importance to th3 theories of 

 the movement of plant food materials through the soil, but 

 practically no actual work has been done on these problems and 

 as yet speculation is merely speculation. It is probable also 

 that the organic materials of the soil have important influences 

 on the surface tension of the soil solution and on the forces which 

 depend thereon. { The detailed elacidation of this question 

 must await a more complete knowledge of the nature and prop- 

 erties of the various bodies which makeup the "organic matter." 11 

 The gravitational movement of soil 

 water. Gravity affects the soil water through the weight 

 of the water itself. Like the hygroscopic forces, gravity (in this 

 form) is always active. It is between, and against, the hygro- 

 scopic adhesions on one hand and gravity on the other that the 

 capillary forces are acting. More practically important, how- 

 ever, are the effects of gravity in saturated or nearly saturated 

 soils. Here the capillary forces are absent (because there are 

 no water-air surfaces) and the hygroscopic ones are unimportant. 

 The water is moving freely under the action of gravity and in 

 accordance with its own characteristic laws of flow. This is the 

 "gravitational water" of the common classification. The main 

 practical consideration here is the rapidity with which this water 

 will flow through the soil interspaces, i. e., the permeabil- 

 ity of the soil. This depends not so much on the total amount 

 of pore space in the soil as on the sizes of the individual spaces. 

 The chief resistances to the flow of the water are frictional and 

 increase greatly with decrease in the cross section of the con- 

 taining duct. It may easily happen, therefore, that a clay soil 

 containing 40 or 50 per cent of pore space will be almost entirely 

 impermeable while a sand with only 10 or 20 i)er cent offers an 

 almost negligible resistance to the flowing water. Other things 

 equal, permeability is greater the coarser the mechanical com- 

 position of the soil and the looser its texture. Mechanical 

 composition is the controlling factor. In saturated soils there 



♦Whether this loss is of water only, or of water plus solute, and in what ratio water and 

 solute are lost, is as yet undetermined. 



tSee the last footnote. 



IIThis knowledge is rapidly being acquired; See Schreiner and Shorey, Bull 74, Bureau of 

 Soils, U. S. Dept. of Agriculture (1910). _^ ... 



