SoilTexture

Soil texture measures the relative proportion of sand, silt, clay in the soil. It is literally the “feel” of the soil. Texture can be assessed by hand to identify the dominant particle type. If clay is dominant it will feel sticky, if silt is dominant it will feel very smooth, and if sand is dominant it will feel gritty. By measuring how strongly the soil sticks together when you manipulate (ribbon) it can help determine the amount of clay there is in the soil by the level of cohesion. Texture is usually directly related to water-holding ability with water holding increasing with clay content.

Soil texture affects the physical properties of soil, and in particular the storage of air and water, Soil Organic Matter content, transmission of water and nutrients, ease of root growth, workability and resistance to erosion. The proportions of sand, silt and clay particles in the soil determine soil texture. Sand particles are the largest (2.0 - 0.02 mm), silt intermediate (0.02 – 0.002 mm) and clay is the smallest (< 0.002 mm). The proportion of sand, silt and clay varies from one soil to another influencing the soil’s characteristics. Soil may also contain fragments of rock, or stones which comprise the gravel component.

From Rural Property Planning (1996), TAFE Rural Program Support Group Adelaide Institute of TAFE

In most soils, changes in soil texture occur with increasing depth. These layers may have either a sharp or gradual change in textural or permeability contrast (i.e. sand over clay).

Characteristics of sandy and clay soils.

Sandy soils

Any soil with more than 70% sand fraction and 15% or less of the day fraction is classified as a sand. As sand particles are relatively large, they tend to function as individuals and do not group or aggregate together. A soil dominated by sand particles will tend to:

  • allow rapid infiltration and drainage of water;
  • have a very low ability to hold water for plant use;
  • be a poor supplier of nutrients due both to the initial low fertility of the sand particles and the leaching resulting from rapid drainage;
  • be highly aerated;
  • exhibit practically no stickiness or plasticity. (Plasticity is that property of a material to be moulded into a shape and then to retain that shape). Consequently, it is not greatly influenced by changes in water content;
  • remain relatively loose and easy to cultivate.

Clay soils

To be classified as a clay, a soil must contain at least 35% of the clay fraction and in most cases not less than 40% (Brady, 1984). Note the strong influence day particles have on soil texture compared to that of sand (sandsized particles must be greater than 70% of the soil mineral mass before the soil can be classified as a sand).

A clay soil could be expected to exhibit the following properties:

  • Air and water movement into and through the soil is slow.
  • It may easily become waterlogged.
  • Generally they have a much higher supply of nutrients when compared to sands.
  • They respond to variations in moisture content often becoming sticky when wet and tough when dry. Some day soils expand and contract upon wetting and drying.
  • They can be difficult to cultivate due to their sticky and tough nature.

They are prone to compaction from surface traffic and/or cultivation.

Loam soils

This is the difficult group to precisely define and describe as it covers all the middle ground between the sands at one extreme and the days at the other. As a result, some loams will have properties similar to sands and others will be more similar to clays. The horticultural industry has long purported that 'loam' is the ideal soil in which to grow plants, providing them with an ideal mix of physical and chemical properties. As can be seen though, loams can be extremely variable. In fact a true 'loam, as defined by the texture tests described in the following section, is rarely encountered.

Field Method to Determine Soil Texture

Soil texture can be assessed in the field by kneading a small ball of moist soil and pressing between fingers to form a ribbon. The feel of the soil ball and the length of the ribbon indicate the texture grade.

  • Take a sample from the horizon to be textured.
  • Place enough soil (about the size of a small egg) in your hand and remove any twigs, large stones or stubble.
  • Use a spray mist bottle to moisten the soil with water until it just fails to stick to your fingers.
  • Let the water soak in and work the soil between your fingers and thumb until it is at the same moisture content throughout (around 2 minutes). You may need to add more water if not moist throughout.
  • Once the soil is thoroughly moistened, try to form a ball – note the feel of the ball or 'bolus'.
  • Press and slide the ball out between thumb and forefinger to form a dangling ribbon.

If the soil forms a ball, the look and feel of the ball can help you identify its primary components.

A Guide to Soil Texture Classes

(after Northcote, 1979)

