Testing the health of your soil
- We asked members to carry out a series of visual soil tests that were designed primarily for farmers and growers with the aim of adapting them to a garden setting.
- Most of the tests were reproduced and adapted with kind permission from the Visual Soil Assessment (VSA) Field Guide by Graham Shepherd and the Visual Evaluation of Soil produced by Bruce Ball et al of Scotland’s Rural College (SRUC). The tests included texture, structure (visual structure test, porosity test), aeration (visual aeration test, smell test, surface ponding and crusting test), rooting depth (potential rooting depth test, fork test), biological tests (visual organic matter test, worm counts).
- Participants found the tests easy and quick to perform (over 90% of participants in all tests) and 70-80% of participants found each test useful.
- Feedback from users indicated that the tests encouraged gardeners to pay more attention to aspects of soil health and provided a useful framework for community gardeners and groups to assess their soil at a new or existing site.
- Experienced gardeners who had been working with the soil for a long time found that the tests didn’t tell them anything that they didn’t know already.
- Although a majority of users found that many of the tests showed that their soil was in good health, some indicated that their soils scored well with some tests, but poorly in others, so it was difficult to draw conclusions.
- A summary score of Texture, Structure, Aeration, Biological activity and Rooting depth with accompanying recommendations would provide more focus and help to translate the tests into practical action.
- Users were also questioned on their soil management practices. The key findings were:
- 83% used home compost in their garden
- 33% grew green manures every year, 42% grew them some of the time
- 70% practised minimal digging and cultivations
- Those that grew green manures were more likely to have good soil structure and porosity than those that never grew them
- As the majority of the group carry out good organic soil practices, it would be interesting to open it out to a wider group who carried out a wider range of soil practices.
- The next step is to assemble the tests into a guide for gardeners with relevant recommendations.
Many of us are aware that healthy soil is important for producing healthy crops, but we are not always sure of what we should be looking for when checking the health of our soil. In particular, the physical conditions of the soil are frequently overlooked. Conventional agriculture too often places a heavy emphasis on the chemical components of the soil and fertilizer requirements whilst neglecting the importance of good soil physical and biological properties to allow a healthy root system and biological life to flourish.
The soil texture refers to the size of the soil particles themselves. There is nothing you can do to change the texture of the soil unless you add soil imported from elsewhere. Particles are classed within three sizes categories: sand, silt and clay. The ideal soil has a mix of all three particles. Soils which are higher in sand content will drain and lose nutrients more quickly but will warm up more quickly in the spring. Soils higher in clay content will hold onto water and nutrients more strongly but often tend to remain colder and waterlogged. There are a number of ways testing soil texture, which can be done using a simple visual test.
The soil structure refers to how the sand, silt and clay particles are organised into aggregates and crumbs. This is one of the soil properties that the gardener can have most influence on. A good soil structure can mean the difference between thriving plants and those that are struggling. Good soil structure can take a while to build up, through growing green manures and additions of organic matter. It can be destroyed very quickly by digging or walking on the soil when it is wet.
An ideal soil retains water and nutrients under dry conditions but drains freely enough to prevent waterlogging under heavy rain. Ponding on the soil surface or the growth of algae are indications that the soil is draining poorly. The presence of blue-grey colours in clay soils is an indication that the soil has been starved of oxygen. Badly drained soil might also smell of rotten eggs.
Organic matter is derived from compost, broken down leaf litter, plant residues and animal manures. It is a vital component for healthy soil. It helps to improve soil structure on both sandy and clay soils, retain moisture and nutrients and provides food for biological life in the soil. Generally, soils that are high in organic matter are darker in colour, whereas soils that are light grey are devoid in organic matter.
A healthy soil should be teaming with biological life that does much to help plants thrive. Fungi, bacteria, other microbes and larger organisms all help to break down organic matter, releasing nutrients into a form that plants can take up. Earthworms help to mix up organic matter into the soil, and create pores to help the passage of water and nutrients. Mycorrhizal fungi add to the root system extending its reach by an order of magnitude. Without biological life, a plant has to work much harder to merely function.
