Expert advice for farmers on achieving sustainable soils

Protecting soil health is vital for the health of crops now, and in the future. This ultimate guide covers the key areas of soil health, how to test your soil, the numerous ways you can improve the quality of your soil and how to manage your soil to maintain good health and ensure sustainable cropping.

Soil structure

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 good structure 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?

Soil drainage

Grey soil colours due to reduced forms of manganese and iron in the topsoil indicate the soil is waterlogged and deficient of oxygen for long periods.

This poor aeration leads to a build-up of carbon dioxide and methane, and reduces the ability of plants to take up oxygen, water and nutrients, particularly nitrogen, phosphorus and potassium.

Poor aeration also slows the breakdown of organic residues, and can induce chemical reactions toxic to plant roots.

What to look for

The colour of the soil is a useful indication of soil drainage, aeration, soil wetness from late autumn to early spring, and soil damage.

A warm, uniform, brown and black soil is a sign of good drainage but grey mottled soil is an indicator of potential drainage issues.

Influence of soil type

Soil type can have a large influence on the drainage capabilities of a soil. Subsoil texture can also play an important part.

Heavy clays often struggle to drain water away efficiently especially if they are compacted.

By contrast sandy soils drain very quickly with all reference farms with sand soils scoring the maximum 2 for drainage, while other soil types were more variable.

However, as a result of easy drainage, sandy soils can struggle with water holding capacity which is a problem for some farmers in drier weather.

Improving drainage

Despite variations in soil type, there are a number of measures that can help improve drainage on all soil types.

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

LEAF (Linking Environment And Farming) carried out on farm soil interviews with a wide variety of farmers across the UK.

Despite a large variety of soil types, many had adjusted their soil techniques in the same way to improve their soil quality.

The following measures were used particularly to improve drainage.

Artificial drainage

Ensure any artificial drainage is properly maintained and repaired regularly.
If previous drainage is insufficient consider the full benefits of introducing or replacing the drainage network.

Cover crops

Deep rooting cover crops can help improve the drainage capabilities of the soil through loosening the subsoil.

Ground cover with shallow rooting plants to hold soil in place on steep or exposed land helps to minimise erosion.

Cover crops can also help to suppress weeds – as a soil sterilant when chopped and cultivated in to the soil which release chemicals. And cover crops are now part of the CAP greening requirements.

Biological improvement

Burrowing earthworms are an important part of a well-drained soil, especially in minimum tilled land. Under minimum tillage, earthworm populations increase and are often relied upon to ensure sufficient drainage.

For further information see Hampden Bottom Farm, College Farms and High Meadow Farm.

Structure

A good structure and minimal compaction are important constituents of good drainage. If drainage is causing a problem on your land, ensure it isn’t rooted in poor soil structure or compaction.

Soil compaction

Compaction is where soil has been squashed into a solid impermeable layer, either at the surface or within the topsoil and subsoil. Compacted layers restrict the movement of air, water and nutrients down the soil profile.

It leads to poor root growth, which stresses the plant and reduces its response to nitrogen and other nutrients. These LEAF videos give useful guidance:

What to look for

Soil can be compacted at many different levels throughout the soil profile and can be damaging at any level. Poor and shallow rooting of crops is an obvious effect of compaction.

Gateways and tracks through fields are particularly vulnerable areas to compaction and care should be taken to minimise this where possible.

Measuring compaction

Dig a hole 50cm deep when the soil is not excessively wet or dry. Look how far the roots and moisture extend down the profile and look for any obvious change in the soil structure.

Where the spade meets resistance is where compaction starts – this will also help identify what the cause is and whether action is required.

Using the table below as a guide, assess the compaction score of your soil. An average of the two scores gives an overall figure.

Indicator

Poor (0)

Medium (1)

Good (2)

Physical

Topsoil compaction (d)

Obvious hardpan, poor rooting

Some restrictions to penetration and root growth

Easy penetration and good root growth

Subsoil compaction (e)

Hardpan and/or soil occurs in large compresses pieces, roots absent

Soil occurs in medium pieces, root penetration with some difficulty

Soil occurs in small pieces, roots penetrate without difficulty

Soil type can have a large influence on compaction, with heavy clay soils being particularly vulnerable, while coarse, sandy soils rarely suffer from compaction.

Improving compaction

Despite variations in soil type, there are a number of measures that will minimise compaction on all soil types.

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

Linking Environment And Farming (Leaf) carried out on farm soil interviews with a wide variety of farmers across the UK.

