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Soils 1: Structure

Course: Sustainable Soils | Last Updates: 7th October 2015

 
Alice Midmer
Project coordinator
Leaf
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Protecting soils and soil structure is vital for cropping now, and in the future. Soil structure regulates erosion and gaseous exchange rates, the movement and storage of water, soil temperature, respiration and development, nutrient cycling, resistance to compaction and supports biological activity. 

Good structure also increases the window of opportunity to cultivate at the right time and minimise tillage costs, tractor hours, horsepower requirements and passes required to prepare seed-beds.

What does it look like?

Good soil structure consists of well-formed aggregates, which are easily broken down between the fingers when moist. Vertical fissures lead roots downwards and the soil is structurally stable. Poor soil structure has hard, sharp edged aggregates that are more difficult to break apart with horizontal “pans” restricting root growth and development.

How to measure it?

There are two tests for assessing soil structure. The Quick Test is recommended for all fields and the Visual Soil Assessment (VSA) is a detailed test for your best and worst fields. 

1. Quick Test

Dig a small pit (40cm x 40cm x 50cm deep). Lift out a section of soil and examine it. 

Assess:

  • Topsoil depth: Generally shallower under permanent pasture than cultivated soil
  • Colour: Soils rich in organic matter will be dark brown in colour, while rusty, grey mottling indicates poor drainage and previous waterlogging
  • Smell: Waterlogged, anaerobic conditions prevent break down of organic matter and manures, denoted by a foul smell
  • Roots: Should be down to 50cm or more in depth
  • Earthworms: 10-15 earthworms is ideal in your sample
  • Cracks, pores and burrows: Look for clear vertical channels between blocks that allow movement of water, air and nutrients

2. VSA

The VSA is based on key soil conditions and plant performance indicators of soil condition, presented on a scorecard. Soil condition is ranked by assessment of the soil indicators alone. It does not require knowledge of the field’s history. Plant indicators, however, require knowledge of immediate crop and field history. 

Soil condition should be assessed using a visual score of 0 (poor), 1 (moderate) , or 2 (good), based on the condition observed when comparing the field sample with the three photographs above.

The scoring is flexible, so if the sample you assess does not clearly align with any one photograph and sits between two, a score can be given, for example, 0.5 or 1.5.

You will need: 

  • One spade
  • One plastic basin (approximately 35cm x 35cm x 20cm deep)
  • One heavy duty plastic bag
  • VSA photographs 
  • Notebook and pencil

When to carry out the assessment

For arable cropping, ideally in spring when the soil is moist or after harvest and before cultivation. Also a second test after the final cultivation to check the condition of the seed-bed. For grassland, ideally in spring or after the final silage/hay cut and before any cultivation. The VSA should be carried out when the soil is at the correct moisture content for cultivation, or is sufficiently dry to prevent compaction by wheeled traffic

If you are not sure when to sample, apply the “worm test”. Roll a “worm” of soil in the palm of one hand with the fingers of the other until it is 50mm long and 4mm thick. If the soil cracks before the worm is made, or you cannot form a worm (for example if the soil is sandy), the soil is suitable for testing. If you can make the worm, the soil is too wet for testing. As long as the soil moisture content is right, test at a similar time each year. This will make you results more comparable from year to year.

Setting up

Allow about half to one hour for each field. The assessment process takes about 10-15 minutes for each sample, and you should sample three or four sites in each field.

Avoid areas such as headlands or loading areas, which may have had heavier traffic than the rest of the field (VSA can also be used, however, to assess the effect of high traffic loading on soil quality).

Make a note of where you carry out the assessments so you can return therefore future monitoring. 

Carrying out the test

Take a test sample. Dig a 20cm cube of topsoil with a spade. If the topsoil is less than 20cm deep, take off the subsoil before moving to the next step.

The drop shatter test

Drop your sample a maximum of three times from a height of 1m (waist height) into the plastic basin. Lay the soil out in the large plastic bag, grading the pieces so that the coarsest clods are at one end and the finest aggregates at the other end. This provides a measure of aggregate size and distribution. 

Soil score

Using the table below as a guide, assess the structure score of your soil. 
 

Indicator Poor (0) Medium (1) Good (2)
Physical      
Soil structure (VSA score) (a) Hard, platy aggregates difficult to break Somewhat blocky Crumbly, loose

What influence does soil type have?

Soil texture and type can have a large influence on soil structure. Where sandy soils often lack structure, self-structuring heavier soils are more prone to compaction. Silt although fertile, can be structurally unstable especially after heavy rainfall events. Mixed soil types (for example sandy loam) can often be a happy medium but soil type is only one of many factors at play when determining soil structure. 

Improve soil structure

Adjusting soil management can have a large influence on soil quality and influence the long-term sustainability of your soil. 

Measures include:

  • Organic Matter: Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability improves, in turn improving the soil’s ability to take up and hold water. Aim to build up and maintain organic matter in your soil. The most obvious way to do this is through regular additions of organic fertilisers such as compost and farmyard manure. Reduced tillage and cover crops are other practices that will increase the organic matter.
  • Cover crops: Deep rooting cover crops, when selected appropriately, can alleviate subsoil compaction and improve topsoil structure. Select the right crop for the right situation and rotation but deep rooting radishes, turnips, mustards and rye can be good options. Appropriately timed cover cropping will reduce the risk of erosion by reducing the amount of time soil is left bare.
  • Non-inversion Tillage: Non-inversion tillage can significantly improve soil stability, workability and soil water holding capacity. Minimising the number of machine passes will reduce compaction, further contributing to better soil structure. Less invasive tillage techniques allow organic matter to build up; help decrease erosion and compaction; increase workability; increase and improve soil biodiversity and cut costs.
  • Grass leys: Grass can be a valuable break crop if a grass ley fits within the rotation. The dense rooting of grasses conditions the soil and improves structure. Once planted, short term grass leys require minimal further cultivations allowing organic matter to build-up within the soil, which is of great benefit once the field returns to arable.
  • Adjust Technique: Over-aggressive and overpowered cultivation and destoning can destroy soil structure. Ensure correct calibration of equipment and assess in detail the impact destoning is having on your soil, by completing a VSA before and after. Is destoning necessary in the rotation or can the practice be reduced?
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