Soil structure monitoring tools
Monitoring is an essential part of 'best practice' farm management to assess where you are currently (eg for benchmarking) and also to help you improve your performance. Monitoring can be done as part of a formalised Environmental Management System (EMS) or as a stand-alone part of overall farm management. This document covers some of the basic issues about understanding soils and suggests other more detailed resources to help with monitoring.
The physical characteristics of soils include all the aspects that you can see and touch such as:
- porosity (the space between the particles)
- stone content
Good soil structure contributes to soil and plant health allowing water and air movement into and through the soil profile, storing water for plant growth and to support machine and animal traffic.
While some soils are naturally better structured than others, some physical characteristics of soils can be changed with good management. It is therefore important to monitor the physical characteristics of soil to understand soil condition. It is also important to ensure that management practices are not contributing to the decline of the soil eg, excessive traffic causing compaction (reducing the amount of macropores, or spaces between the aggregates, therefore reducing the amount of air and water into and through the soil).
Soil texture, structure, drainage characteristics and soil colour
It is the combination of the mineral fractions (gravel, sand, silt and clay particles) and organic matter fraction that gives the soil characteristic known as texture. Texture grades depend upon the amount of clay, sand, silt and organic matter present.
Figure 1: Soil aggregate
The solid part of the soil is made up of particles such as organic matter, silt, sand and clay and together they form aggregates represented in Figure 1. Aggregates are held together by clay particles and organic matter. Organic matter is one of the major cementing agents for soil aggregates. The size and shape of aggregates give soil a characteristic called soil structure.
Soil structure influences plant growth by affecting the movement of water, air and nutrients to plants. Sandy soils have little or no structure but are often free draining. With higher clay contents the soil structural strength increases, but its drainage ability often decreases. Heavy clays can hold a large amounts of water and as infiltration rates are slow, they tend not to be well drained, unlike sand or loam soils with no or a lower clay content. The amount of soil pores and the pore size relate to the drainage capacity of the soil. The larger size and fewer the number of pores the easier it is for water to move though the soil profile as seen in Figure 2.
|Sandy Soil: Note size of particles and fewer pores but larger pore size. Water can freely drain through this soil.||Loam type soil: Note different sized particles, mixture of sands and clays. There are more pores varying in size than in sandy soil. Drainage is slowed by well drained.||Clay soil: Soil particles are small size with many small pores. Water tends to get trapped in the pores due to high tension causing drainage to be often poor and slow.|
It is not just the soil type that affects structure and drainage but also the activities or environmental factors occurring to them. Root and earthworm activity can improve soil structure through creating large pores. Excessive cultivation, removal of crop residues and increased traffic contribute to soil structural decline, through compaction of soils, reducing pore size and breaking down of soil aggregates.
The chemical make up of soils will also determine structure. When high amounts of sodium are present (>6% exchangeable sodium percentage (see soil fertility monitoring tool for details)) clay particles separate and move freely about in wet soil. These soils are known as sodic soils. When sodic soils come in contact with water the water turns milky as the clay disperses and when the soil dries out a crust is formed on the surface. Sodicity can be overcome by applying gypsum.
Slaking is the breakdown of aggregates on wetting, into smaller particles. Slaking generally occurs when intense rainfall hits dry soil, the aggregates collapse as a result of the pressure created by the clay swelling and the trapped air expanding and escaping. This process can block up pore spaces and when the soil dries a crust is formed causing further infiltration and seedling emergence problems.
Soil colour can indicate the organic matter content of soil, the parent material soil is formed from, the degree of weathering the soil has undergone and the drainage characteristics of the soil as in Table 1.
|Soil Colour||Dark brown||Black||Red||Yellow||Grey, Blue/green hues|
|Indication||High organic matter content||Humus|
Lighter coloured soils can generally indicate low fertility for example white sands. Whilst darker soils (like black clays) are quite fertile. There is a large range in between.
Determine soil drainage
The drainage of a soil is an important characteristic to assess, as many plants prefer well-drained soils. If a soil is poorly drained, sufficient oxygen cannot get to the plant roots which can stunt or kill the plant. Soils that are very well drained can limit plant capture of water in drier environments or in dry years due to insufficient water holding capacity. The colour of the soil is the main indicator of how soils drain, refer to Table 1. Other important indicators are the texture of the soil, the presence of buckshot and stones and the dispersability and friability of the soil.
Refer to Baxter and Williams book Part 3 page16 to determine the drainage of soil.
Monitoring physical characteristics
Assessing soil texture
A simple way to determine a soil texture and its characteristics is by hand texturing. When texturing soil it is important to understand the behaviour feel, colour, sound and cohesiveness of the soil, which is achieved by making a bolus (wetting the soil and forming a ball).
