Home_logo

Fact Sheets Physical Indicators

View All Fact Sheets

Making Sense of Physical Indicators

Many indicators of the physical quality of soil measure inherent characteristics of the soil, which means they are largely outside a farmer’s control. These are derived largely from the parent material of the soil and change very little over time or as a result of management. These properties can be measured once and used to group sites that are likely to respond in similar ways to management e.g. sand soils; seasonally waterlogged soils.

Making Sense of Physical Indicators - Queensland

Many indicators of the physical quality of soil measure inherent characteristics of the soil, which means they are largely outside a farmer’s control. These are derived largely from the parent material of the soil and change very little over time or as a result of management. These properties can be measured once and used to group sites that are likely to respond in similar ways to management e.g. sand soils; seasonally waterlogged soils.

Making Sense of Physical Indicators - NSW

Many indicators of the physical quality of soil measure inherent characteristics of the soil, which means they are largely outside a farmer’s control. These are derived largely from the parent material of the soil and change very little over time or as a result of management. These properties can be measured once and used to group sites that are likely to respond in similar ways to management e.g. sand soils; seasonally waterlogged soils.

Measuring Soil Texture in the Field

Soil texture is an estimate of the relative amounts of sand, silt and clay particles in a soil. The physical and chemical behaviour of a soil is influenced strongly by soil texture, which will vary due to the differences in the type and mineral composition of the parent material, the soils position in the landscape, and the physical and chemical weathering processes involved in soil formation. Soil texture affects the movement and availability of air, nutrients and water in a soil and is often used to estimate other soil properties, particularly soil water properties, if no direct measurements are available. A simple measure of soil texture is the way a soil feels when manipulated by hand.

Measuring Soil Texture in the Lab

Particle size analysis (PSA) determines the relative amounts of sand, silt and clay in a soil. These size fractions are the mineral component of a soil and together determine soil texture. PSA is a laboratory alternative to field texturing (see Measuring Soil Texture in the Field fact sheet) and offers a more reliable determination of particle size distribution. There is only an approximate correlation between hand texturing and PSA, because hand texturing relies on qualitative interpretation of texture while PSA measures exact amounts of individual particle sizes.

Measuring Soil Texture in the Laboratory - New South Wales

Particle size analysis (PSA) determines the relative amounts of sand, silt and clay in a soil. These size fractions are the mineral component of a soil and together determine soil texture. PSA is a laboratory alternative to field texturing (see Measuring Soil Texture in the Field fact sheet) and offers a more reliable determination of particle size distribution. There is only an approximate correlation between hand texturing and PSA (McDonald et al., 1998), because hand texturing relies on qualitative interpretation of texture while PSA measures exact amounts of individual particle sizes.
Soil texture is an inherent soil quality property that has a major influence on several other properties that influence agricultural potential (White, 1997). In particular soil texture influences nutrient retention, water storage and drainage. Soils with a higher proportion of sand retain less nutrients and water compared to clay soils.

Bulk Density - Measurement

The soil bulk density (BD), also known as dry bulk density, is the weight of dry soil divided by the total soil volume. The total soil volume is the combined volume of solids and pores which may contain air or water, or both. The average values of air, water and solid in soil are easily measured and are a useful indication of a soils physical condition.

Bulk Density - On Farm Use

The soil bulk density (BD), also known as dry bulk density, is the weight of dry soil divided by the total soil volume. The total soil volume is the combined volume of solids and pores which may contain air or water, or both. The average values of air, water and solid in soil are easily measured and are a useful indication of a soils physical condition.

Water Availability

Of the water entering a soil profile, some will be stored within the rooting zone for plant use, some will evaporate and some will drain away from the plant root zone. Plant available water is the difference between field capacity (the maximum amount of water the soil can hold) and the wilting point (where the plant can no longer extract water from the soil) measured over 100 cm or maximum rooting depth.

There will still be water present in the soil profile, however it is contained in pores that are too small for the plant roots to access. Soil texture, soil structure and hence plant rooting depths are the crucial factors in
determining the amount of water available for plants to access.

Soil Water - Tasmania

Efficient irrigators should aim to minimise the time soil is in a saturated or dry state, and maximise the time when water is readily available to the plant. Soil is like a big sponge — it can only soak up a certain amount of water and it can only do it at a certain rate (infiltration rate). When soil is saturated there is no benefit in applying more water. Excess water only produces plant stress, waterlogging, drainage to watertable below the root zone, run-off and leaching of fertilisers.

