Healthy Trees Need Healthy Soil

Art Morris, NUF General Manager and The Georgia Urban Forest Council's 2018 president, explains why soil preservation and soil building benefit our urban green canopy for Shade Magazine

SHADE: Now that our communities are focusing more on greenspaces, what are some of your concerns?

MORRIS: Over the last several decades we've come a long way in improving public awareness of the benefits of trees and urban greenspaces. The popularity of tree-planting projects, ubiquity of community gardens and prevalence of tree protection ordinances are a testament to the work of organizations like the GUFC. Although we try to preserve trees during development and plant new trees wherever possible, not enough thought is given to what it takes for those trees to reach a healthy maturity. There is an old adage, "Dig a $100 hole for a $10 tree." We should invest at least as much or more effort in protecting and cultivating the soil as we invest in the tree itself. If we unpack that adage, there is a lot of wisdom there.

SHADE: Before trees are planted, what do we need to understand about soil?

MORRIS: Urban and suburban (even rural) soils are entirely different from native forest land. In our planning prior to tree planting or preservation efforts, we should place more emphasis on determining the resources a tree will require from the time of planting through maturity. In almost every case, the limiting factor comes down to soil quality and quantity. Urban trees and forests, almost by definition, are characterized by limited root zones and poor, compacted soils. Many trees survive these conditions, but can we expect themvto thrive? I wonder sometimes whether a tree planted today in an urban or suburban soil will have the chance to reach the majestic size of the trees we currently enjoy, which were planted or seeded 100 years ago.

SHADE: Why is unhealthy subsoil a critical problem?

MORRIS: Tree preservation ordinances throughout the state do a commendable job of protecting trees' critical root zones, but few of them include specific language regarding soil preservation and soil health independent of the tree root zone or replanting plans. In most cases we're left with an unhealthy compacted subsoil on development and redevelopment sites. Even in cases and areas where the native soil is undisturbed, developments on former pastureland or farmland suffer from adverse soil conditions even without further construction impact. The prevalence of poor soils in urban and suburban areas makes me wonder whether we're planting future champion trees or simply hoping that our new plants won't stagnate in poor soil. Sure, mulching, fertilizing and watering will help, but if we're planning for our new plantings to become future champions, we should be doing a lot more.

SHADE: What can you tell us about building healthy soil?

MORRIS: I'm intrigued by the green infrastructure movement; the growing emphasis on incorporation of greenspaces into built spaces is a huge opportunity for us to discuss soil health. I'd love to see community standards for soil preservation and soil building. Even on "undisturbed" sites like former pasture or farmland, we're not dealing with a native soil profile and investment in soil building should be considered. That said, building healthy soils is not easy. At minimum, it requires tilling the subsoil, incorporating composted
green waste or biosolids, nutrient testing and amendments, and then planting the site with appropriate overstory and understory plants to begin the process of nutrient cycling. We'll also need to encourage property owners and managers not to rake leaves, bag clippings and remove carbon from the site.

SHADE: In what ways can healthy soil contribute to stormwater solutions?

MORRIS: Dealing with stormwater is an ever-growing challenge for planners. Rainfall that falls on impermeable surfaces and compacted soils rapidly runs off of the site, ultimately to become someone else's problem. Most municipalities have ordinances to address this, but they may not highlight the benefits of healthy soils with regard to stormwater retention. Notwithstanding the benefits of improved rainwater infiltration of healthy soils, a soil building plan may incorporate berm and swale detention systems to slow surface water flow and further improve infiltration. Healthy soils have a higher infiltration rate, slow the flow of water, reduce erosion, grow healthier plants, reduce leaching of pollutants, and lower the cost of fertilizers and irrigation. Moreover, a focus on building healthy soils will improve the health of community forests while also providing a vehicle for green waste, food waste and biosolids to be utilized within the community.

SHADE: What is your "big picture" for urban greenspace in the years ahead?

MORRIS: Over the last few decades we've come a long way in garnering public support for tree preservation and urban forests. I hope that over the next few decades we can improve the public understanding of the value of healthy urban soils, and even stimulate an investment in building healthy soils as part of overall site and project planning. This is a big task that will require us to nudge public perception of urban "landscapes" away from resource intensive manicured lawns and toward more "woodland garden" setting. We'll need to rethink our tree preservation ordinance language to incorporate soil preservation and guidelines for building healthy soils. I challenge GUFC members, community planners, architects and developers to incorporate soils, as well as trees, into decision making and planning. 

See the Shade article in the 2018 issue here!