SOIL TEXTURE AND 

 ORGANIC MATTER CONTENT 



Sand, clay, and organic matter contents of 

 the surface inch of soil were arbitrarily fixed at 

 their average values to calculate the curves in 

 figure 3. Variation within these parameters can 

 cause estimated erosion to vary from less than 

 half to more than twice the values shown in 

 figure 3. Effects on erodibility are produced by 

 these soil components operating together in a 

 complex manner only approximated by the re- 

 gression model presented in this paper. How- 

 ever, some general trends can be detected (see 

 fig. 4) and are listed below: 



1. — If organic matter is low, clay is more 



erodible than sand. 



2. — Erodibility of clay is inversely related 



to organic matter content. 



3. — Erodibility of sand tends to increase 



with increasing organic matter, es- 

 pecially if the soil contains less than 10- 

 percent clay. 



4. — If organic matter content is high, sand 



can be more erodible than clay. 



Of particular interest is figure 4A which 

 shows relative erodibility as a function of or- 

 ganic matter and sand contents at a constant 

 clay content of 10 percent. Erodibility is about 

 half that of average soil if organic matter does 

 not exceed 8 percent or if sand does not exceed 

 30 percent. When both sand and organic matter 

 exceed these percentages, a further increase in 

 either sand or organic matter results in 

 increased erodibility. 



The apparent adverse effects that organic 

 matter has on stability of sandy soils are be- 

 lieved to be real because comparable relations 

 appeared in the three individual regression 

 equations for Diamond Mountain, basalt, and 

 Trinity Mountains study areas. This phenom- 

 enon is believed to be linked with water repel- 



lence, which has been observed to occur mostly 

 in sandy soils and to be related to organic mat- 

 ter (Krammes and DeBano 1965). Organic 

 coatings profoundly affect the interfacial en- 

 ergy of soil particles. Organic coatings on soil 

 particles reduce the affinity those particles 

 have for water to such a point that positive 

 pressure may be required to force water into 

 the soil (Fink 1970). These coatings may also 

 reduce the forces of attraction between sand 

 particles to such an extent that they repel one 

 another and increase their erodibility. 



Clay tends to reduce or eliminate the ad- 

 verse effects of organic matter on sand. One or 

 more of the following mechanisms may be 

 responsible: 



1. — Organic matter is known to favor aggre- 



gation of clay. Perhaps the stabilizing 

 effects of organic matter on clay com- 

 pensate for its deleterious effects on 

 sand. 



2. — Clay reduces the amount of organic 



matter available for binding to sand 

 particles because it has a much greater 

 surface area per unit weight than sand 

 and a greater capacity to bind organic 

 matter. 



3. — Clay may form the necessary link for 



the aggregation of sand particles. Al- 

 though the organic molecules on ad- 

 jacent sand particles form no bonds 

 with each other, they may form weak 

 bonds with adjacent clay particles. 



The foregoing is, of course, pure speculation 

 but it points out the need for further research 

 into these phenomena and suggests the possi- 

 bility of the use of soil amendments to stabilize 

 organic sands. 



7 



