The living soil ecosystem in some ways resembles a City.

The ecology of the living soil has only recently been properly revealed by research, especially where its relationship to physical and chemical aspects of the soil is concerned.

Recent research has cast new light on the nature of the biological community found in the living soil, with some unexpected results that indicate a much closer link between the physical environment within the soil and biodiversity of the soil community than was previously expected.

The first breakthrough came when the SWEP researchers were analyzing their data to try and identify possible correlations between biological results and other soil tests. At first their results showed an almost random ‘scatter’ with no apparent relationship to any of the usual soil test measures, but when they turned their attention to samples that showed cation percentages that were 70% or more of the ideal proportions (below), patterns suddenly began to emerge that clearly indicated soil physics would have a far greater impact on the biological community than nutrients.

They were then able to show that the total population (of the five microbial indicator groups they were measuring) in well-balanced soil was directly related to the Adjusted Cation Exchange Capacity and that in such soils the component microbial groups were present in fairly consistent proportions (shown below), relative to the desirable total calculated from the Adjusted CEC of the living soil.

It is important to note that the desirable proportions of biological indicator groups in the living soil are percentages of the desirable total population that is calculated from the Adjusted Cation Exchange Capacity. These proportions are only seen in soils with cation percentages very close to or within the desirable range (shown above).

This had important ramifications for soils with less than the optimal balance proportions of exchangeable cations, since many such samples showed active populations far greater than the desirable level calculated from the Adjusted CEC. What was going on in these poorly balanced soils and what changes needed to take place as they were improved?

The problem is that there is no simple progression of change in the biological community of the living soil as there was with the cation balance. For instance, when a soil is low in exchangeable Calcium and high in Hydrogen, you simply add Lime. The Calcium goes up and the exchangeable Hydrogen goes down, while the other cations remain more or less unchanged. By contrast, the balance of biological community is much more complex and dynamic, with many changes occurring in all the groups before they finally settled into a stable balance.

Gradually, however, a picture began to emerge. Poorly balanced soils often seemed to be dominated by one particular group at the expense of others and it seemed to be the Lactic acid bacteria that were often the culprits. Their numbers could be hugely excessive, while other groups were poorly represented. Yeasts were another group that seemed to behave in this way, while photosynthetic bacteria appeared to be the dominant group in soils with very low populations. Eventually, two additional measures were developed to help complete the picture – giving us a view of the living soil that is analogous to what we might see in towns and cities.

The first measure is “Occupancy” – what percentage of the desirable population does the measured level represent? Is the soil a Ghost Town or a bustling Metropolis? Is it under-populated by hard-working families, but over-populated with undesirables?As the soil balance improves, the Occupancy percentage either decreases towards the desirable, or (if the soil has been degraded to a state analogous to that of a Ghost Town) it increases towards the desirable. The question then is “Why is the occupancy level so high or so low?” and this seems to relate to a measure called the ‘Suppression Index’.

This is the extent to which organisms such as Yeast and Lactic acid bacteria inhibit the activity of others (a value of 1.0 would represent total suppression and zero is none at all). You could imagine a high Suppression index being like a high Crime rate in the City – the ‘good people’ become reluctant to leave home and go to work – their activity is suppressed.

And so we can now build a picture of the City in our Soil and see the various conditions that may exist:

1. The Ghost Town: Where occupancy is low and mostly made up of ‘hermits’ who eek out a living on very little. The streets are full of potholes and tumbleweeds, the living conditions (physical environment) so undesirable that only the toughest can survive.

   This would be a highly degraded soil. The total population would be below the desirable (occupancy <100%). The predominant group present in these soils is often the photosynthetic bacteria, so the suppression index may not be high, but the cation balance will be poor and nutrient fertility low or showing a serious imbalance (a high level of available Copper for example).

2. The ‘Wild West’ Town: Here the occupancy level is higher and there are a few productive businesses, but the “shoot-em-up” character of the place still suppresses activity.

   This could be an undeveloped or moderately degraded soil. The occupancy level is usually somewhat greater than 100%, but not extremely high. The suppression index is moderately high and cation balance is moderately poor.

3. The Industrial Precinct: Here occupancy is fairly high, but a substantial proportion of this is made up of undesirables in “low-rent” areas. It’s a bit smelly and dirty and not the place to be walking around at night – fairly productive, but not a pleasant living environment.

   This is typical of intensively farmed soils with high chemical fertilizer inputs. The occupancy level is well above 100%, the suppression index moderately high, and there are some issues with physical properties such as friability, etc.

4. The Affluent Suburb: With tree lines streets, playgrounds for the kids, shops and schools close by and a low crime rate, this is a pleasant place to live.

   In well-balanced soils the occupancy close to 100% and suppression index is low, the cation balance will be close to optimal, with moderate, balanced nutrient fertility.

Painting such pictures of the living soil may appear trite, but they serve to illustrate the complex and dynamic relationships present and how these relationships influence the overall state of affairs. The most important feature in determining how the soil will behave remains the cation balance and its impact on soil physics, but the ability to add biology to the picture and create such descriptions takes the soil test beyond being a page of numbers that can only be understood by a handful of ‘experts’, to being a practical management tool for farmers and gardeners everywhere.

One point that often gets overlooked in all this is that soil biology cannot be managed in isolation. After all, the living soil - like any other ecosystem - is more than just the organisms it contains. Living things need a suitable habitat in which to survive and resources to utilize. For more information on these related fields, you can use the links below. You can also subscribe to our Free email newsletter - The Food Gardener - to ensure you remain up to date with the results of future research in this field.

Further reading and useful links

Using bioactive materials to stimulate soil biology
Here is information on the effects that research shows things like fish emulsion and kelp extracts have on soil biology.

Companion planting
Soil biology is also influenced by the kinds of plants being grown, so companion planting is a very good way to ensure good biodiversity in the soil.

Appropriate use of soil cultivation
Like a fire in a forest, digging the soil always causes a disturbance to the biological community in the soil. If you need to cultivate the soil, it is important to use the right method and apply techniques to aid recovery from the disturbance.

Garden Habitat
Here are ecological principles that apply as much to the living soil as elsewhere in the garden.