A soil that is rich in biological life should have a sweet earthy smell like compost. You should also see plenty of earthworms.
We asked members to carry out a series of visual tests that were designed primarily for farmers and growers with the aim of adapting them to a garden setting. Many of the tests listed here have been reproduced and adapted with kind permission from the Visual Soil Assessment (VSA) Field Guide by Graham Shepherd and the Visual Evaluation of Soil produced by Bruce Ball et al of Scotland’s Rural College (SRUC).
Despite its simplicity, the methodology is based on scientific research and extensive testing has shown that farmers and growers are able to use the tool to accurately assess soil physical characteristics. Many soil physical characteristics, especially aggregate size distribution, saturated hydraulic conductivity and air permeability showed strong correlations with the visual assessment scores (Shepherd, 2003).
- How useful they found it?
- How easy they found it to do?
- How long it took them to do the test?
To evaluate various simple visual methods of checking soil health and adapt them to produce tailored recommendations for gardeners.
Participants were advised to carry out tests in early spring when the soil was moist but not waterlogged. We present the general findings from people’s soil assessments and also their evaluations of the tests.
A total of 373 people signed up to do this experiment, making it the most popular experiment in 2019. Of these, 124 people returned results, giving a response rate of 33%.
Additions to the soil
We asked participants what they added to the soil and how often they added it. We have summarised the findings below:
|Material||% of people||Comments|
A good source of all round, slow-release nutrients. Closes the nutrient loop and
Low in nutrients but a good source of organic matter. Again has the environmental
These may be necessary to supplement the soil or containers where there is insufficient
A good source of organic matter, but very variable in nutrient content.
Chicken manure is very high in readily available nitrogen so must only be applied
Green Waste Compost
This is good for improving levels of organic matter but is slower to release nutrients than
Although rich in nutrients, not many participants used this on their soil. Wormeries generate
Not surprising that few members of an organic organisation are using chemical fertilizers.
The vast majority of participants (69.7%) stated that they did a minimal amount of cultivation or forking when needed. Only a small proportion (12.6%) said that they were no dig. Many people are becoming aware of the benefits of no-dig, but perhaps are not convinced that it will work for them, or lack the time or resources to change their established growing practices. No-dig gardening does require access to a plentiful supply of compost which may be a barrier to some. A significant minority (17.6%) admitted to turning the soil over to at least a spade’s depth every year. From our experience, fewer growers that we encounter at Garden Organic are still using the standard practice of turning the soil over in the autumn and allowing the frost to break up the aggregates. With mild wet winters, this advice is often not appropriate. Firstly, there are fewer frosts that will break up the aggregates. Secondly cultivating the soil in the autumn encourages organic nitrogen to be released into more soluble forms (mineralisation) (Silgram & Shepherd, 1999) which will be washed out by any rainfall, especially if there are no plants to take it up.
Use of green manures
Only a third of growers (33%) used green manures every year, with 42% occasionally growing them. A quarter of the growers (25%) never used them. This is a low uptake for members of an organisation that has been extolling the virtues of green manures for over 60 years.
- How useful they found them
- How easy they were to perform
- How quick they were to perform
- Use - % of participants that found them quite useful or very useful
- Ease - % of participants that found them easy or very easy to perform
- Speed - % of participants took less than 15 mins to do the test
Soil texture could be thought of as the most fundamental test, as it is a key indicator as to how your soil will behave. Both these tests were a modified version of a traditional hand texture test, of which many variations exist already. The VES test had more categories of soil texture than the VSA test. The VSA test aimed to simplify the texture test, and put soils intro categories from poor to good. Loam soils with a balanced mixture of soil particles were given the best rating, then soils with a higher proportion of clay and silt slightly lower, then the sandiest soils most poorly.
|Soil Texture (VSA Score)||Slit loam (2)||Clay loam (1.5)||Sandy loam (1)||Silty Clay (1)||Clay (0.5)||Loamy sand (0)||Sandy (0)|
|% of participants||18||18||19||21||7||13||3|
|Soil Texture||Organic||Sandy||Loamy sand||Sandy loam||Silt loam||Sandy clay||Clay loam||Sandy clay||Silty clay||Clay|
% of participants
VSA test :Use: 75%, Ease: 83%, Speed: 98%
Soil structure test
The soil structure gives a good indication of the physical and biological health of the soil.
