Most of us think of soil as something to be washed out of clothes, swept off floors, or avoided after a rainfall. We use expressions such as “dirt cheap,” indicating a pretty low regard for the stuff.
But that’s just because we think of soil as boring and unimportant. It’s actually an amazing substance that makes life on dry land possible, because soil is where the nutrients and moisture required by plants is stored, and without plants to eat, there would be no animals–and, ultimately, no us.
Soil starts out as rock that didn’t know enough to get out of the rain. Over the years (lots and lots of years), rock is broken down physically and chemically by weathering. This broken-down rock, when mixed with organic matter, produces soil.
Every patch of soil is different, but it can generally be divided into four layers, labelled A, B, C and R. A, the topmost layer, is the most-weathered and leached (leaching is the dissolving and carrying off of chemicals by water). It also contains the most organic matter and plant nutrients. B consists of moderately weathered soil, and tends to accumulate the products leached from A. C contains the parent material for A & B–whatever they started out as before being weathered–while R is not really part of the soil proper; basically, it’s whatever lies under C–usually rock.
An area’s soil profile depends on several factors: the amount of leaching rainfall (total rainfall minus potential evaporation), the temperature (which influences the solubility of the chemicals in the soil), the topography (which has an impact on drainage and erosion), vegetation (which determines humus content), soil organisms (more on those later), the parent material, and, finally, time. Over time, all these factors change, and with them, the soil.
We live in a region with clayey, dark-brown soil that has a dark-coloured A layer and a brownish B layer, underlain by a grayish layer of lime carbonate accumulation. It’s the A layer we’re most concerned with, because that layer grows our crops and our grass and flowers and blows around so vigorously in the meantime. But it wouldn’t be able to do any of those things (except blow) if soil were just weathered rock. It’s the organic component that makes the difference.
You wouldn’t think it to look at, but the soil teems with life. In fact, one square metre of topsoil can harbor one billion organisms. (Feel free to calculate the total number of soil organisms on the planet. I won’t wait.)
These soil organisms come in a variety of flavors. There are the protists (bacteria and algae), microfauna (protozoans, flatworms, etc.–things that feed on other microscopic organisms), the mesofauna (including mites and springtails, which feed on micro-organisms; proturans, which feed on fungi, and pauropods, which feed on decaying plants), the macrofauna (slugs, snails, millipedes, etc.), and, finally, the megafauna, which include various burrowing animals, including the large earthworms.
(In fact, some scientists say earthworms may be the most important creature in the soil. In the process of burrowing through the ground they pass both organic and inorganic matter, aerate the soil, break up the litter of organic matter on the surface, and “stir” the soil vertically. It’s been estimated earthworms completely turn over the equivalent of all the soil on the planet to a depth of one inch every 10 years.)
All of these organisms play an important role in the creation of humus. New humus begins to form when a leaf falls to the ground. First, water-soluble compounds leach out of the leaf, and the microflora (fungi, etc.) attack its structure, softening it. Now the invertebrates (worms, larvae, etc.) fragment it to a mulch, but don’t change it much chemically. That’s left to the primary decomposers, smaller creatures who break the leaf down into simpler compounds. Finally, secondary decomposers break it down even further. Thus the leaf is constantly digested and redigested by waves of increasingly smaller organisms. The final humus may have only one quarter of the volume of the original. It is then mixed with the soil by the earthworms and other burrowers. Production of humus is a relatively rapid process; organic matter in soil can be doubled (or, on the down side, reduced by half) in five to 50 years.
Soil organisms also play important roles in the carbon, nitrogen and sulfur cycles. In the carbon cycle, plants use carbon dioxide, animals eat the plants, animals die, soil organisms eat the dead animals, and the soil organisms then release the carbon dioxide for plants to use all over again.
Nitrogen is essential to protein formation in plants; its availability in the soil is a basic determinant of the soil’s fertility. Soil organisms help make nitrogen available to plants.
Finally, soil organisms break up naturally abundant sulfur compounds in the soil, making them available for plant use, too. The smell of rotten eggs in swamps and marshes is hydrogen sulfide produced by soil organisms in this process.
Generally, we don’t think about soil much. But we should. Recent surveys indicate that one-fifth of Canadian farmland is affected by wind and water erosion, salinity, loss of organic matter and soil compaction. Saskatchewan has 40 percent of the arable land in Canada. It also has the nation’s worst dust storms–and the second-worst in North America (behind Texas). We lose 161 million tonnes of soil annually, more than half the annual soil loss, just to wind erosion. (The rest is lost to water erosion.)
Considering the importance of agriculture to the economy–not to mention its importance to one of my favorite activities, eating–don’t you think it’s about time we started giving soil a little respect?
In other words, it’s time to stop treating it like dirt.
