Figure 9. -Soil erosion on 

 the Diamond Mountain 

 study area in relation 

 to percentage of soil 

 surface protected from 

 direct raindrop impact 

 by plants and litter 

 with 2, 6, 10, and 14 

 percent organic matter 

 in the surface inch 

 of soil. 



0.05 



100 



PLANT AND LITTER COVER (%) 



The Striking feature of this equation is that the amount of erosion increases as 

 organic matter content increases. This is contrary to results of studies on finer 

 textured soils where organic matter favors the formation of erosion-resistant aggre- 

 gates. One would suspect spurious correlation if this positive relation between erosion 

 and organic matter content were observed only on this study area. However, similar 

 relations were observed on other coarse-textured soils in Idaho, up to 500 miles from 

 this study area. 



Basalt .- -lh.e strong interaction between sand content and organic matter content 

 was quite apparent on this area. The regression equation for sheet erosion on this 

 study area is : 



y = 6.615 - 3.58S + l.SOS^ - 19. 7D + 15.0Z)2 - 41. 4£' + 92.1DE + 0.0189ff 



This equation is based on 44 plots and has a standard error of estimate of 0.39 

 and a coefficient of determination of 0.69 (fig. 10). Protective cover provided by 

 plants, litter, and stone (fig. 11) is the most important variable and the effects of 

 slope gradient (table 1) were similar to those on other study areas. The curve in 

 figure 11 is based on average slope (18 percent), average sand content (38 percent), 

 and average organic matter content (9 percent). The effects of variations of sand and 

 organic matter are shown graphically in figure 12. Where sand contents are low, erosion 

 decreases sharply as organic matter increases. But organic matter apparently had a re- 

 verse effect on erosion where sand contents are high, an effect similar to that observed 

 at Diamond Mountain. In soil having a 45 percent sand content, organic matter has 

 little influence on erosion, probably because its aggregating influence on clay is com- 

 pensated by its unfavorable influence on erodibility of sand. 



The net effect is that the fine-textured soils are more erodible if there is little 

 organic matter in the surface soil but, when there is much organic matter, sheet erosion 

 is greater on sandy soils. 



12 



