COMPOSITION OF SOIL WATER L79 



sion, and may take place in any direction. Within the soil 

 liquid, however, there is opposed to diffusion absorption 

 both chemical and physical. Moreover the bodily move- 

 ment of water due to gravity or to surface tension neutral- 

 izes the effect of diffusion in many cases. 



As a matter of fact diffusion in soils does not play a very 

 important part. Movements of plant foods in solution take 

 place for the most part in a general direction up and down; 

 up by surface tension, dow r n by gravity, and are due to bodily 

 movements of water containing the plant foods. Experi- 

 ments have repeatedly demonstrated that plant foods in the 

 form of soluble fertilizers show no effect laterally within a 

 very short distance from the point of application. That is, 

 the plant foods apparently get no chance to diffuse, for if 

 they did the diffusion would be in every direction away from 

 the point of application and the effect would be shown 

 laterallv from the fertilizers. 



142. Composition of Soil Water. — From the preceding 

 sections it has been seen what compounds exist in a soluble 

 form in the soil moisture; how they may be soluble part 

 of the time and insoluble part of the time, due to chemical 

 change. The conditions affecting these various changes 

 have been studied. In addition to this information it may 

 be interesting to know how much soluble plant food material 

 there is in soils. Soil water which contains the soluble 

 compounds may be divided into two classes for convenience 

 of study: Film water and drainage water. 



(a) Film Water is the liquid in the soil which surrounds 

 the soil grains with a thin film, and which furnishes plants 

 with their foods. It bathes the plant roots with their nutrient 

 fluid. Determining the amount of soluble material in this 

 film water is a matter of great difficulty, and the results so 

 far obtained can be considered only as roughly approximate, 

 and even then represent but a few soils. However, it can be 

 said that the following figures show the parts per million in 

 a solution that probably is not far from an average soil 

 solution : 



N2O5 P2O5 K2O CaO 



3 6 33 33 



