THE SOIL AND SOIL FORMATION 67 



To show how these alkaline substances are deposited, 

 one may take a glass tumbler, fill it half full of salt and 

 then fill with water. If the tumbler is allowed to remain 

 in a warm room for some weeks it will be observed that 

 the water creeps up the sides of the tumbler and even 

 over the edge and down the outside, and deposits a layer 

 of salt on the wall of the glass. The water rises over 

 the tumbler, by capillarity through this crust, and where 

 it evaporates it leaves salt. In the same way many 

 soluble salts in the soil are left at or near the surface, 

 where the rising water evaporates. Thus in regions of 

 light rainfall and very dry air, seepage water containing 

 alkali, coming constantly to the surface in a low area, as 

 at the foot of a hill, and there evaporating, often causes 

 such an accumulation of alkali near the surface that 

 most plants will not thrive. Some plants are accus- 

 tomed to growing in soils containing considerable 

 alkali. 



Relation of air to soil and plants. Room for air in 

 soils. While most arable soils contain some relatively 

 large particles, the bulk of these soils is composed mainly 

 of very finely divided particles, %5ooo P art f an mc ^ 

 and less in diameter. Soil materials divided into these 

 very small particles have an immense total surface area. 

 Many of the tiny surfaces are applied so closely together 

 that they exclude air, and even water, from coming in 

 contact with them. There are, however, in ordinary 

 soils numerous interstices among the larger particles, 

 which give room for both air and water. Soils con- 

 taining humus will absorb and tenaciously retain more 

 water than the same soils without humus. Soils hold 

 in their pore spaces and on the surfaces of their par- 

 ticles, by the force called capillarity, considerable water. 

 When this capillary force is only partly satisfied, these 

 interstices serve as channels through which water can 

 be moved from one part of the soil to another part from 



