A Gardener’s Guide to Soil Microbes & Plant Root Exudates
Most Gardeners Understand The Crucial Role That Organic Matter Plays In Soil Health. But Few Know The Role Of Plant Root Exudates.
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Most gardeners know that healthy soil is full of beneficial microbes that contribute to nutrient cycling, disease suppression, carbon sequestration, moisture retention and improved soil structure. They know that adding organic matter is essential for healthy soil as it feeds soil microbes but not everyone understands the importance of plant root exudates.
But not every gardener knows about plant root exudates. Or how essential they are for healthy soil microbes and soil health.
Many corporates are jumping on the soil microbe bandwagon. They are producing commercial products they claim are full of microbes that your soil needs. But in reality this isn’t the answer. There are plenty of microbes on your soil, millions per gram of soil.
The reality is that focusing on creating optimal conditions for these microorganisms to thrive is far more effective.
Let me explain.
The Microbiome: The Foundation of Healthy Soil
Soil microbes include bacteria, fungi, and protozoa. They form a complex ecosystem within the soil. By nurturing a diverse and thriving microbiome, we can create a healthy fertile environment in which your plants can flourish.
The Myth of Adding Microbes
Recognising the interest in soil microbes there has been an increase in the number of companies selling microbes of various types. But unfortunately simply adding more microbes to the soil does NOT magically improve soil health. The addition of microbes misses a simple essential and is frequently totally ineffective. In the right condition most microbes multiple extremely quickly. For example bacteria can double in numbers in as little as 20 minutes! If you do the maths on this you get 21 zeros in the number of bacteria produced in a day! And that assumes you only start with ONE,
Microbes are highly adaptable and will naturally colonise every available niche. There are rarely any shortages of bacteria ready to multiply, though some may be in stasis or dormant, waiting for favourable conditions. Of course it is not just numbers of bacteria we need to consider. We should also think about species. Science has discovered and catalogued huge numbers of soil bacteria but the catalogue probably only runs to 10% of the total number of species. Most have yet to be identified and catalogued.
What we do know about species is that different plant species tend to have different bacteria associated with them, and the array of bacteria will change with the age of the plant, the soil pH, soil temperature and a lot of other variables. I’ll write an article on this topic sometime soon.
However one thing is certain, with the number and variety of bacteria sat there waiting for perfect conditions, instead of introducing external microbes, the focus should be on creating favourable conditions for existing microbes to multiply and thrive. Bacteria are able to respond to their environment extremely quickly so if we improve conditions they will colonise every nook and cranny in the soil.
This basic concept also applies to soil fungi.
Feeding the Microbes: The Key to Success
What holds a numbers explosion of soil microbes is usually the lack of nutrients. So the most effective way to increase microbial activity is to provide them with ample food. While many gardeners know this is best done with composts and animals manures, they don’t always understand exactly what these products do. Many claim they are feeding the soil or plants. The reality is that they are feeding the microbes.
Organic matter in the soil starts to break down very quickly as soil microbes attack it. It starts within minutes, with the microbes breaking down the long chain chemicals in the organic matter to form simpler, shorter chain products. Eventually the breakdown results in the simplest plant nutrients available in soils. But though some simple products are produced very quickly it can take around five years before all the organic matter to totally decomposes in the soil. That’s actually a good thing as it means we have some plant nutrients being produced for years. I’ll write more on that in another article. It’s a fascinating story and worth understanding.
There’s More To Soil Microbes Than Organic Matter
However, it is not all about organic matter. Science now understands that there is another source of food for microbes. For them a more efficient approach is to focus on plant root exudates. Plant root exudates are substances, secreted by plant roots, that are a rich source of nutrients for microbes. It’s one very few gardeners reading this article will have heard about. If you have you are in the top few percent of knowledgeable gardeners. If its new to you, don’t worry, its new to most people and I will explain more below.
Root Exudates: Nature’s Fertiliser
Plants release a variety of organic compounds into the soil through their roots, as mentioned above these are known as plant root exudates. These root exudates serve as a food source for beneficial microbes, attracting them to the root zone and promoting their growth. By maximising root growth and health, gardeners can indirectly support a thriving microbiome.
How to Enhance Root Growth
To encourage robust root development and maximise exudate production, gardeners can implement the following strategies. These strategies support what I’ve been saying for some years and what smart gardeners have done for generations,, without understanding the science behind it.
Maintain Continuous Plant Cover: Avoid bare soil and sow or plant cover crops or intercrops to ensure a constant supply of roots and therefore exudates.
Prioritise Plant Health: Provide plants with adequate water, nutrients, and sunlight to optimise their growth and root development. This way we get more exudates and more microbial activity which provides soil health.
