Soil

Soil profile and soil horizons 

The soil profile is described as the total depth of the soil. Rock layers, water, hardpans and the lower limit of biological activity define the lower limit of the soil profile.

The depth of the soil profile can vary from shallow soil comprised of a few centimetres of soil over rock, to soil several meters in depth. The rooting depth of the soil is the depth to which plant roots can grow and so this part of the soil profile has the largest influence on plant production.

A soil profile can be divided into different layers or soil ‘horizons’. Different soil horizons have their own combination of soil properties that characterise them, including soil texture and colour. Each horizon can be defined by an upper and lower depth. The individual horizons combined form the soil profile.

The top surface soil layer is called the topsoil or the ‘A’ horizon. The topsoil usually has the highest amount of organic matter present of all the soil horizons which often results in this layer being darker in colour than the layers beneath it. This layer is very important for plant growth as it often contains much of the plant available nutrients.

Below the A horizon or topsoil is the B horizon. This layer generally has less organic matter than the topsoil and contains a higher proportion of clay. The depth and water holding capacity of this subsoil influences the plants’ ability to have full root growth. If a subsoil is compacted or has poor water holding capacity, root growth can become restricted.

The C horizon occurs below the B horizon and this layer is the parent material which the above soils form from. If this layer is shallow, plant root growth is restricted by the parent rock material.

Assessing the soil profile:

To assess the soil profile, dig a hole in the soil between 80 to 150cm deep. This is the minimum depth required to effectively analyse the soil profile for agricultural purposes. Measure the depth of the different horizons in the soil profile. Soil colour and texture changes will help you determine where one horizon ends, and the next one begins. Also assess the depth to which root growth occurs in the soil profile. It is also important to note that soil profiles can change across landscapes and so you may need to assess the soil profile across several different locations on your property to identify constraints impacting different areas.

When completing your soil profile analysis, see if any plant roots are present and to what depth – this can inform you if there is a soil barrier preventing plant root growth after a certain depth and you can use this information to help guide what soil management practices you may implement on your property.

Check out this link for a video on how to assess your soil profile.

Soil colour

Soil colour can be a useful indicator to predict several soil properties such as aeration, drainage, and presence of organic matter. It is also a helpful property in determining the soil horizons as described in the section above.

Soil colour is determined by four main factors. These include soil mineral matter, organic matter, the presence of iron and the soil moisture content. When assessing soil colour, it is best to do so on moist soil as the soil can appear lighter when dry.

Soil colour can be determined by assessing soil against a Munsell Soil colour chart. This is an international system which divides soil colour into the hue (the wavelength of the colour), the value (the tone – from light to dark) and the chroma (how saturated the colour is).

A Munsell colour chart is often used to assess soil colour more accurately, however, you can describe soil more simply without this chart by determining if it’s one of the five main colours: red, brown, yellow, grey or black. Other descriptions of soil colour include pale yellow, red-brown, yellow-brown, white, grey-brown and dark grey.

Some indications of soil properties that can be concluded from soil colour include humus being associated with black soils, red soil is associated with iron-oxides and white soils are associated with the presence of silicates or salts.

Some soils may have mottles present which are patches of colour in the profile different to the main colour dominating that soil layer.

Mottles may indicate several characteristics of the soil, such as waterlogging. For simple soil colour assessments, note the colour of the main mottles present and note which soil horizons they are present in throughout the soil profile.

This DPIRD document – “Soil groups of Western Australia: a simple guide to the main soils of Western Australia” – can be used to determine the soil group/s on your property and has a variety of information to guide you through the process. 

Soil texture

Soil texture is the proportion of sand, silt, and clay particles that make up the mineral fraction of the soil that heavily influences the physical and chemical behaviour of the soil.  Sand is a granular material composed of finely divided mineral particles. Silt is a granular particle between sand and clay particle size. Clay is a fine-grained particle; smaller than sand and silt.

Soil texture can influence soil ‘workability’, meaning the amount of water and air in the soil, as well as water infiltration. For example, sandy soil has low nutrient levels and poor water-holding capacity but is highly aerated. Clay soils have higher nutrients and higher water holding capacity but have less air-filled pores. Texture is not a soil property which can be altered but it is useful to understand your soil texture to guide soil management practices.

Soil texture estimation method:

Note – It is useful to determine the texture for each of the different horizons in your soil profile.