Texture Class / Coherence / Bolus Characteristic / Ribbon Length / Other Remarks
Sand
<5% clay
Loamy Sand
5-10% clay
Clayey Sand
5-10% clay
Sandy Loam
10-20% clay
Light Sandy Clay Loam
15-20% clay
Loam
25% clay
Silt Loa m
25% clay >25%silt
Sandy Clay Loam
20-30% clay
Clay Loam
30-35% clay
Silty Clay Loam
30-35% clay
>25% silt
Sandy Clay
35-40% clay
Silty Clay
35-40% clay
>25% silt
Light Clay
35-40% clay
Light Medium Clay
40-45% clay
Medium Clay
45-50% clay
Medium Heavy Clay
50-55% clay
Heavy Clay
>55% clay / Nil to slight
Slight
Slight
Slight
Strong
Medium
Medium
Strong
Strong
Strong
Strong
Strong
Strong
Strong
Strong
Strong
Strong / Sandy to touch
Sandy to touch
Sticky when wet; sandy to touch
Sandy to touch
Sandy to touch
Spongy and smooth to touch no obvious sandiness or silkiness.
Very smooth or silky to touch
Sandy to touch
Plastic and smooth to touch
Smooth, plastic and silty to touch.
Plastic to touch, fine to medium sand seen, felt or heard in a clayey matrix
Plastic, smooth and silky to touch
Plastic and smooth to touch
Plastic and smooth to touch
Plastic and smooth to touch; handles like plasticine
Smooth and plastic to touch, handles like stiff plasticine.
Smooth and plastic to touch; handles like stiff plasticine
StrongPlastic / Cannot be moulded
About 5 mm
5 to 15 mm
15 to 25 mm
20 to 25 mm
About 25mm
About 25mm
25 to 40mm
40 to 50 mm
40 to 50mm
50 to 75mm
50 to 75mm
50 to 75mm
About 75mm
Greater than 75 mm
Greater than 75mm
Greater than 75mm / Single sand grains adhere to fingers
Fingers discoloured with dark organic stain
Many sand grains stick to fingers; fingers discoloured with clay stain
Medium sand grains (dominant size) readily visible
Medium sand grains (dominant size) readily visible.
Greasy to touch if large amount of organic matter is present.
Medium size grains visible in a finer matrix.
Slight resistance to shearing between thumb and forefinger.
Slightly greater resistance to shearing than light clay.
Moulds into rods without fracture moderate shearing resistance.
Moulded into rods without fracture; firm shearing resistance.
Moulded into rods without fracture; very firm shearing resistance

Simplified Guide to Soil Texture Groups

Texture classes are grouped into texture groups and grades as shown below. Texture groups and grades are important for profile classification particularly when using the Northcote Classification.

Texture GroupsTexture Grades

SandsSand,

loamy sand,

clayey sand

Sandy loamsSandy loam,

fine sandy loam

LoamsLoam,

silty loam

Clay loamsClay loam,

silty clay loam,

(fine) sandy clay loam

Light claysandy clay

silty clay

light clay

light medium clay

Medium Heavy Claymedium clay

medium heavy clay

heavy clay

After Northcote (1979)

For the purposes of this course a soil will be defined as a duplex profile if the change in texture in passing from the A to B horizons over a vertical interval of 10 cm or less is greater than one texture group. For example a texture change from a loam to a light clay or a sandy loam to a clay.

Uniform soil profiles must remain within the same texture group throughout the profile. For example the texture could be a sandy clay, a silty clay or a light clay from the light clay texture group,but not a light medium clay because it is in the medium clay texture group.

Gradational soils move from one texture group to another texture group as you move from the A horizon to the B horizon eg a change from a clay loam to a light clay as you move from the A horizon to the B horizon.

Northcote’s Profile Sub-divisions


Laboratory Method to Determine Soil Texture

If you have a laboratory analyses that estimates two of the three soil textural components (i.e. sand, silt or clay), you can use a soil textural triangle such as attached below to determine your soil type (Figure 3).

Can I modify soil texture?

It is likely to bea difficult and expensive prospect to amend the sand, silt and clay content of your soil. However, some techniques may prove beneficial when applied in the right circumstance.

  • Claying

It can be profitable in localised examples to amend sandy soils with clay (usually this must be from an on-farm source to limit transport costs) to treat non-wetting soils or sandy soils low in clay content (this will be discussed more in Topic 6. Growers have also used 'delving' techniques in the past to bring soil higher in clay content to the surface of sandy topsoils. Information regarding the quality of the subsoil should be sought prior to undertaking these types of management, as it is possible to cause problems closer to the surface (sodic soils, boron toxicity, etc.) and reduce plant growth.

  • Soil inversion/mixing

Mouldboard ploughing and other cultivation techniques that invert, mix or reform soils from different layers will influence soil properties. The response can be beneficial or detrimental dependent on the properties of the subsoil being brought to the surface, and vice versa. For example, water-repellent topsoil moved below the surface, may appear to have increased the infiltration of water into surface layers but may result in penetration barriers at greater depth (which may be more problematic in terms of managing the constraint).

  • Stone crushers or crusher dust application

Local supply is required for economic viability. Rock type will influence beneficial properties.

  • Soil ameliorants

Application of gypsum, lime, organic materials and other soil ameliorants can alter surface soil chemical characteristics and structural stability.

Links to information on describing a soil including soiltesting for colour, texture and strength

Better soils website A good general introduction to soils prepared by the Agricultural Bureau of South Australia.

1 / Monitor & manage soils for production.
Soil texture v1 2015