By defining your soil’s texture you will be better able to judge the effectiveness of various fertilisers and soil additives, understand the effectiveness of your irrigation practises better and have a clearer understanding of how your soil will react to temperature changes.
Soil texture is the proportion of sand, silt and clay in the soil.
It is defined as the particle size distribution of the solid inorganic parts of the soil.
Soil Texture normally refers to the solid particles less than 2mm in diameter known as the Earth Fraction.
On an average soil, as shown in the pie chart, the mineral particles only comprise about 45% of a soil’s volume. Air and water provide about 25% each and the active, live parts of the soil (plant roots, organic matter etc) usually comprise less than 5% of the volume.
Larger inorganic particles such as gravel and boulders are defined by average diameter. Particle classes for these large particles were defined in the New Zealand Soil Description Handbook (Milne et al) as follows:
- Boulders >200mm
- Very Coarse gravel 60 - 200mm
- Coarse gravel 20 - 60mm
- Medium gravel 6 - 20mm
- Fine gravel 2 - 6mm
The soil particles of sand, silt and clay have different characteristics influenced by their particle size and chemistry.
The four standard soil particle sizes are:
- Coarse sand 0.2 - 2mm
- Fine sand 0.02 -0,2mm
- Silt 0.002 - 0.02mm
- Clay < 0.002mm
Sand
Sands are generally sharp angular fragments of weathered rock and mainly consist of the mineral quartz with smaller amounts of other minerals. The particle size of sands is - Coarse sand (0.2mm-2mm) - Fine sand (0.02mm-0.2mm)
Properties of Sand -
- Affects the physical properties of soil.
- Sand is a large soil particle.
- Has large pore spaces between particles.
- Has a gritty feel.
- Warms up quickly.
- Has low water holding capacity.
- Has a low ability to absorb and hold plant nutrients.
- Provides good soil aeration.
How sand affects soil characteristics -
Sandy soils can be cultivated earlier in the spring than other soils, so are used for growing early crops. They produce free draining soils, but they may dry out too quicklyin dry conditions.
They are well aerated soils, but too much air may oxidise humus which is needed for soil fertility. Rapid drainage may lead to leaching and lower soil fertility.
Sandy soils don’t form good soil structures and may be blown away when vegetationcover is not present.
Textural Soil Classes
- Sand
- Sandy loam
- Sandy clay loam
- Loamy sand
Silt
Silt is a smooth soil particle formed from weathered rock and mainly consists of the mineral quartz, with smaller amounts of other minerals. Particle size - 0.02mm0.002mm
Properties of Silt -
- Affects soil physical properties.
- Has higher moisture retention than sand.
- Feels smooth and soapy.
- Slower drainage than sand.
- Less aeration than sand.
- Silt particles provide little in the form of nutrient supply or storage, unless it is coated with clay material.
How silt affects soil characteristics -
Pure silt soils do not have good structure so they are easily eroded. Silty soils form compact subsoil horizons. This affects soil drainage.
Textural Soil Classes
- Silt
- Silt loam
- Silty clay
- Loamy silt
Clay
Clays are formed by the chemical weathering of rock minerals. Particle Size - Less than 0.002mm.
Properties of Clay -
- Affects chemical properties of the soil.
- Absorbs and holds plant nutrients.
- Contains very small pore spaces (but has large total pore space).
- Has high water holding capacity.
- Warms up slowly.
- Becomes sticky when wet.
- Has a very large surface area.
- Has a sticky, buttery feel.
How clay affects soil characteristics - .
Clay soils swell when wet; they also crack when dry. Clay soils are fertile soils because the clay particles hold lots of ions. Clay particles “flocculate” (clump together) with the addition of lime and when soil drainage and aeration are improved.
Textural Soil Classes -
- Clay
- Clay loam
- Loamy clay
- Silty clay
- Sandy clay loam
Humus
Formed by the breaking down of organic matter, Humus is the very stable form of organic matter that remains when most of the plant and animal residues in a soil have been decomposed by micro organisms. Particle size Less than 0.002mm.
Properties of Humus -
- Has a negative surface charge.
- Dark in colour.
- Does not exchange ions as easily as clay.
- Important in the formation of soil structure.
- Holds a lot of water.
Characteristics of humus effects in the soil -
- Soils with humus in them are darker so tend to be warmer when they are dry.
- Soils high in humus can be waterlogged so can be cold at times.
- Soils with a low humus content can be susceptible to structural damage from intensive land use. eg cultivation
Soil Organic Matter
The term “organic matter” refers to all the plant or animal material in a soil. The productivity of a soil is related to the amount of organic matter which it contains. In general, organic matter exerts a very important control over the properties of a soil.
