Figure 12. --Effects of changes 

 in sand and organic matter 

 content on amount of soil 

 erosion on the Basalt study 

 area. Soil erosion indica- 

 ted in figure 11 should he 

 multiplied by correction 

 factor indicated in figure 

 12 to correct for variations 

 in soil organic matter con- 

 tent and sand. 



oc 



K 



o 

 u 



0.5 



0.2 



_L__I I I I I L 



I I I I I I I I I I 



30 



35 



40 

 SAND (%) 



45 



50 



Coolwater Hidge . --Plant , litter, and stone cover explains 83 percent of the vari- 

 ance in the log of soil eroded. No significant additional variance was explained by 

 any other variable, probably because of the few plots (15) and the limited variation in 

 these other variables. The following equation has a standard error of 0.32: 



y = 1.293 + 0.105S - 2.045^ 



The curve defined by this equation is plotted in figure 15 along with the data. 

 Erosion is generally greater at Coolwater Ridge than on the other study areas because 

 of rather steep slopes (33 percent average) and a high sand content (57 percent average) 

 combined with a rather high organic matter content (10 percent average). Observations 

 on other study sites indicate that high organic matter content and steep slopes both 

 operate to increase erosion on sandy soils. Another reason for greater erosion at 

 Coolwater Ridge may be due to the character of the vegetation which consists mainly of 

 low-value forbs such as polygonum. This type of vegetation, with its small basal area 

 and low litter production, gives less protection against erosion than an equal areal 

 coverage of grasses or mat-forming forbs. 



Trinity Mountains . --The erosional behavior of these granitic soils is rather 

 erratic. Only about 45 percent of the variance of the log of soil eroded is accounted 

 for by plant and litter cover. The influence of cover on erosion is greatest on soils 

 of high organic matter content. The multiple regression equation is: 



y = -0.666 + 1.714 - 1.82.42 + 8.60F - 18.0.45' + 0.0235u 



This equation is based on 40 plots. The standard error of estimate for this equa- 

 tion is 0.40 but the coefficient of determination is only 0.57 (fig. 14). With plot 

 slope gradient fixed at its average of 23 percent, this equation is presented 



14 



