Getting to Know Your Soil: Colour

Every soil has it's own combinations of characteristics, that affect the nature of your soil and how it performs.  These characteristics can be broadly grouped into three, inter-connected, groupings:

• Physical
• Chemical
• Biological

Lets Get Physical!

The physical characteristics of your soil are the aspects that you can actually see and feel.  They include it's colour, texture, structure, strength, depth and stoniness.

In this post, we're going to look solely at the colour of your soil.

Colour

We touched on colour a bit in the Soil Profiles post, when we discussed organic matter content and how leaching can affect the soil profile.  From that, we learned that colour is very important in helping to determine the different horizons, giving an indication of the amounts of organic matter and certain elements in each, as well as giving us an idea of what the soil's drainage may be like.

The colour of a soil can be affected by it's parent rock, the climate in which it formed, the environment in which it exists, it's drainage, and of course, the organic matter content.

There is a whole industry around producing soil colour charts based on the Munsell Colour System, but what the home gardener really needs to know to make a quick assessment is that soil can be in various shades of: grey, black, brown, yellow and red.  These are all easily recognisable to the layperson and you don't need an expensive colour chart to tell you what they mean!

• brown soils - these soils usually have a high level of decomposing organic matter, but not always.  The colour may also come from iron oxide, indicating that the soil is fairly well drained;
• yellow soils - these soils reflect the colour of iron oxide in a poorly drained situation, where the soil may become water logged for lengthy periods after rainfall;
• red soils - in these soils, iron oxide remains in it's ferric state and is not soluble.  This gives the soil it's red colour and is usually an indication that rain water drains through the soil quickly, allowing air to circulate;
• black soils - the colour indicates a high level of decomposed organic matter, humus, a source of organic carbon.  The darker the soil, the more organic carbon is contained there.  These are generally clay soils and can be either well drained, or poorly drained; 
• grey soils - these are generally due to remaining water logged and therefore anaerobic for long periods of time.  This colour occurs because iron, when in it's ferrous form, can become mobile and leach out of the soil.  Sometimes the soils might have a blue or green tinge, indicating that there is still some iron content;

A topsoil which is light in colour, can be an indication that nutrients, iron, aluminium, clay particles and organic matter has leached down to a lower layer, leaving the soil looking bleached.  This generally only happens to one layer of the soil, referred to as the A2 horizon (see the Soil Profiles post for more info), and can occur when there is heavy clay subsoil or hard pan, preventing the water from draining freely.  In some cases, the subsoil can become saturated.  The water then moves up and down the A horizon, depending on additional watering or rainfall, and steadily leaches out nutrients.

This is why it is important to carefully monitor your watering in the garden, as you may be inadvertently causing leaching of nutrients and thus leaving your plants open to disease and pests.

We also mentioned mottling in the Soil Profile post.  Mottles are patches of bright yellow, orange or red, on a brown, yellow or grey background, that indicate poorly drained soil.  The mottles form when iron oxides have become mobile in a waterlogged soil, then later reoxidize when the soil is once again drained and aerated.  You can generally tell the duration of the water logging by the colouration of the mottles.  If the background is brown with only a few mottles, then the water logging is infrequent, or of a short duration.  If there is a large grey area, then the duration was longer or occurs frequently.

By understanding what the colour of each horizon in your soil profile means, you will better understand it's drainage, fertility and how it should be managed.

Next, texture! :)