For a tree to survive, it needs 4 essential elements:
- Adequate sunlight;
- Nutrient rich soil;
- Adequate water;
If you remove any of these elements from the equation, the tree will not survive. As living organisms, trees are multifaceted. We, as caretakers, need to ensure trees have these vital elements and allow nature to look after the rest.
Trees are lovers of sunlight and feed themselves through a process called photosynthesis. Trees absorb carbon dioxide from the atmosphere and release oxygen in return. Never before has this function become so crucial for the survival of the human race. If you plant a tree in a space with insufficient sunlight, the tree will underperform. Providing a tree with sufficient sunlight will ensure it survives and thrives.
Soil is the foundation of a healthy urban forest — its life depends on it. In Manitoba, we are fortunate to have some of the most fertile soil in the world, so we have a good foundation. Soil is a complex living environment, not just something we purchase at the garden centre. It takes a millennium to produce a good soil structure and mere decades of abuse to degrade it. Soil fertility is a product of good soil care and not adding synthesized fertilizers. Synthesized fertilizers degrade the soil profile by reducing the microbiology of the soil. Nature can provide fertility as long as one follows the five principles of soil.
5 Principles of Soil
- Limit mechanical, chemical, or physical disturbance to the soil. Tilling the soil breaks down the soil profile and degrades its health by killing off the microbiology. Chemical fertilizers degrade soil structure.
- Protect the soil cover. Bare soil is unnatural and doesn't allow for natural nutrient cycling. Covering the soil with a layer of organic mulch protects the soil from the sun's heat and the wind's erosion. As the cover decomposes, it improves soil structure and is nature's way of nutrient cycling.
- Diversify plant material. A diversity of grasses, ground cover, and perennial plants, including shrubs and trees, improves the overall soil structure. The variety of the different roots of each of the plants improve the soil's overall biodiversity.
- Living roots. Having living roots growing in the soil for as long as possible feeds the soil's microbiology with carbon, which is used as fuel for the nutrient cycling that feeds the plants.
- Integrated animals. In the urban environment, it is impossible to let animals roam freely, as they do in the wild. Adding composted manure and bone meal to the landscape, where necessary, can mimic a partial benefit that animals provide.
If a good soil structure is the foundation of a healthy urban forest, then adequate water is necessary for life to survive. Adding too much water (saturated soil) for prolonged periods will lead to the tree dying of drowning. Too little water and the tree will die from dehydration. Adequate water will ensure the tree will thrive! A healthy soil structure will improve the amount of water available to the tree by improving the water infiltration rate and water holding capacity.
Infiltration Rate: The rate for how much water soaks into the soil. The infiltration rate of water improves as the soil structure improves. What is more important than how much rain fell is how much was absorbed into the soil. On degraded soils with low water infiltration, much of the rainfall is runoff that ends up in storm sewers.
Water Holding Capacity: A measure of how much water the soil can hold. A healthy soil structure with a high organic composition will hold considerably more water than degraded soil. Soil that has more water will ultimately provide better growing conditions for the trees.
To check your soil for adequate moisture, push a spade into the ground to the depth of the blade and take a handful of soil in your hands. Squeeze the soil in the palm of your hand. If you can squeeze water from the soil, you have too much water. If the soil falls apart and won't clump, the soil is dry. If the soil holds together when squeezing it, then the moisture is adequate.
It is best to water heavily once every several days, rather than a little bit daily. When watering, consider that you want the water to infiltrate as deep into the soil as possible.
As living organisms, trees need oxygen to survive. Compacted soil has smaller pore spaces for oxygen, which causes stress on the tree. In saturated soil conditions, water displaces the oxygen in the soil pore space resulting in the tree eventually drowning. Soil with a healthy soil structure will have sufficient pore space in the soil for an adequate amount of oxygen, and the tree will survive and thrive.
Rock vs. Wood Mulch (Biomass)
Rock is often used as a soil cover in tree and shrub beds under the myth that it is maintenance-free. This practice is a false claim. Over time, enough sediment settles in the rock, and seeds begin to germinate. Pulling weeds from a rock bed is difficult, and typically landscape workers resort to chemical herbicide applications to control weeds. The soil under these rock beds degrades and becomes compacted over time, resulting in degraded soil structure – the opposite of what we are trying to achieve.
Should I use landscape fabric under wood mulch?
Landscape fabric under wood mulch restricts the movement of microbial life between the soil and mulch cover. It will drastically slow the process of nutrient cycling — a process we want to increase. Landscape fabric is a fossil fuel-based product and will breakdown over time, leaving microplastic waste.