IV. THE REACTIONS OCCURRING IN SOILS " 59 



Where little or no drainage occurs, the continual passage downwards 

 of rain-water and its subsequent rise and evaporation from the surface, 

 may lead to the eventual charging of the surface soil with so much saline 

 matter, dissolved out of the soil, that the growth of plants is prevented. 

 We thus get " brak " or " ajkali " soils. 



This motion, as already stated, takes place in any direction, and 

 may be either helped or hindered by gravitation. If, however, the in- 

 terstices of a soil be completely filled with water, surface tension within 

 the soil becomes zero and gravitation alone acts on the water. 



Whitney has shown 1 that nearly all dissolved mineral sub- 

 stances increase the surface tension, while organic bodies and ammonia 

 diminish it. 



(b) Gravitation. This, of course, always acts in the same direc- 

 tion, and, as has been shown, is the only force acting on the water 

 in a fully saturated soil. It greatly affects distribution of dissolved 

 substances, though its tendency to cause the liquid to sink in the soil 

 is, in many cases, overcome by the surface tension phenomenon above 

 described. 



As a rule, the motions of the water in a soil caused by the two 

 agencies just described, overpower the diffusive tendencies of the sub- 

 stances in solution. Thus, in dry weather, the water from the subsoil 

 is brought up to the soil by the surface tension effect. Evaporation 

 and consequent concentration of the solution takes place, and the dif- 

 fusive powers of the dissolved substances would tend to move them 

 downwards to the weaker solution in the subsoil. This they un- 

 doubtedly do to some extent, but not as rapidly as the liquid moves 

 upwards. The net result thus is, that, in dry weather, the solution of 

 plant food in the upper layers may be more concentrated than that in 

 the subsoil. The roots of plants are probably, therefore, often bathed 

 in a solution of plant food much richer in dissolved matters than is the 

 drainage water from the same soil. For description of the methods by 

 which plants obtain their food from the soil, see Chap. XI. 



Soil " Pans ". Under certain conditions the productiveness of a 

 soil becomes seriously impaired by the formation of a hard, impervious 

 layer, generally between the soil proper and the subsoil. Such forma- 

 tions are known as "pans". They are produced by mechanical or 

 chemical processes. 



A so-called "plough pan" may result from repeated ploughing to 

 the same depth, year after year, especially on heavy land. In this 

 case, the sliding of the base of the plough-share, together with the 

 treading of horses and plough-men in the furrows, so consolidates the 

 top layer of the clayey subsoil as to render it impervious. 



When the subsoil is rich in calcium carbonate a " lime pan " may 

 form, owing to solution of this compound in water containing carbon 

 dioxide and the re-deposition, when the carbon dioxide escapes into 

 the air, of the carbonate of lime. 



Soils containing much organic matter and resting on a subsoil rich 

 in iron compounds, sometimes form what is known as an " iron pan,'* 

 some little distance below the surface. The iron compounds, being 



1 Weather Bureau, U.S.A., Bulletin No. 4, 13. 



