THE FILM WATER 25 



specific gravity of a soil reaches a minimum, the force required 

 to insert a penetrating tool becomes a minimum, while the rate 

 at which a soil warms up reaches a maximum, 1 and the ease 

 with which aeration takes place reaches a maximum. In fine, 

 this critical water content is that at which the soil can be brought 

 into the best possible physical condition for the growth of crops. 

 The practical significance of the optimum water content is far 

 greater than would be supposed from the attention given it 

 hitherto by students of the soil. It is the content of soil water 

 which the greenhouse man should strive to maintain, and which 

 the irrigation farmer should seek to provide, instead of the over- 

 wetting so common to the practice of both. In general farming 

 it is that moisture content at which the farmer will attain the 

 best results in plowing and cultivating, and attain these results 

 most readily. 



With additions of water beyond the critical point, there is a 

 presence of free water in the soil interstices accompanied by 

 important changes in the soil structure. With continued addi- 

 tions, there is a more or less rapid decrease in the apparent 

 volume; there is a tendency for the soil aggregates to break 

 down and the "crumb structure" so greatly desired by agri- 

 culturists is less and less readily obtained, and working of the 

 soil tends in some cases to produce that phenomenon known as 

 "puddling." However desirable the property of puddling may 

 be to the potter or the brick maker, to the farmer it it a bane 

 to be avoided above all things. To overcome it requires his best 

 skill, and it usually takes several years of patient effort to restore 

 a puddled soil to good tilth. 



The force with which the film water is held against the soil 

 grains has not been determined as yet with any degree of pre- 

 cision, but it is certainly very great. If a soil be saturated, that 

 is, if so much water be added that further additions will cause a 

 flow of free water, and the soil be then submitted to some 

 mechanical device for abstracting the water, the moisture content 



1 Heat transference in soils, by Harrison E. Patten, Bull. No. 59, 

 Bureau of Soils, U. S. Dept. Agriculture, 1909. 

 3 



