SOIL MOISTURE 49 



from 60 to 40 per cent, air space. Desert plants or plants coming from 

 dry climates are more tolerant of deficiency in soil moisture and the opti- 

 mum capillary moisture content for these plants is lower. The reverse 

 is true of certain other plants that thrive under moist conditions. The 

 almond is mentioned by Hilgard and Loughridge^^ as suffering from excess 

 moisture when approximately three-fourths of the pore space was filled 

 with water. 



The hygroscopic moisture of the soil varies greatly with the composition. 

 In sandy soils it may be as low as 2 or 3 per cent, and in coarse sands even lower. 

 In ordinary loams it ranges from 4 to 5 per cent, and in heavy clays and adobes 

 it may be as high as 8 or 10 per cent.^" 



The Relative Saturation. — Brown^^ suggested that a more useful way 

 of expressing the water content of the soil would be in terms of its relative 

 saturation. This would take into account the maximum water capacity 

 as well as the actual water content and would afford a more accurate 

 index of the biological or physiological wetness of the soil than the 

 standards of measurement now employed. In commenting upon this 

 question of relative saturation he says : 



"The water content alone is . . . but an imperfect index of the soil con- 

 ditions. Since soils of different mechanical composition have different capacities 

 for water, the same quantity of water produces different changes in humidity in 

 these different soils. For example, the quantity of moisture sufficient to saturate 

 a given mass of sandy soil is insufficient to saturate a like mass of humous soil. 

 The degree of wetness or dryness, of a soil, therefore, really depends on the 

 amount of water which the soil can still take in before being saturated. Con- 

 sequently, a more perfect index to the 'wetness ' or 'dryness ' of a soil is to be had 

 by expressing the water content of the soil in terms of its maximum water capacity, 

 ,, , . Water content 



the ratio ^^^^^^^^^ capacity' ^^^"^ ^^"^^^ ^^^ ^^^^^^^ Saturation of 

 the soil . . . This value, and not the actual water content itself, will be the index 

 to the ' biological wetness' of the soil ... in comparing two soils whose actual 

 moisture contents are different, say, the ratios may indicate that both soils, 

 however, have the same degree of wetness, and so, in relation to the physiological 

 action of the plant are of similar conditions." 



Resistance to Freezing. — Another classification of the soil water is 

 made by Bouyoucos.^^ This classification is based upon freezing point 

 determinations with the dilatometer. Water which freezes at or slightly 

 below 0°C. is termed free water, that which freezes between 0°C. or a 

 little below and — 78°C. is termed capillary or capillary-adsorbed water 

 and that which does not freeze except at temperatures below — 78°C, is 

 termed combined water. These classes do not coincide exactly with those 

 of the classification used before and they serve to bring out some of the 

 features of the water supply of the soil that have been overlooked and 



