SOIL BIOLOGY – build it and they will come or introduce them in?

I got some clarification recently on the question of making soil biology work for us.  Do we take the ‘build it and they will come’ approach or do we actively introduce microbes to ‘enliven’ our soils?  My gut feel says ‘build it and they will come’ – it just sits best with me.  It’s mostly about practices, not inputs.  But, let’s not discount microbial inputs either – there’s a place for all things.  It must be considered too, that cropping and grazing may lend themselves in varying degrees to the two approaches.

I often write about soil biology, because I believe it is an area not given enough attention in farming in the last half century.  There has been much research into the chemical and physical properties of soil, but only more recently has the biology been explored and our production systems are still catching up and working out how to make the biological research practical.

Following are a few things that will help us to most effectively apply the research and which may influence our approach.

Nature prevails

We know that nature always prevails – it is why there is resistance to herbicides and insecticides.   While some may curse this resistance, it is actually heartening that nature has such amazing resiliency.  [private]

In the case of soil microbiology, ‘nature prevails’ also applies.  Dr Karl Ritz from The University of Knottingham says that the native microbes of a soil will eventually overcome introduced species¹.  If we think about it, we can relate to this – this is why legume crop seed is inoculated with nitrogen fixing rhizobia each year at planting time – they don’t live on in the soil well enough to be there for a later crop.  This will likely also be the case for other species introduced to the system – in time they will be overrun by the native species that have adapted to thrive in that environment.

It may be however that a specific, isolated species has such benefit to the production system that there is financial and ecological value in including it as an input each crop or year.  The potential commercialisation of melanitic fungi - (see past blog), may be one such example.  Whilst such microbes may become an ongoing input each year, if the return on investment warrants annual application, then the case for them is clear.  They also tick the box for sustainability and regenerative agriculture as they are a renewable resource – able to be cultured, brewed and multiplied perpetually.

tight and loose relationships

The association between plants and soil microbes vary.  Some associations are ‘tight’.  A tight association between a plant and microbe means that there will likely be more success of colonisation when the microbe is cultured and added to the farming system via inoculation.   Most plant/microbe associations are not ‘tight’.  To use the rhizobia example again – the culture and introduction of these into cropping and pasture systems via inoculation has been very successful because they form tight associations with plants.  Most associations are loose associations, making successful introduction and colonisation less successful.

Most microbes are ‘unculturable’

We might be able to culture some microbes to use for our advantage on our farms, but Dr Ritz states that the fact of current technology is that 90-95% of soil microbes are not able to be cultured.  So, if we are relying on cultured and applied microbes – we are missing a massive amount of organisms, many of which we probably don’t even know their role.

some diseases are not caused by a single pathogen

Some plant diseases are not caused by a single pathogen, but rather the interaction of a number of microbes associated with the plant.  It is likely too then, that the resistance of plants to disease may well be with the help of multiple microbes, so the application of a single species may not be the answer alone to solving disease or plant ill-health.

The system needs energy

There are several variations of soil carbon – and it is both the driver or energy source (as plant sap and labile carbon) for the microbiome and is also an end product of the plant growth process (as humus) – given the help of the soil microbiome.  So soil carbon is both the end product or goal of the process (in terms of building stable soil carbon), and also the fuel for the process.

Carbon pools tend to be termed different things, depending what you read.  But there are essentially three pools.

Humus carbon

I have often had the impression and even reported that humus is the pinnacle of carbon – it increases water holding capacity, is a large holding site for nutrients, and improves soil structure and drainage.  It is a stable form (relatively resistant to microbial breakdown), so is great for increasing soil carbon levels for the long term.  This can then have positive impacts for climate change and will ensure its advantages mentioned above are enjoyed for long periods.  Humus is a carbon form which can no longer break down any further and mycorrhizae play a big part in its formation (from plant sugars).  Whilst humus does all these amazing things, Professor Karl Ritz points out that labile carbon is also very important. 

Labile carbon

Labile carbon is not a long term solution to carbon in our soils – labile means ‘more easily changed’.  It is unstable and readily lost again from the soil. So in terms of storing carbon in soil and impacting climate change, it is not really a solution.  Labile carbon is still very important however for the stimulation and feeding of the soil microbiome, for nutrient availability and turnover, especially nitrogen and for outnumbering potential plant pathogens of the soil.  It is made up of both living and non-living components, such as green manure crops, stubble, plant roots and manure.

Recalcitrant carbon

Recalcitrant carbon is not known to be affected by the soil microbiome. I will mention it however to round out these comments.  It is resistant to decomposition and is highly stable (for thousands of years) and is often in the form of charcoal.  Biochar is a recalcitrant form of carbon.

How to apply the above Learnings

If we understand some of the above mentioned things about the likelihood of success of planning to colonise microbes in our soils and their relationship with plants, it helps us to understand our approach to soil biome health.  Do we ‘build it and they will come’ or do we apply cultured microbes to our soils and crops?  It seems that the build it and they will come approach may be the best overall approach – which will encourage the biology native to each person’s region, soil types and climate, and which should have greater resilience to remain there.  In doing so, we are also nurturing and encouraging a large multitude of species, some of which we may be unaware of and not even able to culture and the benefits in our systems of which are yet unknown.

There may be cases however, where the introduction of specific fungi or bacteria may prove financially viable for that crop or season, and while it may not offer long term colonisation of that species in our soil – the short term ecological and financial benefits outweigh the application and purchase cost.  Pulse crop inoculation has been a good example of this in the past.

How do we build it for them to come?

The way to create the soil environment that will then encourage a proliferation of microbiota has been the topic of many of my blogs.  Building diversity underground requires diversity above ground – whether it be as pastures or in cropping situations.  Things like diverse perennial pastures, pasture cropping to introduce diversity and active root growth for more of the year, multispecies cover cropping to bring diversity to the cropping system, planned grazing and more.

We should also keep abreast of developments in the isolation, culturing and commercialisation of specific strains of microbes.  We now know some of the questions to ask about these products as they come on the market, given what we have learnt today.  Things such as how tight is the relationship of your product with the plant? and does your product prevail among native soil microbe populations?  We must also remember that specific microbe strains will only be a piece in the puzzle – one strain may for example help sequester carbon, but will it help with plant disease issues?

I hope this helps with your approach to your farm.

References

1. Ritz,K. University of Knottingham.  Direct contact.