|Texture Grade||Behaviour of moist bolus (ball formed in palm of hand)|
|Sand||Coherence, nil. Single sand grains adhere to fingers. If you press the bolus between your fingers, holding close to your ear, you will hear the sand grains rubbing against each other.|
|Loamy sand||Slight coherence. Discolours fingers with dark organic stain. Ribbon length 1.0 cm.|
|Clayey sand||Slight coherence; sticky when wet. Many sand grains stick to fingers. Discolours fingers with clay stain. Ribbon length 1.0 cm.|
|Sandy loam||Bolus just coherent but very sandy to touch. Ribbon length 1.3-2.5 cm. Can hear sand grains (see Sand description above).|
|Fine sandy loam||Bolus coherent. Sand can be felt and heard when manipulated. Ribbon length 1.3-2.5 cm.|
|Light sandy clay loam||Bolus strongly coherent but sandy to touch. Ribbon length 2-2.5 cm.|
|Loam||Bolus coherent and spongy. Smooth feel, may be greasy. Ribbon length 2.5 cm.|
|Loam fine sandy||Bolus coherent and slightly spongy. Fine sand can be felt and heard when manipulated. Ribbon length 2.5 cm.|
|Silt loam||Coherent bolus, very smooth to silky when manipulated. Ribbon length 2.5 cm.|
|Sandy clay loam||Strongly coherent bolus sandy to touch. Medium sand grains visible. Ribbon length 2.5-3.8 cm.|
|Clay loam||Coherent plastic bolus, smooth to manipulate. Ribbon length 3.8-5 cm.|
|Silty clay loam||Coherent smooth bolus, plastic and silky to touch. Ribbon length 3.8-5 cm.|
|Fine sandy clay loam||Coherent bolus, fine sand can be felt and heard. Ribbon length 3.8-5 cm.|
|Sandy clay||Plastic bolus, fine medium sands can be seen, felt or heard in clayey matrix. Ribbon length 5-7.5 cm.|
|Silty clay||Plastic bolus, smooth and silky to manipulate. Ribbon length 5-7.5 cm.|
|Light clay||Plastic bolus, smooth to touch; slight resistance to shearing between thumb and forefinger. Ribbon length 5-7.5 cm.|
|Light medium clay||Plastic bolus, smooth to touch, slightly greater resistance to ribboning. Ribbon length 7.5 cm.|
|Medium clay||Smooth plastic bolus, handles like plasticine. Has some resistance to ribboning. Ribbon length 7.5 cm.|
|Heavy clay||Smooth plastic bolus, handles like stiff plasticine. Has firm resistance to ribboning. Ribbon length 7.5 cm or more.|
When texturing soil it is important to understand the behaviour feel, colour and sound cohesiveness of the soil, which is achieved by making a bolus. For example, a sandy loam will only just stick together (slightly coherent) and there will be noticeable sand grains which can be seen and felt and heard if you place the bolus close to your ear and squeeze it.
It is then important to form a ribbon from the bolus to determine the clay content of the soil. The longer the ribbon the higher the clay content. The length of the ribbon is measured against a ruler and along with the behaviour of the soil can be compared with the descriptions on the soil texture
table. The seven steps below and Table 2 allow you to assess soil texture.
Step 1. Grab a handful of soil and feel the dry soil in your hands, look for any obvious gravels or sand particles, note these if they are present.
Step 2. Remove any grass, leaves or root hairs.
Step 3. Moisten the dry soil in your hand with small amounts of water. It is important that the soil is damp but not wet.
Step 4. Manipulate the soil in the palm of your hand, rolling it into a ball (bolus).
Note when doing this any characteristics of the soil:
- Can you visibly see sand grains?
- Can you hear sand grains when you rub the soil between your thumb and index finger and place to your ear?
- Is the bolus sticking together?
- Is the bolus easy to manipulate?
- Is the bolus spongy?
- Is the bolus sticky?
- Are your hands discolouring?
- What colour is being left on them?
Step 5. Using thumb pressure against the middle joint of the index finger, press out a 2mm thick ribbon from the damp bolus, allowing it to break off naturally.
Step 6. Measure the length of the ribbon noting the length and repeat Steps 5 and 6 a further two times to gain an average length.
Step 7. Using the information gained from observing and feeling the soil and the average ribbon length, refer to Table 2 Guide to common soil textures and find the corresponding soil texture.
Breaches of the Water Act
Breaches of the Water Act may occur if appropriate approval has not been sought from the local Water Authority when soil sampling, installing piezometers or neutron probe access tubes. In general, if any soil sample, piezometer, soil moisture tube, is greater than 3 metres deep, or if it intercepts the groundwater, it may have to be registered with the relevant water authority. Individual water authorities could have different approaches to registration. Breaches of the Water Act are an offence that involves a financial penalty.
Registration requires a series of steps that generally involve:
- contacting the local water authority and applying for installation licence
- this licence will require maps of locations, depth, purpose and information regarding near by infrastructure. A dial before you dig, that locates underground infrastructure, power etc may also be required. If the person has never registered a bore, or a hole in the ground so to speak, then it is strongly advised they contact the water authority to determine the steps involved.
- the cost to register - roughly $400 for first hole and an additional $50 for second etc, but this may vary with the water authority.
The registration process exists for several reasons. For example:
- in an attempt to prevent pollution by aquifer leakage, this is why soil sampling is included in the registration process if groundwater is intercepted.
- to provide an identification number and location of groundwater bores, and generate funds to run the groundwater data bases.
- to ensure no illegal groundwater extraction.
The registration process also means that only a registered driller can install the holes or collect the soil samples. If you unsure of the groundwater depth in your region, then using a registered driller removes any risk.
There are generally no exceptions to the rule, however there are variations in how the Act is governed by water authorities, so contact with water authorities is essential. Also, as a low cost risk management strategy, it is recommended to contact the dial-before-you-dig hotline (phone 1100) and ascertain the location of any infrastructure (power, telecommunications, gas etc.).
References and additional resources
Baxter, N. and Williamson, J. (2001). Know Your Soils. Agriculture Victoria - Bendigo. Centre for Land Protection Research. Part 1, 2 and 3.
Charman, P.E.V. and Murphey B.W. (1992). Soils their properties and management. Soil Conservation Commission of NSW. Sydney University Press. Chapter 16. Soils and farming practice. Harte, A.J.
Northcote, K.H. (1979). A Factual Key for the Recognition of Australian Soils. 4th edn. Rellim Tech. Pubs. Adelaide SA Australia.
NATA accredited laboratories - www.nata.asn.au
ASPAC accredited laboratories - www.aspac-australasia.com