1.2 Describing a Soil

Soils cannot be judged simply by looking at the top few centimetres, without any regard for what lies beneath. To obtain a true picture we have to examine the various layers, horizons, which make up a soil profile.

The effective rooting depth is the part of the profile where roots are able to grow and take up water and nutrients therefore, this is the section of the profile that directly influences crop production. The rooting depth of plants will normally extend beyond the top 10cm and frequently extend below 1m. Root density will decline with depth but, wherever possible, soils should be examined to a metre deep, because the sub-soil can be just as important as the surface soil for root growth and plant productivity.

Soil profiles are best described by looking at a pit face, but if this is not possible, samples can be carefully extracted with an auger and laid out in order on a sheet of canvas.

To examine the soil profile

• Dig a hole large enough to have a clear view of one face - the hole needs to be at least 50cm deep.

• If possible, orient the hole so that the side to be examined, the ‘profile’, faces north and receives plenty of light.

• If  the paddock is in crop, dig across the sowing lines to expose the root systems of the plants.

• Remove any soil ‘smeared’ by the digging tools by carefully flicking out small amounts of soil with a knife, until the entire face is exposed.

On examining a soil pit, various layers or horizons may be observed. Each horizon has distinctive characteristics and may have different properties depending on the physical, chemical and biological elements of the horizon.

Top to Bottom

The importance of looking at the whole profile and not making assumptions based solely on the surface characteristics cannot be over emphasized. Sub-surface layers can vitally affect the productivity of soils, and a soil cannot be judged by its surface layer alone.

The effect of water, changing temperatures and chemical action, decompose rock to form the basic soil materials, sand, silt and clays.  In extreme conditions, rock such as granite or basalt, can be transformed into soil in a few years. In a climate like that of South Australia, the process takes much longer. Sandy materials originate from granite and clays (mostly) from basalt. Many South Australian soils have been built up from soil materials moved from elsewhere and deposited by wind or water many thousands of years ago and further changed by wind and weather. The ‘soil profile’, or the various layers in a soil, arise from the way in which this ‘parent material’ has changed under the influence of weather and vegetation since it was deposited. Many factors affect this development, including: the age of the soil, climate, slope, type of parent rock vegetation and soil movements.

The impact of profile development, over time, usually results in a soil with three main layers - (1) topsoil, (2) sub-soil and (3) partially weathered or original parent material. The soil scientist labels these layers the A, B and C horizons; together they form the soil profile.

A Horizon

The A horizon is the most important part of a soil, as it is often the cultivated layer and contains most of the available plant nutrients and soil organisms. The upper part of the A horizon (A1) usually contains organic matter in the form of decayed or partly decayed plant leaves, stems and roots. This usually gives it a darker colour than the lower half of the topsoil layer (A2), which often has little or no organic matter.

B Horizon

This is the zone which receives the materials leached from the A horizon. Because of this, it can differ from the A horizon in colour, texture and structure.

The amount of material brought down by leaching will depend on the porosity of the soil and the rainfall. Materials are leached down in the order of solubility. Soluble salts and gypsum may be leached right out of the profile. Calcium carbonate (lime), being less soluble, generally settles in the B2 layer, with the clay remaining in the B1 horizon. Iron and aluminium oxides are still less soluble and remain in the upper part of the profile, contributing to the soil colour. These oxides are only removed in very heavy rainfall areas. In many areas, the B horizon contains much more clay - also leached from the A horizon.

The depth and water holding capacity of the B horizon greatly affects the value of the soil for plant growth. A friable sub-soil allows water and air to penetrate and plant roots to make full growth. On the other hand, where the sub-soil is dense with no pores, root and plant growth may be restricted.

C Horizon

The C horizon is parent material, either rock, partly decomposed rock or soil materials (sand or clay) deposited many thousands of years ago. The nature and depth of this material has to be taken into account in assessing the value of a soil for plant growth.

If the soil is shallow and the C horizon is rock, plant roots cannot penetrate and growth is restricted. In weathered (decomposed) rock, such as shale, plant roots can generally find their way down through the cracks and crevices and gain access to water and some nutrients. If the C horizon is an old sand or clay deposit, the plant may also access water supplies from here.

Not all horizons are present in every soil. The profile in Figure 1, for example, has only A and C horizons. Due to age and limited rainfall the B horizon has not developed. This soil cannot store enough plant nutrients or moisture and consequently plant growth will be poor.