The VSA structure test categorises soil structure into 3 simple categories: good, moderate and poor. The VES test has five categories: Sq1 = Friable, Sq2 = Intact, Sq3 = Firm, Sq4 = Compact, Sq5 = Very compact. Some people commented that they preferred the ‘higher resolution’ of the VES test.
People categorised their soils in a similar way, using both tests:
|Good Condition VSA2||Moderate Condition VSA1||Poor Condition VSA0|
|% of participants||83||15||2|
|Friable Sq1||Intact Sq2||Firm Sq3||Compact Sq4||Very compact Sq4|
|% of participants||59||24||15||1||1|
VES test: Use: 80%, Ease: 83%, Speed: 97%
In addition to the general soil quality rating giving by the VES test, the VSA test assesses soil health according to a number of other factors including, porosity, aeration, organic matter, earthworm count, surface crusting, surface ponding and potential rooting depth.
A range of pore sizes is essential for maintaining air, water and nutrient movement in the soil and sustaining biological life. Soils are likely to have poor porosity if they are subject to continual traffic, whereas a garden soil cultivated for vegetables is less likely to be subjected to this. It is also likely to receive regular inputs of organic matter through compost, which will help to maintain good porosity and structure.
|Good condition VSA2||Moderate condition VSA1||Poor condition VSA0|
|% of participants||64||33||3|
Consistent with this, a majority of 64% classed their soil as having good porosity, with only a small proportion of 3% having poor porosity.
Visual soil aeration test
When a soil is not aerated properly, normal respiration processes cannot occur and anaerobic organisms start to dominate. Many of these rely on reducing iron or other compounds within the soil, to obtain their energy in the absence of oxygen, resulting in characteristic changes in colour. This tends to happen in soils that are waterlogged for a proportion of the year.
|Good condition VSA2||Moderate condition VSA1 - Mottles absent||Poor condition VSA0 - Many mottles (10-20%)|
|% of participants||92||5||3|
In many soils, particles on the surface adhere together to form a hard cap which is not permeable to moisture and air. This happens more frequently on soils with smaller size particles, such as clay or silt, after heavy rain. Surface crusting decreases the capacity of the soil to absorb small amounts of water, such as dew, under dry conditions. It also means that larger amounts of moisture are more likely to run off the surface and be lost elsewhere.
|Good condition VSA2||Moderate condition VSA1 - little or no crusting||Poor condition VSA0 - Surface crusting 2-3mm thick with significant crusting (crusting is >5mm thick with few cracks)|
|% of participants||73||24||4|
Use: 70%, Ease: 93%, Speed: 97%
Surface ponding is an indication of soil with extremely poor infiltration. It will result in
anaerobic conditions, root death and poor nutrient uptake, leading to yellowing of plant leaves. Luckily, 90% of participants reported their soil did not suffer from surface ponding. Most gardeners are likely to have some choice over where they site their food growing area, so with experience and knowledge of their site, will try and avoid areas that become waterlogged.
|No surface ponding after one day following heavy rainfall when soil is sturated - Good condition VSA2||Moderate ponding for 2 days following heavy rainfall - Moderate condition VSA1||Significant ponding for 4 days following heavy rainfall - Poor condition VSA0|
|% of participants||90||9||1|
Use: 75%, Ease: 98%, Speed: 99%
A healthy soil that sustains good biological life should have an earthy smell similar to compost. If it has no smell, then there are low levels of biological activity. A sour smell or smell of rotten eggs indicates there is anaerobic activity as a result of poor soil aeration.
