164 NATUEE AND PROPERTIES OF SOILS 



capillary capacity except in arid or semi-arid regions. In 

 fact, changes in surface tension through any cause are of little 

 practical importance. 



The finer the texture of a soil the higher is its capillary 

 capacity. This is due to the presence of colloidal material 

 and to the greater number of angles in which capillary water 

 may be held. The amount of internal surface exposed by a 

 fine-textured soil is immensely larger than in one of a sandy 

 character. While texture influences both the inner and outer 

 capillary water the structure of the soil has more to do with 

 the active film-like portion. As a clayey soil is granulated 

 the interstitial spaces are enlarged and an increased capillary 

 capacity results. At the same time, compacting a sand will 

 cause a rise in the capillary capacity of that soil by increasing 

 not only the actual effective surface, but also the number of 

 angles possible for capillary concentration. Further compact- 

 ing will then cause a decrease. 



Organic matter, especially when well decayed, is commonly 

 recognized as having great capillary capacity, far excelling 

 the mineral portion of the soil in this respect. Its porosity 

 affords an enormous internal surface, while its colloids exert 

 an affinity for moisture which raises its water capacity to a 

 very high degree. Its tendency to swell on wetting is but a 

 change in condition incident to an approach to its maximum 

 moisture content, and has a very marked influence on the 

 structure of the soil. The water-holding capacity of muck 

 and peat may range as high as 300 or 400 per cent, based on 

 the dry matter present. Assuming a hygroscopic coefficient 

 of 50 per cent., the capillary figure is still very high. Besides 

 this direct effect, organic matter exerts a stimulus toward 

 better granulation, a condition in itself favorable to increased 

 water-holding power. _ 



The capillary water in any soil, other conditions being equal, 

 tends to vary with the height of the column. This comes about 

 from the effect of gravity on the outer portion of the capillary 