Soil water is held in soil pores (the spaces between soil particles). There are two forms of soil water:
1. Water held tightly to the soil particles (adsorbed water).
2. Water held in the pores between the soil particles (capillary water).

Roots remove water from the soil pores by creating suction. Plants use water from large soil pores first because it is more difficult for the roots to remove water held by the small soil pores. Some plants can extract water from drier soil more easily than others.

Waterlogging

Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space for plant roots to be able to adequately respire. Other gases detrimental to root growth, such as carbon dioxide and ethylene, also accumulate in the root zone and affect the plants.

Plants differ in their demand for oxygen. There is no universal level of soil oxygen that can identify waterlogged conditions for all plants. In addition, a plant’s demand for oxygen in its root zone will vary with its stage of growth.

Waterlogging - Tasmania

Waterlogging can limit agricultural productivity in many areas of Tasmania as the state enjoys relatively high rainfall which normally occurs with an excess of rainfall over evaporation in winter and spring. A range of soil orders experience parts of the year when they are saturated due to high regional water tables, low rates of water conductivity, perched water tables or seepage.

Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space (anaerobic) for plant roots to be able to adequately respire. Other gases detrimental to root growth, such as carbon dioxide and ethylene, also accumulate in the root zone and affect the plants. Plants differ in their demand for oxygen and a plant’s demand for oxygen in its root zone will vary with its stage of growth.

Raised Bed Cropping

Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space for plant roots to be able to adequately respire. Lack of oxygen in the root zone causes root tissues to decompose, and causes roots to appear as if they have been pruned. Consequently, plant growth and development is stalled. If anaerobic circumstances continue for a considerable time the plant eventually dies.

When the waterlogging is identified (see Waterlogging factsheet) the first thing to consider is improving drainage. Options vary from shallow surface drains (i.e. Spoon and W-drains) to more intensive drainage using wide-spaced furrows to raised beds. The efficiency of surface drainage increases respectively as does the degree of management. This fact sheet addresses the raised beds and is a short summary of “A Manual for Raised Bed Farming in Western Australia” (DAWFA Bulletin 4646).

Raised Bed Cropping - Tasmania

Traditionally, Sodosols in Tasmania have been used for pasture production but they are being used increasingly for cereal, pea, poppy and potato cropping. Sodosols are difficult to crop as they usually become wet and unworkable in winter and occasionally flood.

Raised beds are an excellent means of eliminating waterlogging, and are designed to create and maintain:
1. A deepened seedbed that is not dense and does not constrain root growth, with compaction limited to the furrows.
2. A seedbed with more roots and sufficient large pores for good aeration, infiltration and drainage.
3. A short distance and a reasonable height from the bed centres to the furrow base for a substantial hydraulic gradient to stimulate lateral drainage.

Subsurface Compaction

It is estimated that up to 13M ha (70 %) of Western Australia’s agricultural soils have moderate to high susceptibility to subsurface compaction. Subsurface compaction is caused by compression from agricultural machinery traffic with the compacted layer forming between 10 and 40 cm.
In contrast, compaction from stock trampling is confined to the surface 15 cm of soil. In addition to compaction, hard layers can also form as a result of natural soil packing and chemical cementation processes and these may occur throughout the soil profile.
These hard layers slow or in extreme cases prevent root growth and restrict root access to water and nutrients.

Subsurface Compaction - Queensland

  • Soil compaction is caused by compression from machinery traffic or stock trampling, or soils may be naturally hard-setting.
  • Poor root growth, swollen root tips and horizontal root growth can indicate a compacted layer (hardpan), usually between 10 and 40 cm.
  • Compaction can be prevented or reduced by minimising vehicle traffic when the soil is wet, reducing the number of tillage operations and adopting controlled traffic farming.
  • Amelioration of compaction in cracking soils is best achieved by using plant roots to promote the formation of cracks (biological ripping), and inducing drying and shrinking to form cracks. Cultivation when the soil is dry can also hasten the natural breakdown of clods.

