Soils – what trees need to thrive Meghan Midgley, PhD Soil Ecologist, The Morton Arboretum Transportation and Trees Workshop March 2, 2017

Likely problems with roadside soils: Compacted Low nutrient availability High salt loading Due to location and road construction soils are typically degraded by processes intended to facilitate building construction, such as vegetation clearing, topsoil removal, grading, and compaction

Overall goal: mimic natural soils

Overall goal: mimic natural soils What are the characteristics of a forest soil? More pore space Organic matter Structure

Soil compaction is the most prevalent of all soil constraints on road tree growth Roots can’t penetrate

Soil compaction is the most prevalent of all soil constraints on road tree growth Roots can’t penetrate Low water availability

Soil compaction is the most prevalent of all soil constraints on road tree growth Roots can’t penetrate Low water availability Not enough oxygen for roots

Soil compaction is the most prevalent of all soil constraints on road tree growth Low water availability Roots can’t penetrate Not enough oxygen for roots Too much carbon dioxide

Deep ripping alone is a short-term solution!

Soil profile rebuilding is a long-term solution Spread compost 4 inches deep over the surface of the existing soil. The process was developed by a Virginia Tech research group led by Dr. Susan Day. 

Soil profile rebuilding is a long-term solution Spread compost 4 inches deep over the surface of the existing soil. Use a backhoe to scoop a bucket of soil with compost on the top, lift it several feet in the air, and drop it. Breaks up compacted soil into large clods Creates veins of compost down 24 inches The process was developed by a Virginia Tech research group led by Dr. Susan Day. 

Soil profile rebuilding is a long-term solution Spread compost 4 inches deep over the surface of the existing soil. Use a backhoe to scoop a bucket of soil with compost on the top, lift it several feet in the air, and drop it. Breaks up compacted soil into large clods Creates veins of compost down 24 inches Spread 4-8 inches of topsoil on surface and rototill Plant trees Mulch or plant groundcover as desired. The process was developed by a Virginia Tech research group led by Dr. Susan Day. 

Decreases in bulk density persist over time! Creates veins of compost deep in the soil profile that hold soil channels open for root penetration. The introduction of organic matter coupled with root activity can create conditions that will lead to formation of soil aggregates over time—leading to long-term soil quality enhancement. Layman et al. 2016. Urban Forestry and Urban Greening

Which is the best amendment? Road-side soils have low nutrient availability Which is the best amendment?

Scharenbroch and Watson’s CRUD plots Compaction and Remediation of Urban Dirt

Scharenbroch and Watson’s CRUD plots Compaction and Remediation of Urban Dirt

Nitrogen from fertilizer does not stay in the soil

Mulch and compost enhance tree growth most Scharenbroch and Watson. 2014. Arboriculture and Urban Forestry

Biosolids also enhance tree growth

Compost and biosolids are more efficient AND more economical than fertilizer

Road-side soils are inundated with salt Kelsey and Hootman. 1992. Deicing salt dispersion and effects on vegetation along highways.

Salt also reaches sites relatively far from roads Kelsey and Hootman. 1992. Deicing salt dispersion and effects on vegetation along highways.

Salt collects in white pine foliage Kelsey and Hootman. 1992. Deicing salt dispersion and effects on vegetation along highways.

Salt enhances water stress (in an already water-limited environment)

Salt enhances water stress (in an already water-limited environment)

Salt enhances water stress (in an already water-limited environment)

Salt enhances water stress (in an already water-limited environment)

Ionic stress: Sodium interferes with plant metabolic processes Na+ Na+ Na+ Na+

Gypsum may not be the best solution Sodium may be bound to water and clay in soil Calcium in gypsum displaces sodium on clay Sodium can be leached out Calcium may also displace hydrogen on clay, increasing pH in an already high pH environment

Biochar may be a better solution? TBD

Current best solution is to plant salt-tolerant trees

Current best solution is to plant salt-tolerant trees Prevention! Current best solution is to plant salt-tolerant trees

Right tree, right place Don’t plant a large tree in a small space Don’t plant a salt-intolerant tree in the highway median

Right amendment, right tree? Forests with different tree compositions have different organic matter composition and nutrient availability

Soil C:N reflects litter C:N roots leaves P < 0.001 P < 0.001 soil P < 0.001 AM ECM

Maple litters decompose more rapidly than oak litters AM ECM Midgley, Brzostek, and Phillips. 2015. Journal of Ecology

Oak-hickory soils have a distinct organic layer Maple-cherry-tulip poplar soils do not Oak-hickory soils have a distinct organic layer while maple-poplar soils do not

Maple plot: inorganic N economy Oak plot: organic N economy Low chemical quality litter: high C:N High chemical quality litter: low C:N Roots Roots Organic N Organic N AM fungi Enzymes Enzymes Saprotrophs ECM fungi Inorganic N NH4 NO3 Inorganic N NH4 NO3 Phillips, Brzostek, and Midgley. 2013. New Phytologist

Can we match amendments to trees? Low C:N High C:N Leaf compost Composted biosolids biosolids Wood compost

Can we match amendments to trees? Low C:N High C:N Leaf compost Composted biosolids biosolids Wood compost Sugar maple Shagbark hickory Black cherry Red oak

There is no silver bullet There is no one perfect tree or soil program, because there is no one homogeneous urban environment or site. Prevention is the best option.