Knowledge

Apr 12 / 21 Forming Macro Aggregates to Break up Soil Compaction

Darcy M, Lepine

Dealing with Soil compaction in Urban Forestry.

Over the last five years, we listened to hundreds of contractors and worked with several municipalities across Canada. Yes, soil compaction and poor soils continue to be one of urban forestry’s top challenges every season. Although soil compaction is essential to the stability and longevity of all roads, pathways, and sidewalks, it’s a real hindrance to root growth. Our research suggests that plans to reach urban canopy goals should first prioritize treating compaction issues in urban trees over simply planting more trees—we present a quality over quantity argument.

Properly managing soil compaction requires applying amendments that will foster the connection and improve the properties of native soils. Without a connection to the parent soils, in time, trees will die; but arguably worse, they may live but struggle to thrive.

Reaching ambitious urban canopy cover goals requires that every tree planted reaches its maximum genetic potential as quickly and efficiently as possible. Climate change is a race against time. While goals are great, it’s the execution that matters.

Building Ecosystems that Resist Soil Compaction

Biological, Chemical, and Geological properties compose the framework of every ecosystem. Each of these factors are interdependent and affects the other positively and negatively. As so, we see an ordered pyramid for successful ecosystem building. This pyramid of success, we suggest, is built by first addressing the geological, then chemical, and then biological properties of an ecosystem, in that order. three soil properties of every ecosystem

Applying modern ‘standard practices’ reflects the ten plus percent, five-year average mortality seen in most municipalities¹—for instance, digging a hole, filling it with whatever compost is available, backfilling and building a mulch volcano for every different environmental situation, can’t possibly reach the efficiency we need to fight climate change effectively. That’s just looking at the death rate and not the worse case which isn’t being tracked—poor growth and vigour.

Evaluating each landscape independently before planting and applying the appropriate amendments should be standard practice to alleviate soil compaction and grow healthier trees. Not every environment will need to bring in compost to backfill, while others may need extensive core aeration and amendments in the bottom of the planting hole to connect the tree to native soils.

Sometimes mechanical intervention is necessary after planting. However, following the Bio-Geo-Chemical pyramid to ecosystem building can reduce maintenance costs associated with extensive annual pruning and prevent severe issues like root girdling² down the line.

Green Chemistry Affecting Soil Compaction

One chemical method to relieve soil compaction is the addition of humic substances. Humic Substances are the components of ancient oxidized organic matter from when dinosaurs roamed the earth. Think of it like one hundred million-year-old compost. Humic substances like those found in our Hydr8 soil amendment aid in chemically forming micro aggregates and, in turn, macroaggregates from the silt and clay particles in soils. Humic substances complex with ions commonly found in the environment such as magnesium (Mg2+), Calcium (Ca2+), and Iron (Fe2+, Fe3+) create humic colloids³. These colloids bind soil particles together through positive and negative chemical bonds and snowball into larger aggregates.

Furthering Our Understanding of Soil Compaction

Most species of trees planted in urban settings can physically penetrate compacted clay soils, given there’s enough water, oxygen, and nutrients within them. However, a big problem with compaction lies in the anaerobic conditions created by microbes decomposing organic matter that make fermented substances⁴. The stink is the issue for plants, not so much the hardness. Once tree roots reach oxygen-deprived conditions and the presence of ethanol, vertical taproots won’t penetrate this toxic layer and tend to grow horizontally or upward until they reach suitable conditions⁵. They’re left to chase freshwater, often up under our hardscapes. As a result, juvenile trees have a rough time making it past the five-year mark without vertical taproots to anchor them¹. This also makes them prone to drought, weak branches, and blowdown.

Transplant shock combined with compacted parent material, we see, is the number one reason new juvenile trees don’t thrive. Mitigating these two critical factors can reduce transplant and productivity loss by significant margins. It’s critical to understand that if compaction isn’t dealt with first, all efforts to fix ailing trees through chemical or other means after planting are futile and costly. There is no magic bullet, and we could avoid poor growth vigour if addressed at planting. Undoubtedly, trees that don’t penetrate the parent material outside the planting hole will be prone to poor growth vigour and susceptible to increased risk of blowdown and civil liability.

How to Tell When Things are Going Wrong

We can usually visually tell how well trees are growing, or not growing, if they suffer from these issues. We can see trees shoot wispy runners out from the top of the original symmetrical canopy growth. As well, you’ll notice spindly branches growing from the base of the trunk. Overall, you’ll see retarded growth, seen by red dying inner growth (flagging) in evergreens and crispy brown deciduous leaves. (dieback)

Conclusion

Although we have briefly touched base on an essential method of relieving soil compaction, our primary purpose is to understand ecosystem building better. Hopefully, we can improve the idea of standard practices and get away from a one size fits all approach to planting trees in urban environments. After all, trees have limited beneficial timelines of about 30 years in urban settings and won’t pay dividends until they reach significant heights and spread. If we waste time letting them suffer for a decade trying to acclimate to poor conditions, we might as well let our tax dollars rot in the ground alongside.

References:

BACK TO BLOG NEXT ARTICLE