RHS Journals
The Garden
February 2008
Soil: foundation of the garden ecosystem
In the second part of his series examining gardens as ecosystems, Ken Thompson digs into the substance that lies at the root of it all: the soil.
It is a commonplace observation that most of the leaves in your garden survive until autumn. Your hostas may look like confetti, but most herbaceous plants and deciduous trees and shrubs remain largely intact.
However annoying slugs, caterpillars and aphids may be, they stop short of eating everything there is to eat in the average garden. Yet whether you leave them where they fall or add them to the compost heap, the leaves of autumn all eventually disappear. But where to? The answer, as the old saying has it, lies in the soil.
Cycles of materials are a feature of all ecosystems. For some chemical elements, such as carbon and nitrogen, their brief sojourn in plants and animals is part of a much larger cycle that also includes the atmosphere. Others, such as phosphorus, just go round and round in more or less the same place, with rather little lost or gained. Either way, the paths such elements take are characteristic of different kinds of ecosystem and, in this respect, gardens are a lot like woodland and not at all like pasture, for example. In gardens, most plant growth eventually goes through the decomposer food chain – it is only eaten after it is already dead.
And where do most of these decomposers live? In the soil, of course.
Subterranean inhabitants
Soils are complex mixtures of inorganic mineral particles, organic material, air and water, and most teem with life. What lives in a particular soil is largely determined by its physical make-up and chemistry. Heavy soils containing much clay tend to waterlog and contain little oxygen, so may not support much life (neither do overly dry sandy soils and those that are excessively alkaline or acidic). Soils perform vital roles for plants, providing physical support and a reservoir of water on which roots can draw. Nutrients dissolved in the water are also key, and are mostly released by the decomposer organisms, from the dead material on which they feed, in forms available to plants.
Most decomposers inhabiting soils are tiny and unimaginably numerous. Just 10g of ordinary garden soil contains 100 billion bacteria – as many as there are stars in the galaxy. These in turn are preyed upon by millions of single-celled organisms, such as protozoa. Fungi and actinomycetes, which look like fungi but are actually bacteria, are equally abundant, but effectively invisible most of the time. The fruiting bodies or ‘toadstools’ produced by fungi, some of which form symbiotic associations with plant roots or mycorrhizae, do appear above ground, but most thread-like fungal tissue – hyphae – remains below the soil surface. Some garden fungi are actually predatory, setting elaborate traps for passing nematodes.
Nematodes or roundworms eat soil fungi, protozoa, bacteria and dead organic matter, and certain species parasitise larger animals and plants. There may be 20 million in a square metre of soil, but they average only 1mm in length so are more or less invisible.
Among the most numerous large animals in the soil (and by ‘large’ I mean at least visible) are springtails, so named for their springing organ, shaped like a tiny tuning fork, that is clipped below the abdomen and sends the animal spinning into the air, like a flea, away from predators. Springtails eat dead plants and fungi, and are the garden equivalent of krill: 1sq m of soil may contain 60,000. Equally abundant are mites which, like spiders, are arachnids with eight legs; mites include in their diverse numbers herbivores, parasites and predatory species.
Larger organisms
If we move up to animals of a size range that might actually be noticed, by far the most common in garden soils are woodlice, at least as numerous as all other larger ground-dwelling animals combined. Though extremely abundant, there is a range of only about 30 different woodlice species found in the UK. In a study of 61 gardens in the Biodiversity in Urban Gardens in Sheffield (BUGS) project , we found only eight species, of which two (Porcellio scaber and Oniscus asellus) were both ubiquitous and hugely abundant. They look similar, but Porcellio is dull and rough, while Oniscus is more shiny. There are perhaps half-a-dozen species of woodlouse in the average garden, but only three or four are common.
Woodlice are crustaceans, like crabs, and are poorly adapted to life on land. Breathing through gills they are always in danger of drying out, which is why they spend the day in cool, damp refuges, emerging only at night. One or two species of garden woodlice will eat soft, living plants, but the jaws of most (including the most common) are not tough enough, so they mainly eat dead plants, fungi, animal remains and dung.
Unrelated to woodlice, but sometimes looking rather similar and sharing their habits and food preferences, are millipedes. Millipedes sometimes get the blame for holes in plants such as potato tubers, but their jaws are weak and they probably only gain entry after previous damage by slugs or insects.
Continued >