1. Sieve dry soil using a 2mm sieve to remove stones and gravel.
2. Estimate the percentage of stones and gravel in the soil from step one.
3. From the sieved soil, take a small handful of soil into the palm of your hand.
4. Add water to the handful of soil so it can be rolled into a ball and then knead the soil for 1-2 minutes. If you cannot form a bolus with the sample, then the soil is very sandy.
5. Form a ribbon of soil by pressing the soil between your thumb and forefinger. The longer the ribbon indicates a higher proportion of clay.

Check out a video from Soil Science Australia here which shows you how to assess your soil texture.

Soil particle diameters:

• Sand: 0.02 – 2mm
• Silt: 0.002 – 0.02mm
• Clay: less than 0.002mm

It is also important to measure the percentage of gravel in the soil as this influences the available water and nutrients. Gravel in soil occupies areas which would otherwise be occupied by sand, silt or clay particles and therefore has a number of implications for a range of soil properties. Gravel reduces the amount of water and nutrients in a soil. Gravel also has the effect of concentrating inputs such as fertiliser and herbicides. It is important to consider these effects when making soil management decisions, as several soil physical and chemical tests are only assessed using the sand, silt and clay components, omitting the effects of gravel.

Common field soil textures

Table 1. Field texture class descriptions as outlined by DPIRD (2020).

Properties of different soil texture grades. Source: Adapted from Donahue et al (1977) and SA Department of Primary Industries and Regions:

Soil structure 

Soil structure is an important part of assessing your soil profile. Soil structure is how sand, silt and clay particles, as well as soil air pores are arranged. Soil organic matter acts to ‘bind’ these particles into aggregates. A good soil structure with lots of aggregates allows water, nutrients and air to move between the aggregates promoting good plant growth and strong microbial activity. Unlike soil texture, soil structure can be influenced by soil management practices.  

Soil particles may not bond at all and are known as ‘apedal soils’ (such as very sandy soils) and are essentially structureless. This can be due to a low proportion of clay or low presence of organic matter to bind soil particles into aggregates. These soils are associated with rapid drainage and poor water holding capacity.  

Soil aggregates are also known as peds and these can vary in size from 2mm – 500mm. Good soil structure is defined by peds which form a definite shape and are between 1mm and 60mm in size. Soils which are structured into aggregates are known as ‘pedal’. These soils have sufficient clay and organic matter to allow soil particle aggregation and have good water holding capacity and can effectively store water and nutrients, promoting plant growth. Soils with good structure allow good water infiltration throughout the soil profile whilst also allowing it to drain excess water through the profile.

Soil structure can be assessed via determining soil compaction, infiltration and the soil particle aggregation via assessing the degree of slaking and dispersion of a soil aggregate when it is immersed in water.

Soil compaction

Soil compaction can occur in the topsoil and subsoil. Many of the soils in WA are affected by or are susceptible to compaction. Soil compaction is where soil particles become densely arranged and poorly structured as a result of losing soil macropores (air pockets).  

Compaction can restrict plant root growth leading to reduced plant biomass and grain yield. Compaction can also reduce water infiltration throughout the soil profile. Compaction is commonly the result of heavy traffic wheel tracks and soil cultivation for cropping. Livestock can also cause compaction of the topsoil. 

It is also important to consider the time of year you are assessing soil compaction. Soils will be more compacted in summer months as soils are much drier and so measuring soil compaction during the growing season may give you a more accurate measurement. To have reliable comparisons between years, make sure you measure soil compaction at the same time of year.

Making an at-home soil penetrometer:

How to make your own soil penetrometer: Take 50 cm of 3mm fencing wire, using approximately 15 cm of the length to curve into a circle to form a handle. Mark the straight piece of wire forming the penetrometer at every 2 cm.

Measuring soil compaction:

1. Push your penetrometer into the soil.
2. Once you cannot push the penetrometer any further into the soil, remove it and note the depth to compaction.
3. Repeat at numerous locations in the paddock to get an average depth of compaction.

Level of compaction and what this may indicate about your soil. (Source: North Central CMA, 2016 and DPIRD, 2020).

Soil water infiltration

Soil structure and compaction influence how water moves through soils and affects the rate of water infiltration through the soil.

