Food, Farming and Countryside Commission

How to assess soil quality

By Dr Iain Gould

Lincolnshire Institute of Agri-Food Technology

It is important to be able to assess the overall quality of the soils we work with by looking at their chemical, physical and biological condition. Many soil properties that are essential to plant growth are best assessed with laboratory testing (e.g. nutrient levels, organic matter content, pH). However, there are a number of other methods we can use to diagnose soil health in the field.

Anyone can carry out these tests, they are relatively inexpensive and can be applied year-onyear. This makes them great for measuring the impact of changes or introductions made on a farm to soil structure.

Soil physical properties

Good soil structure creates the ideal conditions – sufficient aeration, drainage and water retention – for plant growth. Soil structure is dependent on the arrangement and stability of the materials that make it up. Unlike some of the chemical properties of soil, the physical condition is best assessed in-situ. As such, some of the following simple techniques can be used to great effect.

Two of the more commonly used techniques to assess soil structure are the Visual Soil Assessment (VSA)(5) and the Visual Evaluation of Soil Structure (VESS)(6). These both involve breaking down as-dug soil into their smaller constituents and assigning a scoring system. Where the aggregates, or clods, are still large after breakdown, this could indicate low porosity, poorer structure and as such a lower score. Conversely, a mix including finer aggregates after breakdown could indicate the opposite (Fig 1a).

It is important to note not only the size but also the shape of the soil units. Angular aggregates indicate a poorer soil structure – likely a result of damage by machinery or undesirable conditions. Some structures are known as ‘platy’ – flat structures that break horizontally under pressure (Fig 1b) – these indicate a degree of compaction. This information is all incorporated into the scoring system.

Another simple way to assess soil structure in the field is with an infiltration test. This can be done by pushing a small pipe offcut into the soil (making use of a mallet and a plank of wood), filling the pipe with water and timing how long it takes to drain (Fig 1c). The water will find a route through the soil by exploiting channels and pore space. Greater infiltration means better drainage and rooting potential, likely a result of good soil structure.

Fig 1a: Soil aggregate size distribution following Visual Soil Assessment breakdown

Fig 1b: ‘Platy’ soil structure – horizontal cracking

Fig 1c: Soil pipe infiltration test

Soil biology

Soil is a living system, and the life within a soil is essential to its healthy functioning. Some of the more detailed tests for measuring soil biology can be expensive, however, there are cheaper and simpler methods. For the keen biologist, investing in a microscope and becoming familiar with the important species living in soil is a possibility. Here, we discuss some of the other methods adopted on farms around the country.

Worms are a vital ecosystem engineer in soil – recycling nutrients and influencing soil structure with their burrowing. Worm Counts are a great way to start, and can even be incorporated into other soil structural assessment (like the VSA). A cube of soil the width of a spade is dug from the ground and sifted through for 5 minutes, counting the number of worms. These can be compared year-on-year or carried out in different areas to build up an overall picture of the soil system of a farm/field. Recently, the ‘60 Min Worm’ and ‘30 Min Worm’ initiative, led by Dr Jackie Stroud, proved very popular – see www.wormscience.org.

Another important thing to consider is the microbiology of a soil. This can be tricky, given that microorganisms, such as fungi and bacteria, are invisible to the naked eye. Instead, we can measure decomposition – how much of a food source is broken down by microorganisms over time. Two of the more popular ways to look at this are the Tea Bag Index and the Underpants Test.

The tea bag index (www.teatime4science.org) is a test in which two unused tea bags containing teas of different types are weighed, and then buried for 90 days. The tea bags are then dug out, dried and re-weighed (Fig 2a). The loss in mass indicates the rate of the decomposition of tea carried out by the microorganisms.

The underpants test (www.farmersguild.org/soil-my-undies-challenge.html) is a more visual test based on a similar principle. Burying cotton underpants in different locations can provide a comparison of the rate of decomposition (Fig 2b), indicating the health of the biological community below ground.

Soil chemical properties Soil chemistry is more challenging to measure in the field, which is why it is beneficial to combine the techniques discussed above with laboratory testing. Under current practice, plant-available nutrients are measured by the processing of samples in a laboratory, however, handheld kits are available to measure soil pH and electrical conductivity in the field.

Fig 2a: Tea Bag Index – two teabags buried in adjacent holes

Fig 2b: University of Lincoln staff demonstrating the underpants test – high decomposition (left) and low decomposition (right) after burying in different soil environments