The coarse nature of the soils [textures are predominantly loamy coarse sands or 

 coarse sandy loams), and the relatively high organic matter content appear to preclude 

 use of existing methods of estimating soil water storage (Smith 1944; Shockley 1955- 

 Broadfoot and Burke 1958; Carlson and others 1956; Stone and Garrison 1940). For this 

 reason another approach to water retention prediction seemed desirable for Batholith 

 soi Is . 



The two energy levels for water retention used in developing the retention models 

 were chosen somewhat arbitrarily. Retention at 15 bars is frequently used as an esti- 

 mation of the permanent wilting point of a soil. This concept probably has little 

 value in the Batholith except for grasses and forbs. The roots of many shrubs and trees 

 follow fracture lines or penetrate the matrix of highly weathered bedrock and can be 

 found at great depths (40 to 50 feet) ; presumably they obtain water at these depths 

 after the overlying "soil" has reached the actual wilting point for the species involved. 

 However, the retention at 15 bars is useful and necessary in classifying the moisture 

 regime of soils as specified by the National Cooperative Soil Survey (1970). 



Retention at 1/5 bar has been used frequently for estimating field capacity. In 

 coarse-textured soils, 1/3 bar is usually considered too high an energy level of water 

 retention (too low a water volume) for this estimation. However, there is no unanimity 

 of opinion among soil scientists as to a better figure; 0.10 bar, 0.061 bar (60 cm.), 

 and 0.05 bar water tension have all been proposed, and can be found in the existing 

 literature. Petersen and others (1971) support utilizing 1/3 bar measurements, at least 

 until some other value on the moisture characteristic curve becomes widely utilized. 

 Because of the widespread use of 1/5 bar, the authors concur. The primary intent of 

 this paper is to develop models relating retention to OM, F, and Pb at two commonly used 

 energy levels. 



Because of the rooting nature of plants growing in the Idaho Batholith, the authors 

 make no implication that the difference between 15 bars and 1/3 bar is the available 

 water. 



2 



