We conducting research on whether intensive rotational grazing can increase soil organic carbon and nitrogen content in local ultisols. One thing we have noticed is that the same SOC content can result in red, red-brown, or black soils. So, it doesn't appear to be a SOC threshold that causes the soil to be black. SOC contents in our soils are quite low (typically less than 4% in the upper 2 cm) and rapidly decline in the upper 10 cm.
I can't answer the colour question but advise that if you are interested in quantifying changes in C storage you need to sample to at least 40 cm, or the top of the impermeable layer if there is one.
The soils are black in colour due to the presence of one or more of these:
1. Titaniferous magnetite
2. Organic compounds of iron and aluminium
3. Colloidal hydrated double iron and aluminium silicate
4. Soluble humus
The colour indicate good drainage. Iron found within the soil is oxidized more readily due to the higher oxygen content. This cause the soil to develop a 'rusty' colour. The colour can be darker due to organic matter.
Manganese may be one of the reasons for imparting black colour to soils. Still looking for a comprehensive research data on this aspect.
See the following excerpt of the link below:
“ … Color development and distribution of color within a soil profile are part of weathering. As rocks containing iron or manganese weather, the elements oxidize. Iron forms small crystals with a yellow or red color, organic matter decomposes into black humus, and manganese forms black mineral deposits. These pigments paint the soil (Michigan State Soil). Color is also affected by the environment: aerobic environments produce sweeping vistas of uniform or subtly changing color, and anaerobic ( lacking oxygen), wet environments disrupt color flow with complex, often intriguing patterns and points of accent. With depth below the soil surface, colors usually become lighter, yellower, or redder.”
https://extension.illinois.edu/soil/less_pln/color/color.htm
Thanks everybody! We are collecting soil cores to a depth of 60 so we can calculate C storage. The rocks that are weathering are high-grade metamorphic, typically granitic gneisses. I'm not sure how much manganese is in the soils, but that could be an issue. The soils are clay and clay loams (sometimes silty clay loams) that are dominated by kaolinite, some gibbsite, and quartz. At depth, partially weathered rock fragments can add muscovite and feldspar to the soil. The soils are on hills, and drain moderately well depending on the level of compaction. The ferrihydrite is definitely the source of reddish colors. Five years ago, the first set of soils often showed no variation in color in a 20 cm soil profile because of the degraded nature of the soils. Now, A horizons are easier to denote and the colors are changing. We measure soil SOC and SON using a Leco TruMac Series C/N macrodeterminator. The key to the problem is that soils have the same organic content, are often in the same soil series, but have different colors. The better the aggregate structure, the darker the color, which makes me think that organic iron and aluminum compounds may be the case.
Some nice explanations about black color of soil. I think , soil mineralogy alongside its interaction with humus , presence of redox sensitive elements coupled with geomorphology collectively guide the intensity of black soil coloration. Interestingly , in smectite soils of central India, very often we observe black color of sand , simply due to humus coating... .
PDF enclosed for further reading...
have you tried to analyze correlate the "black colour" of your soil (try absorption readings of 1:10 extract with 0.43m HNO3) with standard analysis for humic acid, fulvic acids and humates? My guess is that the overall quality of your OM decreases with increasing dar colour!
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