Soil Profiles

"dark, crumbly with a hint of stickiness,

3000 years young, 

seeks similar for fun and good times!"

Okay, so not perhaps the kind of profile you were expecting... :)

The soil profile is a 3-dimensional section of the soil.  In this, you can detect different layers (the 'horizons'), which are influenced by different factors in soil formation (see the 'Soil: What it is and where it came from' post) and also drainage and management practises.  A soil profile can vary from as little as a few centimetres, to many metres in depth.

The different horizons can be detected by being able to distinguish the separate colours, structure and textures from one to the next.  In theory...

Soil Profiles

Soil profiles can be categorised into 3 classes, which differ in the change in texture that can be detected when moving vertically through the profile.  Texture and the nature of the soil profile are two of the main features you will need to consider when classifying your soil.

Uniform Profiles

These have very small changes between horizons, or in fact no detectable texture difference at all.  You might have a deep clay loam, all the way to the bedrock.

Gradational Profiles

As you would expect, these show a gradual change in texture as you move down the profile, due to an increase in clay content.  For example, you might have a sandy-clay loam, moving gradually through horizons with increasing clay content, until you have a heavy clay subsoil.  This type of soil is good for growing.

Duplex

The name is again fairly explanatory, with these profile showing a distinct, and often sudden, textural difference between one horizon and the next.  An example would be where you have a sandy loam, over a heavy clay subsoil.

Broaden Your Horizons

Every soil profile contains at least one layer or 'horizon', usually more, which can be divided into the broad groups: topsoil, subsoil and rock.

Within these broad groups, we have different horizons which are named according to where they come in the profile and what their characteristics are.  There are many, many of these depending on which classification system you use and believe me, there are a lot, most countries seeming to have developed their own.

However, the main horizons you will be concerned with are the A, B and C horizons.  I like this Wiki Commons image, which shows them fairly clearly:

Topsoil - The 'A' Horizon

This zone is where the majority of the water and nutrient uptake occurs for plants.

• AO - This layer consists of the decomposing organic matter on top of the soil, for instance the leaf litter in a forest.  It's depth can vary greatly depending on location.  On an exposed sloping site, it may be very shallow or non-existent, in a forest it may be 50cm or so deep;
• A1 - This area contains the most organic matter, being where most of the biological activity occurs, and is normally darker in colour than the lower part of the profile; 
• A2 - Generally similar to A1, but lighter in colour due to their being less organic matter.  This horizon may not occur.

If an A2 horizon is present, this might be an indication of a drainage issue and also of restricted root penetration into the subsoil.

Subsoil - The 'B' Horizon

Plants whose roots are able to access this zone will be more drought tolerant.

• B1 - This horizon may not occur, but where it does it forms a transition between the A horizon and the B2 horizon, perhaps being closer to that of the latter;
• B2 - This is generally where the most clay occurs and is noticeably different in colour.  It might have splashes of bright colour.  Silica, iron and aluminium oxides, clay and organic matter winds up here after being washed out of the topsoil.

The presence of splashes of bright colour ('mottling') is often a result of drainage issues.  These occur where patches of the subsoil remain wet for long periods, leading to that area being anaerobic and starved of oxygen.  Often, they are found with an A2 horizon, but not always.  

Mottling rarely occurs in the topsoil, but would be an indicator of a major drainage issue if it did.

Rock

• C Horizon - This is where the bedrock has become weathered, but will still contain large lumps or shelves of unweathered rock.  There will be little soil development and it's likely to be too deep for you to check!
• R Horizon - This is the bedrock and consists of very large masses of rock which couldn't be excavated by hand.  In some places, this might be on or close to the surface (where there has been no soil development, or where the soils have eroded), but it's likely to be too deep to observe.

What about the 'O' Horizon in the picture?

The 'O' horizon is not always present, nor is it always included in a soil profile  report.  However, it is very important for the continued renewal of soil and, in particular, the organic carbon contained there.

It consists of decaying organic matter and may be non-existent, as in exposed sloped sites, or it may be 50cm or more deep, as in rain forests.

In your garden, this would consist of leaf litter, bits of sticks, mulch and decaying plant life.  It will of course have it's own compliment of fauna, whose role is to further break down the organic matter.

Colour Changes

One of the easiest was to distinguish the different horizons is due to colour changes.  

The A0 and A1 horizons contain higher levels of organic matter than the other horizons and are generally darker in colour as a result.  

The A2 horizon however, is lighter in colour, sometimes being described as 'bleached'.  It usually occurs where there is a heavy clay subsoil or a hard pan, that prevents drainage.  Water accumulates in the A2 horizon until it can drain away, causing leaching of organic material and sometimes clay particles, leading to the lighter colour.

Subsoils usually have a stronger colour, due to higher concentrations of iron, aluminium and clay.  They may also exhibit mottling (see above).

Leaching

This is the movement of material, such as nutrients, clay particles and organic matter, down the soil profile and can be the result of over watering in the home garden or on agricultural land.  It can also occur naturally due to heavy rainfall.

The rate at which leaching occurs depends on the topography, soil type, it's permeability, rainfall and the evaporation rate.

Gradational and duplex soils can show the effects of leaching, with clay particles being concentrated in the subsoil.  Sand and silt particles, which are larger, are left behind.  Organic matter generally only leaches from the A1 profile, leaving the soil looking greyer.

Plant nutrients, which are soluble ions, also leach out of the soil and their loss can make a soil increasingly infertile.  Sandy soils are particularly susceptible, because the large pore spaces allow water to drain freely, and they do not posses an electrical charge like clay, so do not hold onto nutrients.

Leaching is one of the ways in which soil texture and structure can change, sometimes quite suddenly, as in the case of sustained heavy rainfall and flooding.

In some cases, leaching may actually be weak due to annual rainfall not fully draining through the soil profile.  In these case, calcium deposits can build up in a layer at the depth the rainwater normally accumulates.

As a result of leaching over long periods, soil pH can be altered.  For example, soils in areas with a higher rainfall often have a lower, more acidic, pH due to soluble bases being washed away.  These are alkali elements, such as calcium and magnesium, can help to neutralise acids and their loss results in the pH dropping and becoming more acidic.

Soil Texture

The soil texture can also vary from one horizon to the next and assessing this can help you understand how the soil will function, how fertile it is, and what management practises you need to employ.

Soil texture however is a subject all of it's own... :)