In the cool, damp, dark season from late fall to early spring, not a lot is growing outside. Even plants under cloche protection or in the greenhouse will be growing — slowly, struggling to turn the low light levels into useful energy for vegetative growth. This is the right time of year to study soil science and think about how to improve garden soil before new seeds are sown or transplants go out in the spring.
There are four central issues that offer a better understanding of garden soil and how it can be improved: Soil structure, soil components, soil life, and soil pH. Gardeners (and farmers) will benefit from contemplating these four factors on their own and in harmony with one another.
This refers to the ratio of particles that make up the soil itself. They may be sand, silt, or clay. The size of these tiny particles dictates physical aspects of the soil like drainage, compaction, and porosity. Sand is made up of relatively large particles, even if it is as fine as table sugar or salt. Sand is essentially ground up rocks, and can be composed of a number of different minerals. Because sand particles are relatively large, the spaces between each are also large, so water tends to drain through it rather quickly. The large size of sand particles also means that they don’t move around that much within the soil structure.
Silt particles, like sand, are the result of mechanical grinding action, but they are much smaller than grains of sand. Think of finely milled flour in comparison to coarse salt when comparing silt to sand. Silt is so fine in texture that it can wash through soils fairly easily. It’s a major contributor to soil’s ability to clump together or form aggregates.
Clay particles are flat and plate-like. Unlike sand and silt, clay is the result of chemical action, as minerals are worn down by water and acids in the soil. Clay particles can be 1,000 times smaller than sand particles, which is why pure clay has such a unique, plastic texture. It clumps together so well that clay soils can have very poor porosity and drainage.
So when soil experts discuss soil texture, it’s the range from sandy soil to clay soil that they’re talking about. The term ‘loam’ is used to describe the ideal range of soil texture, and is made up of 40% sand, 40% silt, and 20% clay. It clumps together and holds moisture well, and it breaks up easily when it’s dry, instead of becoming hard and packed. Good loam contains particles in such a variety of sizes that the spaces between them are also varied, and this is good for porosity.
The four components of soil play dramatically different roles to the health of the soil itself. Minerals (those sand, silt, and clay particles) make up about 45% of the overall mass of soil. Of this percentage, nearly 90% is composed of the elements silica, iron, oxygen, and aluminum. Minerals play a mainly physical role in soil health.
Water and air are both present in soil, and each account for around 25% of the soil’s mass. Water and air interact with the mineral components of soil in ways that affect drainage, aeration, compaction, and porosity. These are important factors when growing plants, as the roots of all plants need to penetrate soils easily, draw off mineral nutrients, absorb water, and exchange gases. Soil organisms, as we shall see, also depend on the varying amounts of water and air present in soil.
The remaining 5% of the mass of soil is made up of organic matter and humus. Gardeners define organic matter as any dead plant or animal matter that is added to the soil. It is typically made up of compost, animal manure, or crops that have been planted the previous season and then tilled under. Organic matter feeds soil organisms, and fuels the “life” of living, healthy soil. Once it has been broken down, and its nutrients consumed by plants and microorganisms, it becomes humus. Humus adds bulk to soil, and makes it that rich, dark colour – which in turn allows soil to absorb and retain more heat, which also makes the whole system thrive.
The “life” of living soil mentioned above is key to the success of organic growing. Healthy soil should be, as one author titled his book on the subject, “teeming with microbes.” Some soil organisms are present in astonishing numbers, too. Look at the presence of various soil organisms in terms of pounds per acre of healthy soil:
Protozoa – 100 to 200 lbs/acre
Actinomycetes – 800 to 1500 lbs/acre
Algae – 200 to 500 lbs/acre
Bacteria – 100 to 2000 lbs/acre
Fungi – 1500 to 2000 lbs/acre
Earthworms – Up to 900 lbs/acre
Even if you don’t care to imagine what 900 lbs of earthworms look like, they and their fellow organisms play a fundamental role in soil health. Each of these different types of soil life acts to break down the nutrients in organic matter into forms that are accessible to plants. Some, like bacteria, may play multiple roles, and act to fix atmospheric nitrogen in the soil. Others, like earthworms, actually play a physical role in soil health by turning the soil, just like we would with a fork. They come to the surface to feed, and drag all of those nutrients down into the soil substrate where they can be reached by the roots of plants, or converted in other ways by the different microbes that exist at various depths.
Supplying organic matter to the soil (literally feeding the life that lives in it) is the central pillar of organic gardening. Soil with a diverse and functioning life within it is inherently more nutritious to the plants that we grow. The process of growing plants is one of drawing nutrients out of the soil so that we can, in turn, enjoy them as food. So organic growers are in a constant rush to push more organic matter down into that soil.
This is the final consideration of soil health, and it has to do more with chemistry rather than biology. The abbreviation pH is shorthand for “potential Hydrogen.” As water enters soil through rain or irrigation, its molecules become part of a chemical soup. pH measures the ratio of positively charged Hydrogen ions to negatively charged hydroxyl ions. Some of the water molecules form solutions with minerals in the soil and create new compounds, and others bond with clay and humus, creating acidic compounds.
Without going to deeply into it, the end result of these chemical reactions is a fluctuation of acidity or alkalinity in the soil. Nearly all food plants prefer soil that is neutral – not too acidic, not too alkaline. The pH range is measured from 1 (extremely acidic like stomach acid) to 14 (extremely alkaline like lye). 7.0 is considered the happy medium of total neutrality, and distilled water will have a pH reading of 7.0. A number of products are available to the home gardener to help determine the pH of their garden soil.
If soil is too acidic or too alkaline, the nutrients present in the soil become less easily available to plants. At the same time, these extremes make other toxic elements in the soil more available. Soil that is routinely saturated with water, as ours is in South Coastal British Columbia throughout the winter, has a tendency to become acidic over time. Wet, acidic soil, also throws the soil life out of balance, and encourages some anaerobic bacteria, while discouraging other soil creatures. As a result, we must take regular action to neutralize the pH of our local soil. See below for the second part of this article: Soil Amendments & How to Use Them.
Download our Soil Science Graphics from the 2018 Gardening Guide.