Testing the health of our soil

Summary

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.

Background

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.

Here we examine some of the basic physical properties of the soil and how to assess them using simple visual tests. There are a number of visual tests available but many may give advice that is more tailored towards farmers. We wanted to adapt these so that they are more applicable to gardeners.

Soil texture

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.

Soil structure

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.

The ideal soil structure has aggregates that are a small crumb structure that readily crumble with the fingers. It will also have a mixture of fine micropores and larger macropores to allow movement of air and water to the roots. A poor structure will have large blocky aggregates with very few pores.

Soil drainage

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.

Soil organic matter content

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.

Biological life

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.

Visual Soil Tests

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).

The VSA methods were devised by Graham Shepherd for arable and livestock farmers in New Zealand. A number of characteristics are assessed by comparing the soil to photos and giving them a rating of: 0 = Poor, 1 = Moderate, 2 = Good

If you score 0 or 1, you might want to take some of the suggested steps to improve your soil. The rating system is deliberately simple so that the categories are easily distinguishable and can be carried out by anybody.

The characteristics used include texture, structure, porosity, aeration, organic matter, earthworms, smell, ponding, surface crusting and potential rooting depth.

Find out more about this system.

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).

The VES methods were devised by the Scotland Rural College. The aim is to take a soil sample, and match it to the category that most closely resembles the characteristics in the photos on the chart.

The soils are given a structural rating in one of 5 categories:

Sq1 = Friable, Sq2 = Intact, Sq3 = Firm, Sq4 = Compact, Sq5 = Very compact

Find out more about this assessment.

After carrying out the tests we asked participants to assess:

• How useful they found it?
• How easy they found it to do?
• How long it took them to do the test?

At the end of each section are a few general practical tips that address the results of each test. These will be developed further when we have gathered in results and comments.

Aims of this experiment

To evaluate various simple visual methods of checking soil health and adapt them to produce tailored recommendations for gardeners.

Observations

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.

Results

Response rate

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:

Table 1: Materials used by participants

Cultivations

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.

Our experience is that green manures are often low on the priority list for gardeners. Many focus on looking after food crops without seeing growing green manures as an investment in future food crops. Many find the choice of which species to grow and when to grow it, bewildering or they may intend to grow one over the winter, but miss the narrow autumn window for sowing after harvesting summer crops. A guide to choosing green manures is compiled by Garden Organic.

We asked participants to carry out the visual tests on their soil then evaluate them as to:

• How useful they found them
• How easy they were to perform
• How quick they were to perform

We have presented scores for:

• 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

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.

Table 2: Soil texture assessment using VSA test

Table 3: Soil texture assessment using VES test

Most participants found the visual soil tests useful or very useful, with little or no preference given to the VSA or VES methods given.

VSA test Use: 75%, Ease: 83%, Speed: 98%

VES test Use: 75%, Ease: 78%, Speed: 99%

Soil structure

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:

Table 4: Soil structure using VSA test

Table 5: Soil structure using VES test

Generally, people had good soil structure, with few people having structural problems. Using both tests, 83% of people categorised their soil as having good (VSA2 or Sq1/2) soil structure. 15% categorised their soil as moderate or firm, and only 2% as poor or compacted.

VSA test: Use: 79%, Ease: 91%, Speed: 98%

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.

Soil porosity

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.

Table 6: Soil porosity assessment using VSA test

Consistent with this, a majority of 64% classed their soil as having good porosity, with only a small proportion of 3% having poor porosity.

Use: 77%, Ease: 83%, Speed: 98%

Soil aeration

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.

Table 7: Soil Aeration assessment using VSA test

92% of participants classed their soil as having good aeration from the visual test. This is consistent with few participants reporting ponding, surface crusting or smells suggesting anaerobic activity.

Use: 73%, Ease: 88%, Speed: 98%

Surface crusting test

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.

73% of participants stated that the surface of their soil was in good condition with little crusting, with 24% in moderate condition with only slight surface crusting.

Table 8: Surface crusting assessment using VSA

Use: 70%, Ease: 93%, Speed: 97%

Surface ponding

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.

Table 9: Assessment of surface ponding using VSA

Use: 75%, Ease: 98%, Speed: 99%

Smell test

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.

The results of this test was more divided with 60% stating that their soil was in good condition and 38% stating that their soil was in moderate condition. This suggests that many soils had lower than optimum biological activity. Luckily, only very few participants had soil with poor smells, suggesting regular waterlogging and anaerobic conditions.

Table 10: Smell test assessment using VSA test

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.

Although 72% of participants found this test useful, only 63% found it easy to do as it involved digging a deep hole.

Table 11: Potential root depth test using VSA

Use: 72%, Ease: 63%, Speed: 84%

Fork test

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.

Table 12: Soil assessment using fork

Use: 76%, Ease: 95%, Speed: 97%

Biological activity

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.

Table 13: Soil Organic Matter assessment using VSA test

Use: 75%, Ease: 87%, Speed: 97%

Earthworms

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.

Participants sampled a 20 cm cube of soil and counted the earthworms. 65% found that they had less than 20 earthworms in this cube, which was considered poor according to the VSA test. This was cause for concern for many of the participants, but many found this test one of the most interesting, with 51% finding quite useful, and 32% finding it very useful. This test is dependent on the moisture content at the time of sampling, and it is possible that the low rainfall in April 2019, when many would have taken the test, contributed to the low earthworm counts.

Table 14: Earthworm assessment using VSA test

Use: 83%, Ease: 86%, Speed: 82%

Visual weeds test

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.

Table 15: Visual weeds test

Use: 76%, Ease: 95%, Speed: 97%

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%).

Growing green manures also improved soil porosity, with more participants who regularly grew green manures (78%), having soils with good porosity, compared to those who never use them (56%)

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.

This survey was carried out on organic gardeners who, more than likely, all pay good attention to their soil health already. It would be interesting to carry it out on a group with a wider range of soil practices.

Recommendations from soil tests

The results from the tests were divided into 4 categories: structure, aeration, rooting depth and biological life. Download the full report here

Next steps

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.

You can download the full report on testing the health of our soil here.