WATER CAPACITY OF SOILS 71 



The quantities of water, ascertained by a laboratory ex- 

 periment, as capable of being held by any soil are but seldom 

 realized under natural conditions in the field. It is indeed 

 a difficult task thoroughly to saturate a soil with water. The 

 interstices of a dry soil are full of air, and unless the whole 

 of this air is allowed to escape the soil cannot become fully 

 saturated. In the laboratory saturation is best effected by 

 allowing the water to rise into the soil from beneath ; the air 

 then easily escapes through the dry soil above. In nature 

 this proceeding is reversed, the rain falling on the surface 

 and hindering the escape of air. It is thus only after long 

 continued rain that soils are found in a fully saturated con- 

 dition. Illustrations of the amounts of water held by field 

 soils in their wettest condition will be found in Table X. 

 Nos. 1-5 are Wisconsin soils, examined by King thirty- 

 two hours after a rainfall of over three inches ( Wisconsin yth 

 Rep., 152). Nos. 6-8 are soils, from Broadbalk wheat field, 

 Kothamsted, examined by Lawes and Gilbert (J. Roy. Agri. 

 Soc. 1871, no). 



These figures are on the whole somewhat lower than the 

 percentages by weight given in Table IX as the result of 

 laboratory experiments. 



maximum amount of water it is capable of holding. This mode of work will 

 lead to more accurate results than experiments made in the laboratory on 

 powdered soils, not only because the soil is taken in its natural state of 

 consolidation, but also because, if the cubic foot is measured in a moist 

 condition, the considerable errors which sometimes arise from the change 

 in volume of the soil on wetting are entirely avoided. If a cubic foot of 

 moist sand weighs when perfectly dry no lb., and the sand has a specific 

 gravity of 2-62, then the sand contains 32-6 per cent, of its volume of inter- 

 spaces, and would hold when saturated that proportion of water. On the 

 other hand, a cubic foot of clay, weighing when dry 75 lb., and of a specific 

 gravity 2-50, will contain 51-9 per cent, its volume of interspaces. These are 

 nearly extreme cases ; ordinary soils (peat of course excluded) will fall between 

 these limits. 



