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Soils 6: Biological Health

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

Alice Midmer
Project coordinator
Biography >>

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.

Biological Health: How do I measure it?

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 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 video

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