2 24 SOIL CONDITIONS AND PLANT GROWTH 



desirable now that the soil is recognised to possess marked 

 colloidal properties. The modification is rather in the con- 

 ception than in the grouping, but it is important because of 

 the necessity for clear ideas on the subject. 



We start from the fact recognised by Briggs and Shull, 

 and demonstrated by Keen (146^, b) that the state of the water 

 in the soil is the same whether we are considering the first 

 per cent, or the last : there is no break over the whole range 

 of moisture content from dryness to saturation. 



It follows, therefore, that such divisions as the foregoing 

 must indicate changes in the force by which the water is 

 retained, not changes in the water itself Broadly speaking, 

 there are three types of forces which come into play : — 



1. Intimate surface forces of very high magnitude {e.g. 

 1000 atmospheres or more) which, however, are capable of 

 acting effectively only on a small amount of water ; 



2. The ordinary surface forces commonly spoken of as 

 capillarity, capable of acting on considerably more, but not on 

 an indefinite amount of water ; 



3. Gravity, which acts on all the water independent of its 

 amount. 



All these forces act, but the relative importance and 

 effectiveness of each depends on the amount of water present. 

 While no sharp points of demarcation exist there are, for any 

 soil, percentages of moisture for which one force exerts a 

 predominating effect. 



The first attempt to divide soil water on the basis of the 

 forces retaining it was made by Briggs and Shantz in 191 2 

 (550- ^ powerful centrifuge was employed which threw out 

 all water held by forces of less than 1000 times that of 

 gravity, which is equivalent to about one atmosphere. The 

 percentage of water held at this point is called the moisture 

 equivalent. 



A considerable advance was made by Shull (264). He 

 used the attractive pull by seeds as a means of estimating the 

 force with which water was held by the soil, and he calculated 

 the pull of the seeds by studying their absorption of water 



