Many years ago, I made the mistake of referring to soil as "dirt." It is definitely not something one should do in the presence of soil scientists and it illustrated my ignorance of an important discipline in science.
Dirt, for lack of a better definition, is the inorganic constituents of soil. Soil, on the other hand, is a living community of organisms ranging in size from the microscopic to the massive.
Indeed, if you go out into your backyard and grab a handful of top soil, you are likely holding more organisms in the palm of your hand than there are people in Canada. Everything from bacteria to fungi to insects to earthworms can be found in that handful.
We depend on this thin layer of top soil for our existence. Soil depth around the globe on average is only a metre deep. Yet within this thin layer of soil lies all of the nutrients and minerals necessary for plant growth.
The chemistry which occurs within soil is quite remarkable. Ions move in capillary fashion within and between layers. Various organic functional groups chelate cations. Water transport can be both inhibited and accelerated by soil structure.
We understand a great deal about soil but there is still so much more to learn. Some 500 years ago, Leonardo da Vinci said: "We know more about the movement of celestial bodies than about the soil underfoot." This is probably still true today but we are learning.
The king of the underground, though, has to be the common earthworm. Not a creature with a majestic pedigree. Most of us probably think of earthworms as slimy and disgusting if we think of them at all.
But they are fascinating creatures.
The best guesses put the number of different species of earthworm at around 3,000. We don't really know the exact number because there are many places in the world where the structure and nature of soil has yet to be explored.
Furthermore, while earthworms have no eyes, they are sensitive to light and generally avoid it. The only time earthworms rise to the surface is during rain when there is the potential to drown in the soil. Light avoidance makes it hard to dig for earthworms.
Interestingly enough it is the rhythmic vibrations of the rain on the surface which induces this response and many animals, including humans, have learned to use vibrations as a way of attracting earthworms. (There is a world championship for worm charming.)
Earthworm species can range in size from under one centimetre to over three metres in length. The Giant Gippsland worms from Australia are typically a metre long, contracted, and two centimeters in diameter but some specimens reach three metres contracted and over four metres when extended, weighing in at more than a kilogram. In South Africa, seven-metre-long earthworms have been reported.
However, for all their diversity and wide range, the most fascinating aspects of the earthworm might be their capacity to act as chemical engineers or maybe "chemical factories" would be a more apt description within the soil.
At a simple level, earthworms and their gut microbes process nitrogen, phosphorus, and potassium. A typical household fertilizer might be 10-10-10. Earthworm castings are typically around 3-1-1. Comparatively, castings are not as rich in nutrients as an industrial fertilizer.
However if you were to add up all the earthworms under an acre of land, it could easily be more than a million which means a lot of castings. Perhaps more to the point, the nutrients are embedded in the soil and not just at the surface. This facilitates transport to roots.
Earthworms also churn up the soil - which can be both beneficial and detrimental depending upon the soil type. In doing so, they break down animal and vegetable matter helping to produce an amorphous material called humus. It consists of the remnants of cellulose, starch, lignin and other biomolecules not consumed by other organisms.
Humus helps to harbor nutrients and moisture within the soil column.
Earthworm biochemistry also provides nitrates into the soil through the decomposition of urea. This enriches the soil content as both ammonia and nitrates can be assimilated by plants.
They are able to breathe in carbon dioxide and through calciferous glands they can fix it as calcium carbonate or limestone pellets. Understanding this process and replicating it in a laboratory setting might provide a mechanism for carbon sequestration.
Earthworms also produce a unique set of surfactants, called drilodefensins, which prevent protein clumping and enzyme malfunction. Such compounds might represent drug targets for human use.
Charles Darwin once said of earthworms: "It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures."
Certainly there is more science in soil than meets the eye.
