134 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Expressing the water present by weight in 100 parts of dry soil 

 exaggerates the differences between soils. This is most marked 

 when peat is compared with a sand, for we really compare the 

 water in seven volumes of wet peat with one volume of wet sand. 



Retention of Water by the Soil. A soil, though protected from 

 evaporation, does not remain fully saturated, but loses water 

 through capillary action and the action of gravity. If a wide 

 tube of sufficient length, filled with coarse sand of uniform sized 

 particles, is saturated with water and allowed to drain, we find 

 two distinct layers; a short column of sand fully saturated, 

 and a long column above it fully drained, and containing a nearly 

 uniform quantity of water. 



If the particles of sand are not uniform in size, as in an 

 ordinary soil, then we find three layers, but not sharply distinct as 

 in the preceding case. The highest layer is fully drained, and 

 the content of water of the soil increases, until the lowest layer 

 is fully saturated. The water in the fully drained layer not only 

 coats the particles but fills the finest of the interspaces. The pro- 

 portion of interspaces occupied by water increases toward the 

 bottom of the tube, until finally all the interspaces are filled and 

 the soil is saturated. 



The following experiment of King 1 exhibits the differences 

 between two classes of soil material. Tubes 10 feet long and 6 

 inches in diameter were filled with sand prepared by sifting 

 through sieves of different degrees of fineness. The sand was 

 saturated with water, and allowed to drain, protected perfectly 

 from evaporation, for in days. The water in each 6 inches of 

 the columns was then determined. Two of the series are shown 

 in the table . 



The particles in tube I are nearly uniform ; the water content 

 is nearly constant until the ninth foot is reached, when it suddenly 

 increases. The particles in tube II are of varying size; the water 

 content increases constantly from the top to the bottom. 



The amount of water retained by a drained soil depends upon 

 the smallness of the spaces between the particles, and also on 

 1 Report Wisconsin Station, 1893. 