The soil in Figure 2 has all three horizons, but the C horizon is relatively unimportant because of the depth and good structure of the A and B layers. A soil such as this will have a good reserve of minerals, will be capable of storing large amounts of moisture and will allow unrestricted plant growth.

Soil Colour

The colour of a soil is perhaps its most obvious characteristic. Along with texture and structure, it forms the basis for soil classification. A change in soil colour in a profile is often linked to water and may help identify a perched water table or leaching.

How do Soils Obtain their Colour?

Two main factors influence colour in the soil:

• Mineral matter

• Organic matter

Climate also affects colour. In the warm, moist areas where weathering is more intense, the soils are more highly coloured than the soils of cool, dry climates.

Mineral Matter

Soils are formed by the breakdown of rocks. Sometimes these rocks give their own colour to the soil. This has occurred in the red soils of the Gawler Ranges.

However, more usually the colour results from compounds formed during the breakdown of the parent material. Red, yellow, grey and bluish-grey colours result from compounds of iron. Under average conditions of air and moisture, iron forms a yellow oxide. Where soils are better drained or drier, the colour changes to red. In waterlogged soils, where air is lacking, we get reduced forms of iron giving a grey, green, or bluish-grey colour.

Figure 3:  
What does soil colour tell us ?

(Source A Cass CRC Soil & Land Management)

Organic Matter

This imparts a brown colour to the soil. Humus, the final stage in the breakdown of organic matter, is black. Therefore, a soil high in organic matter will vary in colour from brown to black depending on the level of humus. The well drained soils of the warmer, moister areas are brown, due to larger amounts of organic matter; but the poorly drained soils, with larger amounts of humus, are black.

The intensity of the colour can also vary with the distribution of the organic matter and humus. The higher the sodium content of the soil, the darker the organic colour becomes. This is because sodium causes the organic matter and humus to disperse more readily and spread over the soil particles.

Usually colours seen in the field result from the mixing of colours formed by mineral and organic matter. A red-brown colour may be formed from a mixture of red iron oxide and brown organic matter. Organic matter build up in white sand can be seen by a greying in the top 20cm.

Like other soil properties, colour must always be observed throughout the whole profile. A soil may be reddish-brown on the surface and appear to be fertile. However, this colour may only occur to a depth of a few centimetres and then change to a mottled yellow-grey indicating that anaerobic conditions may exist for part of the year. Growth on such a soil would be poor.

Colour is objectively assessed by comparing the colour of a freshly broken surface of moist soil with the standard Munsell Soil Colour Charts.

Soil Texture

The proportions of sand, silt and clay particles in the soil determine soil texture. 

The particle size and percentage of particles of each component determine the feel of the soil and the physical properties relating to each texture class. 

In the field, soil texture is easily determined by working the soil in your hand.

Figure 4b

Relative soil particle size

Texture affects all physical properties of soil, particularly the storage of air and water, the soil organic matter level, the movement and availability of water and nutrients, ease of root growth and its workability and resistance to erosion. The proportion of sand, silt and clay varies from one soil to another influencing the soil’s characteristics.

Assessing Soil Texture

Soil texture is easily assessed in the field by observing the behaviour of a small handful of moist soil, kneaded into a ball and pressed into a ribbon. The soil is wet slowly, whilst kneading, to a moisture content such that the ball just fails to stick to the fingers. Kneading should continue for a minute to ensure that fine clay aggregates are completely broken down. The soil ball is pressed out between the thumb and forefinger to form a ribbon. The feel of the soil ball and the length of the ribbon indicate the texture grade.

By texturing a soil we are really establishing the percentage clay present. The special properties of clay and its impact on root growth and soil productivity are described later.

The ability to work the soil after rain or irrigation and the susceptibility to compaction and erosion is determined by soil texture. Figure 6 relates the physical properties of soil to the texture class.

Figure 5:
Behaviour of a moist soil ball and its texture grade

Source: A Cass - CRC for Soil and Land Management

Having established the texture of your soil using Figure 5, refer to Figure 6 and match your soil texture class to its physical properties. Do not forget that soil structure can moderate the effect of texture.

Figure 6: 
Physical properties of soils in different texture classes.

Source: A Cass - CRC for Soil and Land Management

Sands are naturally in a compacted state and are rarely further compacted by traffic. 

Source: A Cass – CRC for Soil and Land Management

1.2 Describing a Soil

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