78 THE WATER-SUPPLYING POWER OF THE SOIL 



ultimate controlling force determining the rate of water entrance into 

 our osmometer becomes the capillary resistance just referred to. This 

 control is no longer simply the relation between the water attraction of 

 the sugar solution and that of the soil films adjacent to the membrane, 

 though of course this relation is still the proximate control. The water- 

 attracting force of the first soil layer has been greatly increased through 

 partial drying and can not decrease except by income of water from 

 more distant regions of the soil. Thus, at a certain stage, the rate of 

 intake by the osmometer becomes limited by the rate of arrival of 

 water in the active soil layer against the membrane. 



In our experiments it appears that the gradually or rapidly lowered 

 rate of water absorption is brought about on account of this last 

 consideration, and the rates actually recorded, after the first short 

 time interval, are the highest possible rates at which water can move 

 from more distant regions to the actively supplying soil layer. Why 

 does this fall in rate continue for so long a tune in the moister soils 

 and for so short a period in the drier ones? For answer to this question 

 we need only to revert to the consideration of the ease with which the 

 first decrement of water is removed from the soil in its assumed static 

 condition. In the drier soils we may suppose that the water films 

 are, at the start, so thin that their capillary force is nearly as great 

 as the force exerted toward absorption by the osmometer. This con- 

 dition may be supposed to obtain throughout the soil mass. Hence, 

 when absorption begins there can occur but a relatively small amount 

 of thinning of the water films nearest the membrane, and as the wave 

 of movement progresses outward there can arise but little increase in 

 the resistance offered to movement of water from soil film to soil film. 

 Such a soil should very soon come into a state of dynamic equilibrium 

 and its rate of supply should then be long maintained. This is exactly 

 what seems to be exhibited by our soils below the critical point of 

 optimum water content. 



With the moister soils, on the other hand, there should be, at the 

 outset, a much greater initial and early change in the thickness of the 

 proximate soil films, and with progressive outward propagation of the 

 disturbance thus inaugurated there should be a long-continued increase 

 in the resistance offered to movement of moisture from more distant 

 regions to the active layer. In the end, no doubt, an absorption rate 

 should be maintained, but much longer time must elapse before this 

 condition might be reached in the wetter soil than is required in the 

 case of the drier one. This, also, is in agreement with the experimental 

 findings set forth above. 



From these considerations it was suggested that the initial inter- 

 position of a dry soil layer, between the osmotic membrane and the 

 supplying soil, might diminish or remove altogether the phenomenon 

 of high initial rate and prolonged decrease. This was tested in two 