Despite a large variety of soil types and farm types, many had adjusted their soil techniques in the same way to improve their soil quality.

The following measures were used particularly to minimise compaction. Further information on the farm-specific implication of these changes can be found in on LEAF’s website – scroll down to the case studies under the Simply Sustainable Soils section.

Importance of timeliness

Allowing soil conditions to dictate cultivations timing will help minimise compaction. Working wet soil is the most common cause of problems.

It is sometimes impossible to avoid working wet soil while continuing to crop, but sometimes not cropping and allowing the soil to recover can be more economic over a whole rotation.

Traffic

Low-profile floatation tyres are a straightforward and often necessary precaution to minimise compaction. Using tracks on machinery further minimises compaction and can increase the cropped area.

Crab wheels are often used on particularly heavy harvesting equipment. Fertiliser spreading techniques can be manipulated to minimise machinery passes and alleviate compaction.

Tramlines

Minimising the compacted areas by adhering to tramlines is another technique to help keep overall compaction to a minimum.

Control traffic farming or GPS steering could be a farm-wide technique that would help keep tramlined areas to a minimum. Alternatively, adjusting tramlines every year and subsoiling them out could help spread the compaction.

Minimum tillage

Minimum tillage techniques can help minimise compaction. The reduced number of passes necessary in minimum tillage is a large contributor to this. The build-up in earthworms and in particular their casts, as a result of minimal soil disturbance, can further alleviate compaction.

Hardcore gateways

Gateways, especially in livestock fields, are liable for compaction. Hardcoreing the area is a relatively straightforward and long-term solution. Be aware of the effects of tracks through fields for feeding and the impact these can have on the soil. Minimise where possible.

Poaching

Poaching via livestock is a common cause of compaction and is difficult to alleviate in grassland. Ensure necessary movement of stock in wet conditions and consider bringing stock inside where appropriate.

Consider the advantages of permanent pasture for winter grazing. One famer valued the deeper roots and denser swards in preventing the land from getting as wet.

Rolling

Rolling grassland often leads to unnecessary compaction. Consider relying on natural fissure drainage instead where possible. Using ballast rollers is another way to avoid unnecessary compaction.

Organic matter

Humus, the dark-coloured organic material in the final stages of decomposition, is relatively stable reservoir of plant nutrients and water.

OM content can have a large influence on the soil structure and this in turn can impact compaction and drainage issues. A soil rich in OM is also preferred by much soil biodiversity as it provides a healthy living environment.

Similarly, soils rich in OM are intrinsically healthy and nutritious for crops. In this way, OM is an integral part of soil health but can often be overlooked in favour of more obvious soil qualities. Soils that are rich in OM are generally dark brown/black in colour; light brown is generally sign of lower OM content.

This video provides useful guidance:

Measuring OM

Testing for organic matter percentage can be undertaken in a laboratory. It can also be assessed visually using the table below as a reference.

Indicator Poor (0) Medium (1) Good (2)
Nutrient balance and exchange      
Soil organic matter status Organic matter levels are low, soil is crusty, cloddy, hard. Light brown in colour. Organic matter levels are moderate, some crusting and clods. Brown in colour. Organic matter levels are high. Soil is friable, with good soil structure. Dark brown in colour.

Sandy soil has inherently low organic matter while peaty soils have naturally higher levels and in some situations rich organic peat is used to help boost the content.

Clay and loamy soils are somewhere in between, depending on individual composition, use and management. Land under grass will often have higher organic matter levels than arable land with comparable soil type.

Despite these variances, OM is important in all types of soils and with the appropriate soil management can be improved.

Improving OM

Despite variations in soil type, there are a number of measures that can help enhance the organic matter status of your soil and its availability to crops and grassland.

Adjusting soil management practices appropriately can have a large influence on the soil quality and can particularly influence the long term sustainability of your soil. These farm case studies illustrate some of the options.

Test OM %

Few farmers measure the percentage of OM in their soils because it is unlikely to change significantly from year to year. However gaining a specific measurement can help track whether its increasing and where it varies across the farm.

An organic matter measurement should be added to regular soil testing every five years or so to get a clearer picture and assess impact of practices.

Increase organic additions

Addition of OM as farmyard manure (FYM) or slurry is widely adopted on farms and often the turning point in a soil’s performance.

Different types of organic matter additions provide different benefits to the soil but all can help improve the content and general soil health. Slurry and FYM are largely added for their nutrient value while box muck and compost are valued for their benefits to soil structure.