Diversify Plant Species: A diverse range of plants will attract different types of microbes, fostering a more robust and resilient soil microbiome. This is why I like the idea of mixing crops in beds. This isn’t companion planting, it’s just following nature and being natural.
Minimise Soil Disturbance: Excessive cultivation can disrupt the soil structure and harm beneficial microbes. I know not everyone understands or likes No Dig, But if you opt for no-till or minimal-till gardening practices the soil will benefit because the microbes suffer less disruption. .
Necromass: The Forgotten Nutrient Source
A Forest of Fungi
In addition to root exudates, another important source of microbial food is necromass. Necromass is the dead organic matter in the soil. It would be easy to confuse this term with compost or domestic animal manures, but it’s not quite the same. It is all of these things and also the remains of all the soil microbes. And we shouldn’t underestimate how many bacteria, fungi and other soil inhabitants die in the soil each minute. It is millions. Don’t forget bacteria can double their numbers in 20 minutes and the potential number we’d have if conditions were perfect has 21 zeros after the first number (that number is actually 4.7 but that’s a detail most of use don’t need to worry about).
To define it more succinctly necromass refers to the dead organic matter, including the remains of plants, animals, and microorganisms, that accumulates in the soil. Necromass provides a steady supply of nutrients for microbes, contributing to soil health and structure.
The Role of Fungi in Soil Health
So far I’ve focused on bacteria but fungi also play a particularly important role in soil health. They form extensive networks of hyphae, which can help to improve soil structure, increase water retention, and facilitate nutrient uptake by plants. Mycorrhizal fungi establish symbiotic relationships with plant roots, providing them with essential nutrients in exchange for carbohydrates.
Promoting Fungal Growth
To encourage fungal growth, gardeners can follow a similar strategy to what I suggested for bacteria:
Avoid Excessive Cultivation: Cultivation / Tillage can disrupt mycorrhizal networks.
Use Organic Matter: Incorporate organic matter into the soil to provide a food source for fungi.
Avoid Chemical Fertilisers: Excessive use of synthetic “chemical” fertilisers can suppress fungal activity.
Maintain plant growth on plots at all times.
The Importance of Soil Biodiversity
A diverse microbiome is essential for healthy soil. A variety of microbes, including bacteria, fungi, and protozoa, work together to perform different functions. By promoting biodiversity, gardeners can create a more resilient and productive soil ecosystem.
Practical Tips for Improving Soil Health
Composting: Composting organic waste can provide a valuable source of nutrients and improve soil structure.
Cover Cropping: Planting cover crops between main crops can help to maintain soil health, prevent erosion, and suppress weeds.
Mulching: Mulching with organic materials can help to retain moisture, suppress weeds, and improve soil structure.
Avoid Over-Watering: Excessive watering can lead to nutrient leaching in some cases but can also create anaerobic conditions that can harm beneficial microbes. this happens when the soil pores get filled with water which excludes soil air.
Test Soil Regularly: Regular soil testing on farms and market gardens can help to identify nutrient deficiencies and adjust management techniques accordingly. This isnt practical in garden or allotment situations but still worth a mention here!
Avoid Chemical Pesticides: Chemical pesticides can harm beneficial microbes. Consider using organic pest control methods instead.
What Can History Tell Us About Plant Root Exudates?
There is a historic precedent that involves root exudates that were should consider.
Traditionally cereals farmers harvested in late summer and left the stubble overwinter to be ploughed and sown in spring. Over winter the stubble was full of “weeds”. And they were the growing crop that kept the microbes happy.
Sometimes sheep would be let in to the stubble to have a graze on green stuffs, which coincidentally prevented the weeds from seeding and provide “sheep borne nutrients” as the sheep recycled what they ate!
Which Plants Produce The Best Root Exudates?
As for which weeds provide the best roots … I’ve no idea but would plump for those with a big root system that remains active as temperatures reduce.
Perhaps this is something that researchers have already investigated, they have certainly researched how to measure plant root exudates. It’s a question I must check out.
I’m sure there’s a formula about the best roots for exudates, waiting to be discovered.
One day it could be as well known as E = mc2 :-)
Could it be ….. E=rt2
E=Root mass x temperature2
Plant Root Exudates … My Conclusions
By understanding the critical role of microbes in soil health, and focusing on creating favourable conditions for their growth, gardeners can cultivate thriving plant communities. Instead of relying on external inputs; adding organic matter, prioritising root growth, maximising plant diversity, and minimising soil disturbance are key strategies for nurturing a healthy soil microbiome and achieving sustainable gardening success.