Subsurface Compaction – New South Wales

The majority of agricultural soils in Australia have developed subsoil physical constraints, in particular compaction. Subsurface compaction is caused by compression from agricultural machinery traffic with the compacted layer forming between 10 and 40 centimetres. In contrast, compaction from stock trampling is confined to the surface 15 cm of soil. In addition to compaction, hard layers can form as a result of natural soil packing and chemical cementation processes and these may occur throughout the soil profile. These hard layers slow or in extreme cases prevent root growth and restrict root access to water and nutrients.

Soil Compaction - Tasmania

Soil compaction is the process of increasing the density of soil by packing the soil particles closer together causing a reduction in the volume of air. Soil water acts as a lubricant increasing compaction when a load is imposed on the soil.
Compaction usually results in less plant root proliferation in the soil and lowers the rate of water and air movement. As a result, the amount of water available to the crop is often decreased. Slower internal drainage results in poorer subsurface drain performance, longer periods of time when the soil is too wet for tillage following rainfall or water application, increased denitrification and decreased crop production. Increased compaction also adds to the energy consumption by tractors for subsequent tillage.

Controlled Traffic Farming

It is estimated that up to 13M ha (70 %) of Western Australia’s agricultural soils have moderate to high susceptibility to subsurface compaction Subsurface compaction is caused by compression from agricultural machinery traffic with the compacted layer forming between 10 and 40 cm.

Controlled Traffic Farming - NSW

In cropping soils compaction is caused by compression from agricultural machinery traffic with the compacted layer forming between 10 and 40 cm. Compaction results in decreased pore space around the root zone of the plant, creating an environment high in density and strength. Less pore space reduces soil water holding capacity, and water that is available to plants is held more tightly in the smaller pores. Therefore the plant has to exert more energy to extract water rather than using that energy for producing yield. Low soil porosity can also be detrimental to soil biological activity. There are limited options for ameliorating subsurface compaction apart from physical tillage of the compacted zone. However, management practices can be implemented to reduce or avoid subsurface compaction.

Controlled Traffic Farming - Queensland

Much of Queensland’s cropping and grazing lands are affected by, or susceptible to, subsurface compaction. Subsurface compaction generally occurs between 10 and 40 cm depth, and is caused by the compression of the soil by agricultural machinery or stock traffic. A compacted soil lacks the interconnected air spaces that are essential to the movement of water, gases and plant roots. This can limit plant yield and increase the energy required for cultivation.
There are limited options for ameliorating subsurface compaction. However, management practices can be implemented to reduce or avoid subsurface compaction.

Soil Structure Decline - Tasmania

Soil structure describes how mineral particles and organic matter are arranged to form aggregates, as well as how pore spaces are arranged within and between aggregates. Soil structure is the clods and aggregates that you can see rather than soil texture which you can feel. Soils with degraded structure can result in low yields and are difficult to manage due to a restricted range of soil wetness for tillage operations. If a soil has poor structure this can lead to problems with drainage due to the blocking of soil pores resulting in a decrease in the rate at which water can infiltrate the soil and drain through the soil. Compaction can lead to reduced aeration when wet, particularly on heavier textured soils such as Ferrosols, resulting in restricted volumes of soil available for root growth. The ability of plants to penetrate the soil is also reduced when structure is poor, which affects access to both soil nutrients and moisture, and so crop yields. Soil compaction leading to increased soil strength can limit plant growth by restricting root elongation as well as limiting the range of tillage options for soil preparation. Poorly structured soils are more likely to form a surface crust after heavy rainfall and are more easily eroded by wind or water.

Seedbed Soil Structure Decline

Surface soil structure decline generally results in one of two things – hardsetting or crusting. A surface crust is typically less than 10 mm thick and when dry can normally be lifted off the loose soil below. Crusting forces the seedling to exert more energy to break through to the surface thus weakening it. A surface crust can also form a barrier reducing water infiltration.

Seedbed Soil Structure Decline - Queensland

Hard-setting or crusting soils are usually indicators of poor soil structure, which leads to poor water infiltration, poor crop/pasture growth and difficulties when cultivating.
Where structural decline is due to sodicity, gypsum application can improve soil structure and lead to increased crop yield.
Increasing soil organic matter and decreasing traffic by stock and machinery can improve soil structure in lighter textured soils.

Grdc_logo_inline_cmyk_(small) Logo_daff_(small) Sbi_logo Uwa Avon Ag_dept Ghost_media