Soil description

Soil horizons are layers within the soil profile and are approximately parallel to the ground surface. A soil profile is made up of several horizons and each is distinguished from the horizon above or below by being different in one or more characteristics. These differences include colour, texture, structure, consistence, and coatings.

Soil horizons are usually given letter codes (horizon notation) to identify the type of horizon. For example, the topsoil is referred to as the A horizon and subsoil horizons are referred to as B horizons. C horizons are those which have been slightly altered by soil forming processes and generally occur deep in the soil. Other horizon notations include  R for rock, O for organic material such as leaves or peat, and E for a layer leached of  organic matter, iron or clay. 

Each of these horizon notations can be made more specific by adding additional codes. E.g. Oh is an organic horizon of strongly decomposed material, Bt is a B horizon containing clay translocated from the horizon above, Bg is a pale wet horizon with at least 2 % mottles.

Please see the Soil Description Handbook by Milne et al. (1995) for all horizon notations and their definitions.

The colour is always described using moist soil.

Colour is described using a Munsell Soil Colour Book. This book contains pages of colours rather like a paint chart.

The soil is matched to the colour and the colour name and colour code are recorded.

10YR colour table (right-hand image): This is one of several pages from the Munsell Soil Colour book. The code specifying the colour in the top right corner is 10YR 8/8 and the colour name (not displayed) is 'Yellow'.

First the colour of the matrix is described. Any mottles are then also described using abundance, size, shape, contrast and colour.

Soil texture is the relative proportion of sand, silt and clay in a sample of soil. 

The texture of a sample can be assigned to one of several texture classes depending on the proportions of sand silt and clay in the sample. Examples of texture classes are 'silt loam', 'clay loam' and 'clay'.

Texture can be estimated by gradually wetting a soil sample and rubbing it between the fingers. The percentages of sand, silt and clay are estimated based on how gritty or sticky the sample feels.

A texture triangle (right-hand image) is then used to read off what the textural class is. In practice, only two of the three sizes need to be estimated, the third is derived by difference. E.g. if sand is 50 % and clay is 30 % then silt must be 20 %.

Peds: the building blocks

Soil is made up of particles and small aggregates, these can clump together to form larger aggregates (peds) which are the 'building blocks' of the soil. 

Soil structure is the:

• size
• shape
• degree of aggregation (if any) of the structural units (peds, clods),
• the way they are arranged in the soil,

and includes the spaces between and within the aggregates.

 
Types of Structural Units

• Soil aggregate 
  A general term for any distinct lump or clumping of soil particles, including peds, casts, clods or fragments.
  
  
• Ped
  An aggregate formed in the soil and may be an aggregation of smaller peds and is:
  -  separated from other similar aggregates by an obvious pattern of cracks or partings, or
  -  has distinct surfaces over at least one-half of the aggregate, or
  -  has coatings over at least one-half of its surface.
  
  
• Clod  
  An aggregate 100 mm or more in length and formed by soil disturbance such as cultivation.
  
  
• Fragment
  An aggregate less than 100 mm in length and formed by soil disturbance such as cultivation.
  
  
• Cast
  A ped of biological origin e.g. a worm cast.