Soil Structure

In order to understand how your soils behave, it is important to understand how your soil fits together and what it is made up of. Soil structure refers to the size and make up of the soil particles and clumps.
In order for soils to be porous and provide a good medium for plant roots to grow and get the nutrients that the plants need, the structure has to allow for an even distribution of the air, water and mineral particles and soil organic matter (which provides the glue that sticks soil particles together) through the depth of the soil.
Clay particles, organic matter and in some cases the presence of calcium and iron compounds help to bind the soil together into structural units.
Soils can restructure due to cultivation, natural fracturing processes when clay shrinks and biological activity.
Soil texture will determine how you manage your soil structure and also what the risk factors are, however understanding your soil and how it works is the crucial first step to excellent soil management.

Well structured soil allows the free movement of air and water through fissures (cracks) between the structural units. Pores within the units also allow the movement of air and water
Good soil structure is the key to maximising productivity reducing the risk of greenhouse gas emissions, and locks up carbon, irrespective of the farm type
A soil with poor soil structure has a high risk of generating run off. The risk of runoff is greatest when poor soil structure is near the soil surface
Soil structure deteriorates when structural units are deformed, producing a dense single mass of soil (or large soil units). This occurs when pressure is applied to a wet and soft soil
Pressure squeezes the soil units together and reduces pore space within the units. A dry soil can withstand pressure without deforming soil structure
Some soils are unstable when clay, calcium, or organic matter content is low. Unstable aggregates disperse when wet, forming a solid mass as the soil dries. When this occurs at the immediate soil surface the soil may form a cap

Soil texture refers to the relative properties of clay, silt and sand. These are the main components of all soils but they occur in differing amounts.

Sand 

• The largest particles found in soil
• These have the largest airspaces between individual particles through which air can freely circulate and water can easily drain.

Silt 

• Has smaller particles than sand but bigger than clay
• Air pockets and water channels are more restricted than in sandy soils

Clay  

• Smallest particles found in soil
• Spaces between individual particles are also small
• Air and water movement is restricted

Rub some moist soil between finger and thumb to assess soil texture. Sand feels gritty and when moulded into a ball soon breaks up. Silt feels smooth, silky, or floury while clay feels sticky when wet, looks shiny when smeared, and will hold a ball shape. Soil texture cannot be changed but knowing what it is and how it behaves will help plan management.

This diagram which comes from RB209 explains in more detail how to hand texture your soil. Click on the image to enlarge it.

Grab a spade and dig a hole!

By digging a hole in the soil and taking out a wedge, you will be able to see first-hand what the structure is like and if there are any problems.
Good soil structure is characterised by well-formed porous blocks with rounded edges, easily broken between the fingers when moist. Vertical fissures lead roots downwards. Soil with good structure is hard to damage. Poor soil structure has much harder, sharper blocks which are more difficult to break apart. Horizontal fissures restrict root growth and development, these soils are easy to damage.

You will need a spade approximately 20 cm wide, 22 -25 cm long and (optionally) a sheet, sack or tray (if not using the ground to lay your samples on), a small knife and a digital camera.

Samples can be taken any time of year but the soil should be moist; too wet or dry makes it difficult to get a representative sample. Roots are best seen in an established crop or for some months after harvest.

When selecting an area to sample make sure you select an area showing a uniform crop or soil colour or where you suspect there may be a problem. Within this area plan a grid to look at preferably 10 spots on the soil. For small experimental spots 3 or 5 samples per plot would be sufficient.

To carry out simple quick tests that assess soil structure based on the appearance and feel of a block of soil dug out with a spade, follow the instructions below.

N.B. Scores may fit between categories if they have properties of both. Scores of 1-3 are within an acceptable range, scores of 4 or 5 indicate compaction and a change of soil management is needed.

Material adapted from ‘Visual Soil Structure Quality Assessment’