If there’s one sure way to rark me up big time, New Zealanders, it’s by calling soil “dirt”. I have never understood that kind of language.
That word is linked to dirty and things that are unclean, and it totally ignores the fact that soil grows huge forests, it feeds us, cleans water and allows photosynthesis by providing fertiliser for plants, shrubs and trees.
The late Professor Thomas William Walker (soil scientist at Lincoln University and my colleague on the long-ago Maggie’s Garden Show) used to be one of those fervent proponents of healthy soils and all the lifeforms it supports. Soil Man and Bug Man had heaps of conversations about the science of it.
There are millions of critters and bacteria and fungi in a spade-full of soil, all doing their job and keeping the balance. You could call this Unique Soil Biodiversity.
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If we just start with fungi (the lifeform that literally runs the planet) we will not just cover your compost bin and the leaf litter on top of the mulch. These recyclers and composters come in thousands of different species and link up with so many tiny microbes and invertebrates that without them, nothing would rot into fertile organic matter. Simple as that!
But the fungi that form those huge mycorrhizal systems are probably the least understood creatures (yes, I call them creatures) that connect all the trees in a native forest and act as an intelligent support system.
Do plants talk to each other? Yes they do… by using an intricate network of fungal mycelium.
What is the most expensive species of mycorrhizal fungus? Well, that surely has to be truffles.
When you want to think about soil organisms of the invertebrate kind, it becomes quite clear that most of them are actually not well known by gardeners, let alone the public at large. When looking through literature and my bug collections, I realised there are orders I rarely come across and one order I have never even heard of: pauropoda.
Imagine a miniature version of a millipede – just a millimetre long in the top layers of soil under mosses or beneath rocks and in rotting branches and logs.
We don’t even know how many species we have here in New Zealand, and their life cycle and reproduction are still poorly understood, meaning they could be parthenogenetic (with just females reproducing all by themselves); also, they have no eyes but do have branched antennae.
It is believed that some species may be chewing on fungi whereas others will eat more solid food, whatever that may be. There you go! We don’t even have a clue as to their role in the ecosystem.
I can fill this whole column with fascinating bits of info on critters such as the proturans (one of the oldest groups of invertebrates on the planet) and the tardigrades (water bears) that are famous for their ability to survive being thrown into liquid nitrogen (cryptobiosis) and stay alive in that hostile environment for decades, while their normal life cycle spans just one year.
These weird and wonderful, slow moving and microscopic beasties are also roaming the soil in your vege garden, feeding on nematodes (the carnivorous species) or plants (vegetarians).
The problem is that you are unlikely to find a lot of these beasts unless you garden with a microscope in your pocket.
The creatures we do find readily are the larger and more obvious groups such as millipedes (Diplopoda). The majority of these slow-moving but elegantly walking multilegged soil dwellers are shredders of dead plant materials. You’ll find millipede species in the top layers of soil and under logs, mulch and tree litter layers as well as your compost bin.
They are useful to us because making dead leaves, fruit and branches a lot smaller really speeds up the decomposition process as fungi and bacteria get straight into the job by invading the tissues.
One exotic variety (spotted snake millipede) has found its own way of creating dead plant material, by literally tunnelling into ripe strawberries, which will rot soon after the invasion.
Most gardeners will be familiar with slaters, woodlice or pill bugs (Isopoda). These are terrestrial versions of our crustaceans (lobsters, crabs and shrimps) but few people are aware that we have a large number of marine isopods.
In your garden they act as omnivores of dead stuff, but often specialising in decaying wood. That is a hard material to chomp on, so they prefer to get into wood that’s already softened by all kinds of borer beetles, bark beetles and fungal decay. Once again, their mission in life is to break it all down into smaller pieces that can be broken down further by caterpillars and other recycling invertebrates.
The calcium carbonate skin of slaters is hard but not entirely sealed to avoid losing water through evaporation, so they tend to stick to moist habitats (bark mulch and logs). Some of our native species also prevent desiccation by rolling themselves up into a small ball.
Their skin might be tough, but it doesn’t prevent predation by the slater’s main enemy: the slater spider.
These pink and cream-coloured predators have very sharp mandibles that can penetrate the slater’s defences. Often the slater spiders can be found in the same habitat as the slaters themselves, on top of the soil.
Another common soil and compost dweller is the amphipod (landhopper). These shrimp-like, jumping critters are Crustacea that have adapted themselves to live on the land, especially in dark, moist places. They, too, are fabulous recyclers and part of the composting system of our forests. Although most of them live in or on soil (under logs, branches and thick layers of fallen leaves), some have been found doing their job in the tops of trees, under mosses and in epiphyte communities.
Perhaps the most numerous critters in the top layers of soils and in litter and compost are the Collembola. We know them as springtails and the 250 or so different species in New Zealand are perfectly adapted to living in damp, dark places: under bark, in soil, amongst wet forest litter and in our domestic compost bins.
They come in pink, blue, white and purple (that’s as far as my taxonomic knowledge goes) and sometimes float in large rafts on puddles of residual water after a downpour. In my compost bin, I always applaud the millions of white collembola that literally occupy themselves by delivering the ultimate finished product: organic matter, ready for inclusion into the soil.
With all this abundant life in the soils around our forests and gardens, there is a more than ample opportunity for predatory critters. Some of the most gorgeous protein hunters are the turbellaria or flatworms.
Imagine a very flat cross between a medium-sized earthworm and a slug, cruising on a pad of slime at night in conditions of high relative humidity.
Their colours vary from brown to black and purple, blue or bright orange, often with a “racing stripe” of complementary colours on its back. They can stretch and contract their bodies with ease.
Flatworms do not have a mouth, nor a traditional gut system. As they slither over the soil, prey species (worms, slugs and other small invertebrates) are literally digested by the mucous on the underside of the body.
During the day, you can find them under rocks and paving stones, under branches and old planks as these habitats provide the best relative humidity. In dry summers, the flatworms can move deeper and deeper into the soil.
When you find them, it’s best to leave them where they are, because any disturbance or capture often results in rapid decline and death of the flatworm as they simply “auto-digest” themselves!
The king of the castle in terms of invertebrate predator on the soil has to be a centipede (Chilopoda). We have many different species in half a dozen orders, ranging from fast-moving, stout hunters to very long and thin predators that move much slower through tunnels, cracks and crevices in the soil.
Their colours are usually in the brown to light yellow range and the diagnostic feature to look for is the single pair of legs for each body segment (a millipede has two pairs of legs per body segment).
Most centipedes are harmless (unless you look like a live invertebrate) but our famous native giant centipede (up to more 15cm in length!) does not have a sense of humour when it comes to human interaction: it can and will bite, and can cause a considerable amount of distress and pain.
So, when you start digging in the “dirt”, there is a whole different world down there that looks after your soil, your organic matter and your garden’s fertility.
And you know what? I haven’t even covered the class Insecta yet!
Ruud Kleinpaste, aka The Bugman has been presenting television and radio shows on entomology and natural history for more than 30 years. He lives in Christchurch.