The Importance of Soil Biodiversity

I had the pleasure on Monday evening to give my very first talk to a gardening club. It was all very professional with a projector for my (first ever) Powerpoint presentation and a microphone strapped to my head. I felt, after a while, like I was giving a TED talk. 

The club in question was the very organised Kilternan Gardening Club (I was invited a year ago by their chairperson Ingrid Goodbody), where I met some very keen and knowledgeable gardeners. I know from chatting afterwards that some poor souls read our newsletter/blog, so I would like to say hello and to thank everyone for their warm welcome.

The talk was built around some bullet point notes which I displayed on the screen with some colourful pictures to keep everyone amused. I then filled in with some freestyle waffle which I think, for the most part, I got away with.

Stupidly I had left my supporting notes at home so, as I said last night, each slide that came up was nearly as much of a surprise to me as to the rest of the room. I hope that the group doesn't mind if I try to give the gist of our conversation here as there may be some parts that are of interest to our online community.

Your Soil is Alive!
As we all know, soil is a living breathing thing. Yes, it is mostly made up of inanimate material like ground up rock (45%), water (25%) and air (25%), but the remaining 5% of organic matter is literally teeming with life. In a spade full of soil, there are many billions of bacteria, hundreds of metres of fungal hyphae, and millions of amoeba and protozoa.

These billions of microscopic organisms are feeding and excreting their waste products into the soil. They become food themselves for tiny arthropods and nematodes, who in turn are on the menu for earthworms and larger creepy crawlies. 

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Birds or small mammals feed on the larger soil dwellers, which is where this web of life bursts out of the soil and into our world. I find it amazing that, in a few small steps, we go from single cell bacteria up to flying creatures - with the bulk of the action taking place in the first few inches of soil.

Which came first, the chicken or the egg?
We are all used, I think, to the idea that a fertile soil grows healthy plants. We know that the manure or garden compost we spread on our beds rots down and provides nutrients that are taken up by plant roots. 

It could be argued, however, that this process is actually the other way around; it is the plants that feed the soil rather than the soil feeding the plants. Yes, we have many interdependent cycles going on in the soil; but I think we can say that there is a definite starting point. 

We must accept that all life on earth is reliant on the sun and, as plants are the only organisms capable of processing solar energy (and making food), that all life therefore relies on plants.

Who's the boss?
Apart from dead plant material rotting down and returning nutrients to the soil, plants actually influence the soil around them in a very different and much more direct way. 

During photosynthesis a plant chloroplast takes carbon dioxide and water and, using solar energy, transforms them into carbohydrate sugars (carbo-carbon, hydrate-water). The plant uses this fuel to build stem, leaf and root cells, but also keeps a significant proportion for another, less visible purpose. 

Plant root tips secrete a carbohydrate solution into the soil to smooth their passage and to provide food for the soil life around the growing roots. Remember the billions of microscopic bacteria and metres of fungi? They are the first link in the chain of beneficiaries (which includes us) who owe their existence to the plant's unique ability to photosynthesise. So who's feeding who: is the soil feeding the plant or the plant feeding the soil?

The plants are in charge
Here's the really clever bit. The living, dying, excreting and respiring being done by the bacteria and fungi around the root produces nutrients that are in an ideal form to be taken up by the plant. The plants are therefore providing sugars to fuel their own nutrient-processing workers. 

Better still, plants will secrete different sugars depending on what species of the 60,000 different soil bacteria they want to attract. 

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An example of this is the legume family (e.g. peas, beans and clover): they specifically target nitrogen-fixing bacteria, which they house in nodules on their roots. This is the reason why we don't need high nitrogen feeds for legumes because they, or more accurately the bacteria in their roots, can make it themselves.

Good guys VS bad guys 
Both friendly soil bacteria and fungi protect plants from infection by pathogens (pathogens are disease causing organisms that can be bacteria, viruses or fungi) by forming a protective layer around the roots. The protective layer is formed by their own bodies, so they really just crowd them out - but either way, it works. 

Soil bacteria also produce a layer of slime that enables them to stick to roots and soil particles; this slime traps pathogens and prevents them from moving about or entering the root cells. 