Soil infiltration tests measure the rate of water movement into a soil. Soils which have a low infiltration rate are susceptible to runoff and water erosion and can have less water available to be used for plant growth. Soils with high infiltration rates can become waterlogged, resulting in the reduced presence of air-filled soil pores which can lead to restricted plant growth.

Method for measuring the rate of water infiltration:

You can make your own infiltrometer using a piece of PVC pipe 150mm in diameter and 110mm long.

1. Remove any stubble or vegetation from your sampling area.
2. Put the pipe 20mm into the soil ensuring it remains as level as possible.
3. Press the soil around the edge of the pipe to prevent water leaking at the edges.
4. Pour 500mm of water into the pipe, starting a timer from when all water has been poured in.
5. Measure how much water has been absorbed by the soil in 6 minutes and then multiply by 10 to get the value in mm/hour.

Interpretation of results:

Table 9. Interpretation of water infiltration results (Source: NQ Dry Tropics, 2019)

Soil water repellence

Water repellence is a common problem in soils across Western Australia. Soils which exhibit water repellence are known as hydrophobic soils. Soils become water repellent when waxy organic matter accumulates at the surface of the topsoil. The waxy organic matter or hydrophobic compounds which lead to soils becoming repellent originate from microbial and plant sources.  

Soil texture largely influences development of water repellence, with sand textured soils being more susceptible to developing this constraint. This is due to sands having a smaller soil surface area which can become coated with hydrophobic compounds. Therefore, fewer hydrophobic compound coated sand grains are required for a sandy soil to become water repellent than a soil with a higher proportion of clay, which has finer particles and a much higher overall surface area. Clays therefore require more hydrophobic compounds for a sufficient proportion of particles to become coated causing it to become repellent.

The previous section looked at water infiltration of soils. As you have probably already guessed, water repellence has a large impact on infiltration rates of water into soils. Water repellence not only reduces overall water infiltration, but it also alters the path in which water infiltrates the soil. Instead of entering the soil evenly as it would in a non-repellent soil, water preferentially infiltrates old plant root channels and macropores and in sloping landscapes, the water will run off and pool in flat areas or depressions, eroding soils along the way. Uneven soil wetting and runoff can lead to uneven or delayed crop and pasture establishment and losses of crops and pastures can be significant.

Testing for water repellence:

1. Using a water dropper, drop one drop of deionised water onto the surface of the soil from a height of approximately 1.5cm.
2. If the drop forms a sphere on the surface of the soil, some degree of water repellence is present.
3. Measure the length of time the drop of water remains on the soil surface – using the table below to determine the degree of water repellence:

Soil aggregation

Slaking is when wetting of a soil aggregate occurs and the pressure of air becoming trapped in the soil pores causes the aggregate to break down into smaller particles. These smaller soil particles then block pores in the soil matrix resulting in the formation of a crust on the soil surface. This process occurs due to a lack of soil structure and physical bonds with organic matter and plant roots.

Dispersion is where moist or wet clay in a micro-aggregate of soil breaks up into individual clay particles, creating a milky appearance as seen below. These individual clay particles penetrate soil pores, sealing the soil surface.

For a step by step video on testing for slaking and dispersion, check out this video from Soil Science Australia

Testing for slaking and dispersion:

1. Collect some soil samples from the soil horizons you are interested in, particularly the topsoil (0-10cm) and the subsoil.
2. Using a shallow dish or glass jar, put a small amount of distilled water (about 30-40 mm depth or just deep enough to cover the soil aggregates) in the container,
3. Place 3 completely air-dried soil aggregates 4-6mm in diameter (approximately pea-size) from your samples into the water.
4. Slaking can be assessed after 5 minutes. Slaking can be scored from 0 to 4 as follows:

Table 2. Slaking score as determined by the degree of slaking of a soil (Source: NSW DPI, 2000).

5. Dispersion can be assessed at 10 minutes and 2 hours and is scored depending on the degree of severity as below:

Table 3: The degree of dispersion from 0 to 3. (Source: NSW DPI, 2000).

What do these results mean?

Slaking

Table 4. The potential implications and possible management strategies for soil affected by slaking based on the degree of slaking present. (Source: NSW DPI, 2000).

Dispersion

Table 5. The potential implications and possible management strategies for soil affected by dispersion based on the degree of dispersion present. (Source: NSW DPI, 2000).