有機物質
Organic matter tends to make the soil color darker. Humus, the final stage of organic matter breakdown, is black. Throughout the stages of organic matter breakdown, the colour imparted to the soil varies from browns to black. Sodium content influences the depth of colour of organic matter and therefore the soil. Sodium causes the organic matter (humus) to disperse more readily and spread over the soil particles, making the soil look darker (blacker). Soils which accumulate charcoal exhibit a black color.