342 CHAPTER XXI 



The quantity of capillary water retained varies greatly 

 according to the nature of the soil, and is also largely a func- 

 tion of the total surface exposed. Thus sands retain the least 

 and clays the most. Compacting the former and granulation 

 of the latter increase the capillary capacity in both cases. 

 Humus greatly increases the capillary capacity of all soils. 



Gravitational Water. — This is free water, removable 

 by drainage. The presence of gravitational water causes a 

 water-logged condition in soils, hence is objectionable if main- 

 tained for any length of time. Immediately after rain all 

 soils contain more or less gravitational water. In semi-arid 

 and arid regions, however, the excess of water is rapidly dis- 

 tributed in the lower soil layers, where it assumes the capil- 

 lary form. The poor growth of plants in water-logged soils is 

 chiefly due to improper aeration, which is dealt with more 

 fully under the subject of drainage. 



The Wilting Coefficient. — This is used as a basis for 

 comparison of the capacities of diiferent soils for supplying 

 water to plants. The wilting coefficient of a soil is its total 

 moisture content at the point where plants just wilt. Per- 

 manent wilting occurs when the soil still contains a certain 

 amount of capillary water, and is due to the fact that the 

 remaining moisture is either unextractable by the plant, or 

 not extractable at a sufficient rate to meet the demands of the 

 plant. Wilting is soon followed by death of the plant, unless 

 water is added to the soil. The moisture contained in the soil 

 at the wilting point is for all practical purposes unavailable 

 to the plant. The available moisture, therefore, is the amount 

 present in excess of the wilting coefficient. 



The wilting coefficient varies greatly in different soils ; 

 in sands it may be as low as 5 per cent., or even less, while 

 in clays it may be as high as 30 per cent. In general, the 

 finer the texture the higher the wilting coefficient. Under 

 normal conditions, however, the wilting coefficient of a par- 

 ticular soil is the same for all plants. 



It will be obvious that the content of available moisture 

 is the only sound basis for comparing the efficiencies of dif- 

 ferent soils in regard to moisture supply — a sand containing 

 10 per cent, of water may supply considerably more available 

 moisture than a clay containing 30 per cent. 



The relation of the various soil constants, and the dif- 

 ferent forms of moisture present, is made clear in the follow- 

 ing diagram (adapted from Hosier and Gustafson). 



