ECOLOGICAL RELATIONS OF SOIL 



219 



lary in size. The relation of noncapillary to 

 capillary porosity is important, since it de- 

 termines the ease of circulation of soil fluids. 

 Widely different patterns of soil textures 

 may present similar porosities. 



Porosity and texture are the major items 

 that determine soil consistency, a soil attri- 

 bute that depends on the pattern of pore 

 space, the mechanical units, and the com- 

 position of the material present. Soil con- 

 sistency is not constant for a given soil; 

 rather, it represents the condition at the 

 moment and is especially influenced by 

 moisture. Viscosity and plasticity of the soil 

 are also closely related to its water content. 

 Fine soil texture makes for a high degree of 



suiting alteration in color, and changes in 

 all the other physical and many chemical 

 attributes of the soil. Heat conductivity is 

 changed, water percolates more slowly, ad- 

 sorptive powers increase, and the associated 

 biota is altered; the whole character of the 

 soil is made different as a result of an in- 

 creased rate of hydrolysis. 



THE SOIL CLIMATE 



In sunlight, temperature at the soil sur- 

 face varies more than in the air above or in 

 the soil below. Daily fluctuations penetrate 

 the upper layers, and seasonal variations go 

 deeper. The depth penetrated depends on 

 insolation, on atmospheric conditions, and 



Table 16. Soil Temperature Gradient at Tucson, Arizona (From Sinclair, 1922) 



plasticity; coarser soils are more friable. 

 Like soil consistency, soil structure refers to 

 its condition and is shown most diagram- 

 matically when the soil is air-dry. Soil 

 structure depends largely on the presence 

 of aggregates of soil particles; these tend to 

 disappear with increased moisture, and in 

 the presence of sufficient water, soil struc- 

 ture almost vanishes. 



The most obvious physical characters 

 that differentiate the soil horizons are struc- 

 ture, consistency, porosity, texture, and 

 color. These affect and are affected by 

 chemical processes— by hvdrolysis, for ex- 

 ample. Active and continued hydrolysis in- 

 creases the proportion of clay with a re- 



on the local soil characteristics. As would 

 be expected, dark exposed soils absorb heat 

 more rapidly than do somewhat similar 

 ones of lighter color. Decided changes are 

 especially characteristic of the surface and 

 upper layers of the soil in deserts. Table 16 

 gives an indication of the magnitude and 

 steepness of the daily and annual tempera- 

 ture gradient in the upper 2 meters of arid 

 soil. The resulting soil climate is closely re- 

 lated to and derived from the atmospheric 

 climate; even so, microclimatic conditions 

 in soil may bear only a distant relation to 

 the ecoclimate above its surface. 



Movement of moisture through the soil 

 and its storage there are affected by the 



