420 Mineral Nutrition of Plants 



capacity, the permeability is about 0.2 cm. per hour or 1/40 of the value 

 at saturation. At a tension of 200 cm. or approximately 0.2 atmosphere, 

 the permeability drops to 0.0013 cm - P er h°ur which is 1/6000 of the 

 value at saturation. Measurements of this type obtained a number of 



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DATA 



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d VIRGIN PEAT A 

 v " " B 



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SOURCE 

 LA RICHARDS (1931) 



it 



SJ RICHARDS 8 

 BD WILSON (1938) 



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100 200 300 



SOIL MOISTURE TENSION - CM. OF WATER 



Figure 3. Relation of unsaturated permeability of soil to soil moisture 

 tension. The data points illustrate the rapid decrease in the permeability 

 with a small increase in tension. 



years ago by L. A. Richards for mineral soils (100) and by B. D. Wilson 

 and S. J. Richards (/J5) for peat soils are plotted to a linear scale in 

 Figure 3. At saturation, the permeability of many normal soils will lie 

 in the range from 1-10 cm. per hour. This range is far above the values 

 shown for unsaturated soils, even at the low tensions of 10-20 cm. of 

 water. It is evident that as air replaces water in the soil pores and the 



