^858 EXPERIMENT STATION RECORD. 



The quantity of water present in the soil at any time is due to the 

 combined action of the factors described as affecting the humidity of 

 the soil, as well as to other influences. In general, the average quan- 

 tity of water in natural soils is greater in proportion to the fineness of 

 the particles and the quantity of clay and humus present. Of the 

 different soil constituents humus holds as a rule the largest quantity 

 of water, then follow in order clay, loam, and quartz. Soils having a 

 separate grain structure show a much higher average humidity than 

 crumbly soils. Stones diminish the i^ercentage by volume of water in 

 proportion to their quantity. With an increase in density we gener- 

 ally find a rise in humidity, other things being equal. 



As may be seen, the variations of humidity in natural soils corre- 

 spond in general to those of water capacity, because as a rule evapora- 

 tion and permeability practically equalize each other, for generally a 

 high evaporation is associated with a correspondingly low permeability, 

 and vice versa. Nevertheless, these factors, especially evaporation, can 

 under certain circumstances weaken or completely nullify the influence 

 of the others. For instance, the values lor the average humidity of 

 soils of varying compactness are more nearly the same than those found 

 for the water capacity under the same circumstances, because with the 

 increase in density the evaporation increases, involving a more rapid 

 loss of the stored-up water.^ That evaporation is in many cases the 

 most important factor influencing the humidity of the soil is shown 

 by the fact that a soil of uneven surface and dark color contains less 

 water than one of smooth surface and light color.^ The stirring of 

 the upper layer of soil at first increases the evaporation, but after a 

 short time the loosened layer dries out and serves to prevent evapo- 

 ration from the lower layers by diminishing the direct influence of the 

 agencies of evaporation on the soil and by retarding the capillary rise 

 of water to the surface.^ 



Variations in the humidity of the upper parts of the soil are caused 

 by the accumulation of water in an impermeable layer in the subsoil. 

 If this impermeable layer is near the surface, the water may come 

 quite to the toi) of a level soil, in which case the soil is saturated. If 

 the ground water is farther beneath the surface the influence on the 

 humidity of the upper layers depends entirely on the conductivity^ of 

 the layers. The higher the water rises by capillarity the greater the 

 influence of the ground water on the humidity of the upper layers. 



INFLUENCE OF THE DEPTH OF THE SOIL. 



Experiments on this subject* have shown that the absolute humidity 

 of the soil always increases with the thickness of the soil layer, and 

 that the ijercentage by volume behaves in the same way uj) to a certain 



IE. Wolluy, Forsch. Geb. agr. Phys., 5, pp. 1, 145. 

 ''Ibid., 3, p. 117; 12, p. 386. 

 'Ibid., 3, p. 32.5. 

 <Ibid., 16, p. 1. 