Some soil bacteria and fungi also produce disease-inhibiting compounds (the most famous of which is the penicillium fungi, I'll leave you to guess the compound) that help keep disease under control. It has been proven that plants grown in a vigorous, healthy and bio-diverse soil need very little, if any, disease controlling products.

The best stewards of your soil
If we think of the spread of the feeding roots of a large deciduous tree, we can get an idea of the sheer volume of sugars being injected into the soil and the amount of soil life being supported. 

We can also see that the soil life is at its most active when plants are growing in it, processing energy from the sun and feeding that energy below ground in the form of carbohydrate sugars. 

It is therefore true to say that the best stewards of the soil are the plants themselves, as it is they that provide the power that drives the soil food web. It follows then that we gardeners should strive to keep our soil covered with crops, whether edible or green manure, for as much of the year as possible.

The distribution network
The relationships between soil bacteria, mycorrhizal fungi and plant roots go back 450 million years which, as far as we are concerned, is almost the beginning of time. This is important because it means that all three have evolved together and have settled into a mutually beneficial equilibrium that is the cornerstone of life on earth. 

For this reason, it may be helpful to view them as a single organism as, when this engine is working at it's optimum, we grow the best crops. 

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A mycorrhizal fungi network is directly linked to plant roots on a cellular level. While fungi and plants are completely different organisms (they are not even in the same kingdom) they are viewed as a secondary root system for the plant. 

The word mycorrhiza comes from the Greek mukès' (fungus) and 'rhiza' (root), so literally means 'fungus root'. And what a root system this is: a mycorrhizal network can cover 700 times more soil than the plant root alone, so is a massively powerful nutrient and water gathering system.

A very particular set of skills
I have made this silly comparison before, but like Liam Neeson in the movie 'Taken', mycorrhizal fungi have a 'very particular set of skills'. Unlike plant roots that don't join those from a neighbouring plant, mycorrhizal hyphae link to form a vast network; this is how they are able to cover such huge areas. 

Fungi are also able to break down rock, which means they are effective miners of minerals. They can then transport these minerals (using their extensive network) directly to the plant roots. The network also moves water around, so can keep plants irrigated in dry conditions when the plant roots would not otherwise be able to reach a water source.

Drainage contractors
You may have noticed in the first couple of paragraphs that good soil contains about 25% air. This is important because the soil life needs air to breathe and do their work; this is the reason we turn our compost heaps to introduce air for the composting bacteria. 

The many larger soil dwellers all contribute to keeping it 'open' and aerated, but the earthworm is probably best known and most effective. Earthworm burrows are extremely efficient at draining soil but also for depositing nutrients in the rootzone, as the worms digest food and leave their casts behind them. 

There are a number of different species of earthworm: some surface feeders and some that feed on organic matter in the soil, but all produce nutrient-dense casts which are a perfectly balanced plant feed.

No Dig needs a healthy soil
Obviously one of the core principals of 'No Dig' is not to dig - and thus, to not interfere with soil life and activity as much as possible. One key 'no dig' gardening technique is to spread bulky organic matter over the soil surface. 

When I say 'organic matter', I mean mulches like garden compost, municipal compost, well rotted manure, leaf mould, seaweed or composted wood chip. This material is broken down and incorporated into the soil by the aforementioned soil life, so it makes sense that the more biodiverse your soil is, the better it will do its processing job. 

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Remember also that the life in your soil takes time to do the processing work. which makes applications of compost or other mulches a naturally slow release feed. Mulching is then the ideal solution, because soil-protecting mulches lose very few nutrients over winter (because the processors are less active) but will power up in spring. 

Unlike chemical fertilisers which are mostly soluble and wash out quickly, nutrients in bulky organic feeds are locked up until broken down and digested by your new soil friends.

Conclusion
There is more interesting stuff to go into here but for today, I am out of time. My overall point is that your soil is a living, breathing thing that needs to be cherished and looked after. I hope I have shown how plants are also key to a healthy soil and how they provide the vital elixir from which all other life blooms. 

As gardeners, I think the lesson is to keep your soil growing plants for as much of the season as possible - whether you are growing edible crops, sowing green manures or other ground cover plants. If it is too late to sow a crop and you have bare soil, cover it with a mulch to protect and feed.