Soil colour – visual health and chemistry

Soil colour conveys a wealth of information. Not only are soil colours a key diagnostic feature of horizonation (and therefore classification) but they can also tell you about soil organic matter, clay mineralogy, salt and carbonate accumulations, and drainage conditions. Most people know that dark brown or black soils are good for growing plants. Why? Because they contain large amounts of organic matter, or are composed of dark minerals, or both. Organic matter improves soil structure and nutrition while dark minerals are often nutrient rich. Some examples:

  • Dark soils often contain more organic matter than pale soils
  • Black clays, red clays, and white clays are composed of different minerals
  • White crystals (salt) or white powders/nodules (carbonate) indicate chemical limitations
  • Red, yellow, black and brown soils indicate the soil is adequately drained while grey, green, and olive coloured soils indicate waterlogging
  • Coloured splotches might mean the soil gets waterlogged occasionally

But how is this useful? Soil colour is very useful for two pieces of information – soil capacity for sustaining plants and waterlogging. Soil capacity is fairly simple: in general, dark soils have a greater capacity than pale soils. Pale soils can be improved to a degree by adding organic matter but will always be limited by their inherent mineral composition.

Waterlogging is perhaps more important. I live in an area with hot, dry summers and very little stream flow. How can I tell if the garden (or, say, the basement you’re thinking of building) is prone to waterlogging? One way to find out is to dig a hole and see what colour to soil is. At the house I’m living in, the soil turns solid grey within about half a meter. So, yes, the soils under my house probably get waterlogged quite frequently. What does this mean for plants? The soils I’m on are also rather heavy somewhat alkaline clays, so any plant that likes well drained or acid soils is not going to be happy. Citrus, for example, would not thrive at my house.