Most of the benefits of having organic matter in a soil result from the actual process of its decay. These benefits are the release of plant nutrients from decaying material and the formation of a dark brown finely divided substance called “humus”. Humus can increase the soils ability to store water and retain plant available nutrients.
Importance of Organic Matter in the Soil -
Organic matter improves the “tilth” and structure of soils. The secretions of bacteria which decay organic matter help to bind soil particles into aggregates. The cementing effect of organic matter helps to bind sandy soils and so lessen their movement by wind.
Organic matter is spongy and is able to hold large amounts of water. Soils that contain much organic matter, especially humus, are darker in colour than those with little organic matter. The darker colour enables the soil to absorb more heat from the sun, warm up faster in the spring and allow earlier plant growth.
Organic matter contains fairly large quantities of nitrogen, phosphorus, potasssium and other essential plant elements. During the decay of organic matter these elements may be released in a form which plants can use. The decay of legume plants results in an increase of available nitrogen in the soil.
The decomposition of organic matter which has a low nitrogen content (e.g wheat straw) may cause a temporary lack of available nitrogen in the soil. This is due to the increased bacterial numbers using the available nitrogen.
Various organic and inorganic acids are formed in soils when organic matter decays. These acids help to dissolve soil minerals. The colloidal nature of the decomposed organic matter (ie humus) results in a high cation-exchange capacity - higher than in mineral colloids (eg clay). These colloidal particles have a strong ability to hold on to plant nutrients and so reduce the rate of loss by leaching.
Organic matter is a source of food and energy for many of the soil organisms - including earthworms and useful soil bacteria.
Soils with a high organic matter content are less liable to be eroded. This may be due to:
- Organic matter cementing the soil particles into clumps which are less likely to be blown away.
- Organic matter improving the soil structure so that water will penetrate into the surface rather than running off the surface.
Sources of Organic Matter
Organic matter can be sourced from:
- Crop residues ie roots, leaves and stems of plants in various stages of decay. These make up most of the organic matter in a soil.
- Green manure crops. Leguminous crops help to increase the nitrogen content of the soil.
- Animal manures.
- Peat and organic fertilisers.
How to define your Soil Texture.
Soil Texture is worked out in the field by manipulating a moistened soil sample between your thumb and fingers. Working out soil particle size in the field is a fun part of soil mapping where you get to get your hands dirty!!
Try this simple exercise:
Take a sample of the soil from your garden or paddock on your property or borrow a handful from a garden area if you don’t have access to your own area. Take a small sample, moisten it slightly from a water bottle, and work it into a ball between your thumb and fingers,
How long do you have to work a soil between your fingers to get it to a putty like consistency?
Clay soils can take several minutes. Silty soils can be worked up very quickly.
The first picture is of a silty clay soil. The soil took a long time to work up so it could be easily moulded. It contained approximately 40-45% clay.
Will the soil adhere to your thumb as a ball?
Clay soils cannot be easily “flicked” from your thumb. Silty soils can be “flicked” off easily.
Does the soil form “peaks” on your finger and thumb when you pull it apart?
Clay soils form large peaks between your finger and thumb. Silty soils do not form peaks of any significance.
This is a clay soil - see how it peaks easily?
Can you clean the soil off your finger when you rub it across with your thumb?
Clay soils leave their yellow colour on your finger. Silty soils can be cleaned right off so that all you see is your finger.
Does the soil feel “soapy” or “buttery” when worked up?
Silty soils feel “soapy”. Clay soils feel “buttery”.
This is a buttery feeling clay soil but unfortunately we can’t give you the touch sensation via your internet yet! However, try this with a clearly silty soil and a clearly clay soil and you will understand the difference in touch.
Will the soil dilate (spread out) in your hand when you shake it?
Silty and sandy soils spread out across your hand when shaken. Clay soils stay intact. This is due to both particle shape and particle chemistry.
With a bit of practice, these field tests become very useful as a guide to soil texture.
Soil Texture can also be more accurately determined in the laboratory using a variety of mechanical sifting and other technical processes.
There are a number of standard diagrams, usually triangles or pyramids, that explain the standardised naming of soil texture according to the proportions or percentages of sand, silt and clay in the mix.
A standard soil texture diagram.
Here is a simple exercise to help you to understand how to work out the texture name for a soil from the percentages of sand, silt and clay in the Mix. Try it now ; the answer is below.
If a soil comprises 60% silt, 20% sand and 20% clay, what is the official description for its soil texture?