If organic materials are not readily available, there is always the possibility of swapping materials with neighbouring livestock farms. Application method of organic matter could have an influence on impact.

While most farmers broadcast manures, Low Field Farm was investigating umbilical cord spreading to minimise the impact of tractors, while manure at Llysun was drip fed to minimise volatilisation.

Ploughing straw

Ploughing straw back into the land can provide a really important supply of organic matter.

Since the straw burning ban, this has formed an essential constituent to many soil management plans.

Cover crops

Cover crops have many benefits. They can help reduce erosion as well as aid drainage but they can also form be an important source of organic material. Instead of leaving stubble or your soil bare, consider the benefits of including a winter cover crop.

Grass leys

Putting land into grass, or as a break crop, is a highly effective way to build up organic and restore soil structure and health. Introducing livestock on the farm to graze pasture also provides valuable FYM benefits.

This video shows the positive impact on yields and reducing run off:

Min-till

Not inverting the soil and disturbing it less via min-till is another technique to help minimise OM loss via oxidation and promote its build-up. Many arable farmers carry out some form of min-till during their rotation; strip-, min- or zero-till because of the benefits to soil quality and organic matter content.

Soil pH and nutrients

Grass and crop yields can be reduced by up to 20% where soil pH is below target. If the pH is wrong, nutrients applied to the crop may not be available to the crop, instead being lost through the soil, incurring costs and run-off issues.

The pH determines the relative acidity or alkalinity of a soil and is important to understand, and assess, in order to maximise crop growth. pH is measured on a scale of 1 to 14 (but less than 4 and more than 9 is uncommon), with 7 being neutral. Below 7 is acidic and above 7 alkaline.

Nutrients in the soil need to be managed so that the supply to the crop is matched by demand. N, P and K are all key nutrients which should be tested regularly, every three to five years. The availability of these nutrients is affected by soil pH in different ways and it is important to be aware of these impacts.

How to detect deficiency

Deficiency in nutrients and acidity cannot be detected visually in the soil. Poor crop performance is often the first sign that the soil is lacking nutrients.

Regularly testing soils and adjusting lime and nutrient applications accordingly can ensure good growing conditions are maintained and sufficient nutrients provided.

For continuous arable cropping, experts advise that the maximum availability of nutrients from the soil is achieved at pH6.5. In order to maintain an appropriate pH, soils should be tested every 3-5 years and treat acidic soils with a liming material. On soils where acidity is known to occur, more frequent testing may be needed.

How can I measure pH and nutrients?

  • Twist a sampling auger/soil corer down to 7.5cm in grassland fields or 15cm in arable fields.
  • In a bucket, collect 25 cores of soil while walking the field in a ‘W’, avoiding gateways/feeding areas.
  • Transfer a sample to a plastic bag and label.
  • Send to soil laboratory. Where there are different soil types it’s best to send multiple samples from one field. Sample every three to five years and (ideally) not within 6 months of manure, fertiliser or lime application.

What influence does soil type have?

Some soil types and conditions allow land to remain neutral or alkaline without the need for lime additions. Other soil types require regular applications of lime to remain within optimum parameters.

In addition, heavier land requires more lime per acre than lighter land to raise the pH the same amount.

The rock underneath the soil can play a part here also. For example, soil over limestone retains certain level of alkalinity inherently.

Similarly, soil type can have an influence over the natural fertility of the land. For example, sand is naturally less fertile whilst peat and silt retain more nutrients.

Despite this, some form of additions are utilised in all conventional farming systems to maintain optimum conditions.

Improve soil pH and nutrient status?

Despite variation in soil type, there are a number of measures that will improve chemical conditions for plants on all soil types. Adjusting soil management practices appropriately can have a large influence on the soil quality and influence the long-term sustainability of your soil.

  • Awareness of the conditions A comprehensive soil history of the P, K and pH levels throughout the farm and from individual fields is important to achieve before remedial action can take place. Similarly any further field history can help gain a fuller understanding of conditions.
  • Lime Applying appropriate amounts of lime will raise the pH effectively. Quantity of lime required will depend on your soil type, the pH and cropping. Seek specialist advice or see RB209 for details.
  • Soil mapping Soil mapping is a technique that is becoming more popular throughout the UK. Detailed soil maps give a fuller picture of soil health instead of relying on field averages.
  • GPS steering This allow more accurate targeting of inputs when and where required, particularly for N, P and K. Whilst this can reduce your overall fertiliser use, it will primarily ensure that nutrients are applied where they are needed most. This can help uniform the yield across a field, increasing overall production.
  • Organic fertilisers Substituting some synthetic fertiliser requirements with organic replacements has the added benefit of aiding soil structure.
  • Livestock Grazing livestock will add valuable FYM to the soil and in some situations farmers on sandy soils have added pigs into the rotation to increase soil fertility.
  • Clovers enrich the soil quality as they are leguminous and fix nitrogen from the air. Including them within your grassland, provides a valuable addition to the grazing quality and will enhance the soils nutritional composition by returning nitrogen to the soil.

