{"id":4844,"date":"2021-06-16T15:00:57","date_gmt":"2021-06-16T19:00:57","guid":{"rendered":"https:\/\/seeds.ca\/schoolfoodgardens\/?p=4844"},"modified":"2023-01-04T12:41:40","modified_gmt":"2023-01-04T17:41:40","slug":"soils-a-crash-course","status":"publish","type":"post","link":"https:\/\/seeds.ca\/schoolfoodgardens\/soils-a-crash-course\/","title":{"rendered":"Soils: A Crash Course"},"content":{"rendered":"

Written by: Christina Pizzonia<\/strong><\/p>\n

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Soil\u2014we\u2019ve all seen it, heard about it and, at the very least, stepped on it. But what is soil actually? (hint: it\u2019s not just \u2018that brown stuff in the ground\u2019). How can gardeners improve the quality of their soil? Continue reading to find out!<\/span><\/p>\n

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Where does soil come from?\u00a0<\/b><\/p>\n

Contrary to popular belief, the soils we know (and love) weren\u2019t always here. In fact, while the Earth is ~4.5 billion years old, the reddish-brown soils common today only emerged around 400 million years ago!<\/span><\/p>\n

The formation of soil started with bedrock (basically – different kinds of rocks, like igneous, metamorphic or sedimentary). Over time, bedrock was broken down into smaller and smaller pieces through physical and chemical means in a process known as \u201cweathering.\u201d The result? What we now recognize as soil!<\/span><\/p>\n

Different factors, including the climate, timespan, topography and overall biodiversity can influence the kind of soil produced from a given type of bedrock. In fact, scientists can actually use these factors to predict the kinds of soils that will be present in your neighbourhood in the future!<\/span><\/p>\n

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The horizons<\/b><\/p>\n

Once well-formed, most soils can be split into different \u201chorizons\u201d or \u201chorizontal layers.\u201d These are illustrated in the soil profile below:\u00a0<\/span><\/p>\n

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Plantlet.org<\/a><\/p><\/div>\n

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The top-most layer is known as the O horizon, and is found in most soils. It\u2019s composed mainly of organic materials, including leaves, twigs, leaves, mosses, etc.\u00a0<\/span><\/p>\n

The next layer is known as the A horizon (surface\/topsoil), and is composed predominantly of minerals (from bedrock). The A horizon may also contain some organic matter from the O horizon.\u00a0<\/span><\/p>\n

The following layer is the B horizon (subsoil), and contains the accumulation of minerals from the upper horizons.\u00a0<\/span><\/p>\n

The C horizon (substratum) sits just beneath the B horizon, and is composed mainly of broken down rocks (\u201cparent material\u201d to scientists).\u00a0<\/span><\/p>\n

The bottom-most horizon is the R horizon, or bedrock. This layer is made up of the original rocks from which the parent material was made.\u00a0<\/span><\/p>\n

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The four major ingredients<\/b><\/p>\n

\"BoromirThe first (and most well-known) ingredient of soil is organic matter, making up only ~5% of soil by volume. Organic matter (i.e. material derived from decaying plant, animal and microbial residue) is generally darker in colour, and is an excellent source of essential nutrients. Microorganisms thrive in organic matter, converting it into substances that can be easily uptaken by plants. Perhaps the most famous interaction between microorganisms and organic matter in soil involves the nitrogen cycle\u2014microorganisms \u201cfix\u201d the nitrogen in organic matter, transforming it into a form that can be used by plants! Fun fact: A teaspoon of healthy soil actually contains more organisms than there are people on earth!<\/span><\/p>\n

Water and air (both make up around 25% of soil each) are another two ingredients. Water is found in the pore space of soil, and acts as a transporter for essential nutrients (plants uptake most nutrients from soil using a water-based transport system). Air is also critical, as it provides oxygen (air is ~20% oxygen) to organisms living within the soil.\u00a0<\/span><\/p>\n

And, of course, last but not least, minerals. Soil minerals make up the bulk of soils (~45% by volume), are divided into different main classes based on the size of their particles. Clay particles are the smallest (< 0.002 mm), followed by silt (0.05 – 0.002 mm), then sand (2 – 0.05 mm). Different types of soil contain different ratios of these three types of particles.\u00a0<\/span><\/p>\n

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Nature.com<\/a><\/p><\/div>\n

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If you want to find out the approximate percentages of each particle type (sand, silt or clay) that makes up your garden, you can! Check out <\/span>this video<\/span><\/a> to find out how (it\u2019s super simple, I promise!). Now, if you\u2019re dissatisfied with the results, and want to change the composition of your soil to better suit your gardening needs, that\u2019s possible too\u2014and you wouldn\u2019t be alone in that goal. Sandy soils contain a lot of pore\/open space and can barely hold water\u2014while silty and clay-based soils (due to their smaller particle sizes) are easily compacted and can restrict water\/nutrient uptake by plant roots. The solution to both problems is to add organic matter. Adding organic matter to clay\/silty soil can improve infiltration and pore space (i.e. spots for air\/water to reside), while adding organic matter to sandy soil can drastically increase the water-holding capacity of the soil. That\u2019s what I call a win-win.\u00a0<\/span><\/p>\n

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Other Classifications<\/b><\/p>\n

In addition to being classified based on their particle type (sand, silt or clay), soils may also be classified based on their relative acidity. Chemists measure the acidity of a substance using something called the pH scale, which typically ranges from 1 to 14.\u00a0<\/span><\/p>\n

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Substances with a higher ratio of hydronium (positive) to hydroxide (negative) ions are said to be acidic, with a pH ranging from 0 to ~6.5, while substances with a high ratio of hydroxide (negative) to hydronium (positive) ions are said to be alkaline, with a pH ranging from ~7.5 to 14. Substances with an equal ratio of both ions are said to be neutral, with a pH of ~7.\u00a0<\/span><\/p>\n

To test the pH of your soil, you can purchase pH paper from your local hardware store. Most plants grow best in slightly acidic soil, with a pH of about 6.5 to 6.8, through depending on what you\u2019re growing, a healthy range can be anywhere from 6.0 (e.g. potatoes, strawberries, blueberries, etc.) to 7.5 (e.g. cauliflower, cabbage, etc.). If the pH of your soil is significantly higher or lower, plant roots may have a difficult time absorbing essential nutrients\u2014 while concentrations of certain metals (e.g. iron, aluminum, manganese, etc.) increase. Although trace amounts of metals are a necessary component of any soil, high concentrations can have an adverse effect on plant growth.<\/span><\/p>\n

Safely improving the pH of soil takes time (think: at least a few months), but it <\/span>can<\/span><\/i> be done. If your soil is too acidic (pH < 6.0), it may be due to high levels of precipitation or a sudden influx of organic matter (pine needles, peat moss, oak leaves, etc.). To increase your soil\u2019s pH, you can add liming materials (dolomitic limestone, wood ash) or calcium hydroxides.\u00a0<\/span><\/p>\n

If your soil is too basic (pH > 7.5), it may be due to the addition of nitrogen fertilizers or simply from the parent material itself. The addition of elemental sulfur can rectify this imbalance.\u00a0<\/span><\/p>\n

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Closing Thoughts\u00a0\u00a0<\/b><\/p>\n

As we\u2019ve seen above, soil is super complex – and is composed of many different parts all working in harmony. Like it or not, soil is essential for life. It can even:\u00a0<\/span><\/p>\n