THE FORMS OF SOIL WATEB 219 



face, while its colloids exert an aj03.nity for moisture which 

 raises its water capacity to a very high degree. Its ten- 

 dency to swell on wetting is but a change in condition 

 incident to an approach to its maximum moisture con- 

 tent. The following data, taken from a compilation by 

 Storer/ give an idea of the capillary capacity of the soil 

 organic matter : — 



Percentage 

 of water 



1. Humous extract from peat 1200 



2..Non-acid extract from peat 645 



3. Vegetable mold 309 



4. Peat 190 



5. Garden loam, 7 per cent humus .... 96 



6. Illinois prairie soil 57 



7. Field loam, 3.4 per cent humus .... 52 



8. Mountain valley loam, 1.2 per cent humus . 47 



Even after allowance has been made for the increased 

 hygroscopic coefficient incident to an increase in organic 

 matter, the effect of the latter Is very strongly evident 

 on the capillary capacity of a soil. Besides this direct 

 effect, organic matter exerts a stimulus toward better 

 granulation, a condition in itself favorable to increased 

 water-holding power. 



147. Determination of capillary water. — The capillary 

 water in a sample of field soil may be determined by mak- 

 ing a moisture test in the ordinary way for the total water 

 contained. This represents the hygroscopic plus the 

 capillary water. A determination of the hygroscopic 

 coefficient on another sample yields a figure which when 



iStorer, F* H* Agriculture, Vol. I, p. 106. New York. 

 1910. 