Biological health

Alongside the influence physical and chemical properties have on the quality of soil, biological health also has an important part to play. Earthworms are a major indicator of biological soil quality and often play an important role in the improvement of soil health.

Increased numbers of earthworms help break down crop residue into soil organic matter and allow infiltration and soil aeration. A diversity of plant residues in turn support a wide variety of surface invertebrates.

What to look for

The easiest way to determine biological health and functioning is to assess the level of earthworms in soil. Where there are particular problems in specific fields, for example, where plants are underperforming, you may be even more interested in improving soil quality.

Observing the breakdown of plant residues, beetle and burrowing insect activity and smell are also good indicators of the biological health of your soil.

Earthworms are an excellent indicator of soil biological health and organic matter. The number of earthworms on your farm will depend on many different factors, including soil type, weather and land management, and there may even be big differences in parts of the same field.

Numbers of earthworms – your underground money makers – can be increased by reducing or eliminating cultivations, adding organic materials and growing green manure crops.

Earthworms prefer a near neutral soil pH, moist soil conditions and plenty of plant residues on the soil surface. They are sensitive to certain pesticides.

How to monitor earthworms

Carry out a quick visual assessment by looking in your soil pit for earthworms and their burrows. A more detailed test is set out below.

You will need:

  • Notebook to record results
  • Mustard powder
  • Watering can
  • Sample tray

Mix up a suspension of English mustard powder (50g in 10 litres of water) and pour over a 1sq m area. Count all the worms that appear from that area in the next 30 minutes. The higher your score, the better the soil quality. 10-15 earthworms is an indication of good health.

Using the table below as a guide, assess the biological health score of your soil. Use an average of the scores for an overall figure.

Indicator Poor (0) Medium (1) Good (2)
Biological health      
Earthworms (i) Low numbers, no casts or holes Moderate numbers, few casts, holes or worms High numbers (10+/sq m). Lots of casts and holes in tilled clods
Living organisms (j) Little or no observable soil life Some moving soil organisms Soil is full of soil organisms
Smell (in spring) (k) Pungent (sulphur) odour Some odour, mineral odour Sweet, “earthy” odour
Plant residues (l) Little or no plant residues Some plant residues, slowly decomposing Residues in all stages of decomposition

Biological healthy soil is linked to soil organic matter status, structure, compaction and drainage. A healthy soil encourages biodiversity and the biodiversity in turn provides benefits to the soil.

Soil type therefore can have an influence, but this is often as a result of its influence over other factors.

For example, sandy soil often has low levels of earthworms as a result of inherently low organic matter and organic peaty soil often provide a good habitat for them.

Land under grass generally has higher levels of biodiversity than arable land. This is as a result of the minimal disturbance, allowing the biodiversity to build up and develop.

How to improve biological health

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

Keep records

The majority of farmers did not measure biodiversity and therefore were unable to assess whether certain practices were beneficial or not. By keeping simple records of numbers of earthworms and plant residues, it is possible to track changes and diagnose problems that may not be obvious at first.

Minimum tillage

Minimum tillage techniques are more beneficial to earthworms as they involve less soil disturbance.

Ploughing kills 33% of all worms and so can be detrimental to populations. Ploughing at any stage in the rotation can minimise the benefits felt through adoption of minimum tillage at other stages.

This LEAF video shows how min-till benefits biodiversity

Reduce destoning

Destoning is one of the most damaging operations to earthworm populations and soil quality and many producers have stopped growing potatoes in their rotation as a result. Benefits of min-till can be overrided by destoning.

Return plant residues

Returning straw to the field and incorporating it into the soil is beneficial to the biological health of the soil. Since the straw burning ban, straw is never baled at College Farm – instead it is incorporated and is thought to be the most beneficial soil management practice on farm.

The benefits of incorporating straw are shown in this LEAF video


More information

See LEAF’s website for the LEAF Simply Sustainable Series including a PDF on sustainable soils