ABSTRACT 



Volumetric water retention at 1/3 and 15 bars was evaluated for strength of 

 relation to the interacting effects of percent organic matter, percent fines 

 (cSOum diameter) and soil bulk density. Samples used in the models were re- 

 stricted to soils formed from granitic parent materials from the Idaho Batholith, 



These soils are unique in ihat they are coarse-textured and have a broad 

 range of organic matter content. Where a broad range of textures exists, tex- 

 ture (aside from pore size distribution) has often been found to be the single 

 variable most useful in predicting retention. Perhaps by reason of its more 

 limited range (particularl\' in the clay fraction), texture appeared to have only a 

 secondary effect in the Batholith soils. In contrast, retention was strongly af- 

 fected b>- organic matter in the coarse-textured soils here, possibl\' because of 

 the broad organic matter range and the enhanced potential for organic matter to 

 exert its retention effects. Organic matter alone explains 57 percent of the 

 variance in water retention at 1/3 bar, and 62 percent at 15 bars tension. In- 

 corporating the interacting effects of the remaining two variables explains an 

 additional 17 percent of the variance in water retention at 1/3 bar, and an addi- 

 tional 11 percent at 15 bars. 



Tables and equations for the two retention models are presented in the 

 Appendix. The final models were subsequentl\- adjusted by least squares to two 

 new data sets of geographically diverse soils from the Idaho Batholith. The ad- 

 justed retention models operate well for predicting water retention at both 

 energy levels. The value of the models is discussed in light of these findings. 