|Good condition - Distinct earthy sweet smell VSA2||Moderate condition - Slight earthy smell VSA1||Poor condition -Unpleasant smell VSA0|
|% of participants||60||38||2|
Use: 75%, Ease: 93%, Speed: 98%
Potential rooting depth
A majority (55%) found their soil to have moderately poor potential rooting depth in the range 20 – 40 cm. A smaller proportion (37%) found their soil to have a moderately good potential rooting depth in the range 60 – 80 cm. The soil provides a significant potential to store water, reducing the need to water plants during dry periods. Plants that root more deeply will have access to much larger reservoir of stored water, so will be more resilient in dry weather.
|Potential rooting depth cm||Good (>80)||Moderately good (60-80)||Moderate (40 -60)||Moderately poor (20-40)||Poor (<20)|
|% of participants||0||37||0||55||8|
This was added to the VSA test as a simple and quick method for gardeners to assess a depth at which the majority of roots could be found. 64% found that they could insert the fork to a depth of 30 cm before reaching resistance, whereas 32% found that the fork met some resistance when only pushed into a depth of 15 cm. This will restrict the growth of plants and their ability to tolerate drought. Some people commented that this test was subjective as it depended on strength of person and type of implement. It also requires further development, as it is a subjective guide that hasn’t been calibrated with soil physical measurements, unlike the VSA tests.
|Good condition VS=2 -Fork can be pushed to the depth of the prongs||Moderate VS=1-Fork meets resistance when poked in half way||Poor condition VS=0 Fork can't be pushed in more than a few inches|
|% of participants||64||32||4|
Use: 76%, Ease: 95%, Speed: 97%
Soil organic matter
A simple visual soil organic matter test can be performed by assessing the soil colour. Overall, soils that are high in organic matter are darker, than those lower in organic matter. Comparing uncultivated to cultivated areas can give an indication as to the effect of your soil practices on organic matter levels. 74% of participants classified their soil as having good organic levels. This is consistent with many areas that have been cultivated for vegetable production, where generous amounts of compost are applied.
|Good condition VSA2 - Soil similar colour or darker than uncultivated areas||Moderate condition VSA1 - Soil slightly lighter than uncultivated areas||Poor condition VSA0 - Soil significantly lighter than uncultivated areas|
% of participants
Use: 75%, Ease: 87%, Speed: 97%
It is difficult to assess the biological activity of the soil with a simple visual test, but observing and counting earthworms can provide a good indicator.
|no. of worms in 20cm cube of soil||Good (>35)||Moderately good (29-35)||Moderate (22 -28)||Moderately poor (15-21)||Poor (<15)|
|% of participants||7||6||17||5||65|
Use: 83%, Ease: 86%, Speed: 82%
This test was also added to see what extent weeds could be used as an indicator as to the condition of the soil. Although some weeds commonly appear under certain conditions, such as creeping buttercup in compacted soil, there were too many inconsistencies for this test to prove useful. This test divided opinions, with 50% saying that it was quite useful or very useful, 33% not very useful and 17% no use at all.
|Soil condition||Good condition VS=2 Fertile soil: Chickweek & Nettles||Moderate condition VS=1 Heavy soil: Perennial thistles, Dandelions, Lighter soil: Corn spurrey||Poor condition VS=0 Compact soil: Creeping buttercup, Moss, poor drainage|
|% of participants||57||33||10|
Correlations between soil practices and soil visual test scores
We grouped participants by various soil practices to see how this influenced the visual soil tests. In many cases, it was difficult to make a comparison, as the vast majority of people carried out a similar practice. For example, nearly, nearly all participants added some form of organic matter to their soil either as home-made compost, green waste or leaf mould. However, as there was a more even mix of people that used green manure plants regularly or never used them, we were able to demonstrate differences in some visual test results between these groups. Although the effects were not that large and not statistically significant (using chi squared test at 5% probability), they were consistent. Amongst the participants that used green manures every year, more of them (92%) had soil structure in good condition than participants that never used them (76%).
Other practices have small effects on soil conditions. No dig slightly improved soil structure and increased the worm count, but these effects were only small and could be due to natural variation.
The next steps will be to take some of the tests used here and put them together with some recommendations for gardeners to provide a soil testing guide for gardeners. Putting the tests together to make separate scores in the categories: Texture, Structure, Aeration, Rooting depth and Biological will allow targeted recommendations to be made for each category.