Article: McGrath, D., Henry, J., Munroe, R., & Williams, C. 2020. Compost improves soil properties and tree establishment along highway roadsides. Urban Forestry & Urban Greening, 126851.
Cities are increasingly recognizing the many services that trees provide, such as reductions in carbon dioxide emissions and air pollution and the prevention of erosion. One way to increase tree coverage in cities is to plant trees along highways. While this would provide many benefits, it can be challenging; roadside construction often compacts soil, which makes it harder for roots to access the water and nutrients that they need from it. These soils are also often missing “topsoil”, or the top layer of soil that tends to have more organic material, which is once-living material now decomposing that can hold water and nutrients well. The poor quality of these soils means that not as many of the planted trees are successful, and they may need continued attention and care over time, which can be impractical.
One way to improve the success of roadside plantings is to improve soil quality before the trees are even planted in order to improve their odds of success. In doing so, it is important to loosen up the soil, which allows water and nutrients to flow more freely through it towards roots. It may also be important to add organic material, such as compost, which helps store water and nutrients in soil so trees can access them more often.
To learn more about the importance of decomposing organic material in efforts to improve soil quality for roadside plantings, Dr. Darby McGrath and her colleagues at the Vineland Research and Innovation Centre in Ontario, Canada, conducted a 5-year study to help determine the extent to which adding organic material – specifically compost made from food waste – might improve the quality of soil and thereby improve the growth and success of these roadside trees.
Testing Out the Treatments
McGrath and her team selected two sites along Highway 406 in Thorold, Ontario for this experiment. At each site, they established several treatments to test whether or not loosening up the soil (using a rotary spader) would improve soil quality and tree growth. In addition, they added different amounts of compost: 10%, 25%, and 50% on a per-volume basis (that is, volume of compost per volume of soil) to soil that had been loosened up as well. They copied each of these five treatments (no treatment, loosening soil, and then loosening soil plus three levels of compost) in each of their experimental sites, so each site had two blocks of treatment. They used compost made from food and yard waste.
After establishing these planting beds and measuring different properties of the soil in order to determine the starting conditions, the team planted 6 or 7 Freeman’s maple seedlings in each of the 10 conditions in both sites. The specific type, or cultivar, of Freeman maple (Acer x freemannii) they chose was Autumn Blaze ‘Jeffersred’, which is popular for planting and moderately drought tolerant, meaning that it could potentially thrive without additional maintenance.
Getting the Dirt on Compost
After 5 years, in 2019, the research team did find that soil conditions changed as a result of these treatments. In particular, the density of the top layer of soil was lower in the treatments that received more compost (25% and 50%) than in the other treatments, and the amount of organic material in the soil was higher in these treatments as well.
As for trees, the 10% and 25% compost treatments seemed to be the best for tree growth and success. More trees survived in both the loosened control and in the 25% compost treatment than in all other treatments. Trees growing in the planting beds that had added compost grew more over time, and trees growing in the 25% compost condition grew most in diameter, though trees in all compost treatments grew more in diameter than the trees that did not have added compost. In this way, the research team clearly identified that adding compost does improve tree growth in these difficult planting locations, and that between 10% and 25% compost is ideal.
As is so often the case in research, not everything went according to plan. Despite putting trees in plastic collars to discourage animals, they did find rodent damage on some trees. They also noticed that the compost they received was not “finished” – that is, it was still actively and quickly decomposing. When compost is still active like this, it heats up due to all of the microbes in it actively breaking down materials. This is kind of like how we get warmer and break a sweat when we are active. This heat caused additional stress on the trees, which could probably explain the poor success rate of trees in the treatment with the highest levels of compost.
These unexpected occurrences reminded the team that outside factors can stress and kill trees. Therefore, in studies like these, it may be wise to not only consider the number of trees that survive, but also any other measurements that might say more about how well the trees actually grew in these conditions. It is important to measure changes in the dimensions of trees as well to get a sense of how these treatments influence their growth, not just their chances of survival.
Applied research like this study by McGrath and her colleagues is important in that it helps cities come up with evidence-based ways to find solutions to improve the environments more and more of us are living in now. In this case, they present solutions to multiple environmental issues, by proposing a way to support methods of increasing tree coverage (thereby trapping carbon dioxide, improving our air quality, reducing erosion, and more) while also providing a good use of compost made from food scraps and organic wastes.
To learn more about other applied research projects and resources relating to greening the landscape, check out the Vineyard Research and Innovation Centre’s Greening the Landscape website.