With all the smells, colours and textures found in a garden, it is easy to forget that about the unsung hero beneath our feet: soil. Soil is a crucial resource for food production, providing the necessary support, nutrients, and environment for growing and harvesting food. A single spoonful contains more life than humans on earth! It is the very foundation of any garden and just like for houses, with weak foundation, everything crumbles.
In this article, we’ll explore why soil matters so much for growing a healthy and sustainable garden and break it down into it’s 3 main components: physical, chemical, and biological.
Physical Properties of Soil
What is Soil?
In simple terms, soil is the mix of organic matter (decomposed or decomposing material), minerals, gases and liquids. It can be only a few centimeters on top of bedrock, like in some places around Nova Scotia, or several meters thick like in the Amazon. Between the surface where plants emerge and the bedrock below, soil is actually separated into several layers: organic layer, topsoil, subsoil, parent material. In gardening, we usually deal with the organic and topsoil layers, so let’s take a closer look at how these layers affect plant growth.
What type of soil do I have?
There are 4 mains types of soil: sand, silt, clay, and loam. Sand particles are the largest compared to silt and clay. They allow good air flow through the soil and drain quickly, which means nutrients are likely to be washed away. Clay has the finest particles that can pack down tightly. This slows drainage but also decreases aeration. On the plus side, clay tends to hold nutrients well. Silt is in between sand and clay, holding moisture and air in between sand and clay. Loam is likely what you think of as the ideal garden soil. Sand and silt have a large proportion compared to clay. Loam also tends to contain more organic matter.
Testing your soil
An easy way to test your soil is to grab a handful, wet it, and notice how it feels. If it is gritty, you likely have sandy soil. If it is very smooth, it would be silty, and if it is slimy, there is more clay.
Another way to do it is to add a handful of soil to a jar, top it with water and add a couple drops of dish detergent. Shake it well and let it sit. After 1 minute, the sand will have settled. After 2 hours, the silt will settle and when the water becomes completely clear, the clay has settled. You can also let your jar rest until the water becomes clear and look at the results then. Take a look at the proportion of each layer and determine how to amend your soil.
Texture vs Structure
Texture refers to the relative proportion of different mineral particles (sand, silt, and clay) in the soil. This is what we determined in the previous section.
Structure refers to how these particles clump together. It influences aeration, drainage, and root penetration. Ideally, moist soil should make 1/2 inch clumps when squeezed in your hand. This structure has good moisture retention, with excess water running off. Roots and microorganisms can also move freely throughout the soil and the good aeration allows oxygen to penetrate in the roots for plants to breathe.
The ideal soil texture and structure allows water and nutrients to be absorbed into the soil, drains excess water, and has good aeration for movement of plant roots and microorganisms.
Chemical properties of soil
The chemical properties of soil refer to the different substances in the soil and how they interact.
Nutrient Content
One of the main soil functions is to provide nutrients to plants. Both macro and micronutrients are essential elements for plant growth. Macronutrients are needed in large quantities and include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients are needed in small quantities but play a vital role in plant growth. It is important for vitamins and minerals to be present in the right quantities because excess elements can cause toxicity and low levels can cause deficiencies in plants.
Soil pH
Soil pH is a measure of how acidic or alkaline a soil is. It’s an important factor in gardening because it influences the availability of nutrients to plants and activity of living organisms in the soil. Optimal soil pH is generally considered to be between 6 and 7.5, although individual plants can have preferences for more acidic (blueberries) or alkaline soils. Outside of optimal pH, various elements may not be available in the right quantities for plant growth or plants may not be able to absorb them. For example, when pH becomes too alkaline, elements like iron become less available causing yellowing on leaves (called chlorosis).
An easy way to test soil pH is with a pH soil tester or a multi-purpose soil tester (that measures light, moisture and pH), which are widely available in garden centres.
Soil Biology
The last pillar of soil health is the life contained in it. Soil organisms (called soil biota) include mammals, earthworms, insects, fungi, and bacteria. Bacteria, viruses and fungi are what we call microbes. They all play a role in either nutrient cycling (ie making nutrients available to plants), nutrient uptake, or soil structure.
Nutrient cycling
Nutrient cycling is the process where nutrients move through the soil, plants, animals, and back to the soil. When plants and animals die, their bodies become organic matter in the soil. Microbes like bacteria and fungi break down this organic matter into simpler substances. Nutrients like nitrogen, phosphorus, and potassium are released into the soil in forms that plants can use.
Soil life is responsible for supplying approximately 75% of plant-available nitrogen and 65% of available phosphorus. 1
This decomposition process contributes to soil formation and what can make your garden self-sustaining to an extent. When plants lose their leaves or die back in winter, leaving that plant material in your garden beds will feed the soil organisms which will in turn provide your plants with nutrients during the following season.
Nutrient uptake and soil organisms
In addition to making nutrients available, microbes can also help plants absorb nutrients.
Some bacteria form a partnership with plant roots, specifically legumes. The bacteria enter the roots and form little colonies that can be seen when pulling up pea plants for example. The bacteria transform nitrogen gas found in the air we breathe into ammonia which is used by the legume plant. In exchange, the plant provides the bacteria with sugars it makes through photosynthesis.
Another example of this type of partnership is between mycorrhizal fungi and plant roots. The mycorrhizal fungi attach to the roots and form an extensive network of very small root-like structures. Because plant roots are quite thick, they cannot extend as far into the soil as the fungi. The mycorrhizal extend the area where plants can reach nutrients and also connect plants with each other. These networks allow plants to exchange nutrients and communicate, such as sending signals of stress (drought, pests) to other plants.
Microbes also help plants defend themselves against pests and diseases by boosting their immune system.
Soil structure
Burrowing animals like small mammals and other large organisms affect soil in very visual ways. Their tunnels, which we can observe with the naked eye, have an impact on the texture and structure of soil. They physically create space that improves aeration and the animals leave behind organic matter in the form of droppings, nesting material, fur, food remnants, etc.
Other organisms like bacteria secrete substances that form soil aggregates, changing the structure of soil on a tiny scale.
Conclusion
To have a self-sustaining and thriving garden, all three soil characteristics need to be healthy and balanced. As you might have noticed, each property affects the others. For example, improving the physical structure of soils helps retain nutrients and allows organisms to move freely. So when deciding what amendments would create healthy soil in your garden, take into account each aspect of soil health.
Luckily, increasing soil fertility does not have to be complicated. For most gardens, adding organic material and some compost will be enough to create healthy soil for growing plants. You can do this when you plant out your garden. Mix a little compost in your hole and mulch around your plant with organic materials like leaves or grass clippings. With time, organisms will break down the mulch, releasing nutrients and drawing them from the soil surface deeper into the soil profile. The movement of these organisms will in turn improve texture and structure.
Be cautious! It can be easy to overdo it and think that adding more compost will improve soil health faster. But, if there are more nutrients than soils can handle, the excess will run off. Adding too much compost, can be a waste of time and money and may affect nearby soils and waterways.
When it comes to adding compost, less is more.
Nurturing the three pillars of soil allows us to spend less time maintaining our garden and more time enjoying it!
To learn more about soil health, and easy steps to boost it, take a look at this article.
References
University of Minnesota Extension. Soil Biology. https://extension.umn.edu/soil-management-and-health/soil-biology#:~:text=Soil%20biological%20processes%20are%20responsible,food%20and%20a%20favorable%20environment.
Wisconsin Horticulture, Division of Extension. Mycorrhizae. https://hort.extension.wisc.edu/articles/mycorrhizae/
