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Soil Remediation Protocol Cover

Soil Restoration Protocol

Introduction

Urban soils are typically mechanically engineered to be highly compacted and contain a low amount of organic matter. This drastic alteration in the soil profile usually results in poor establishment and growth of vegetation, including trees, shrubs and ground vegetation. To assess soil compaction there are several different techniques. The technique that we use for our research is soil bulk density. Soil bulk density is the dry weight of soil contained within a known volume (unit - g cm-3 or Mg m-3). What makes measuring soil bulk density important is that many studies have been conducted on the density of soil and how it influences or even restricts root growth. Therefore, based on other studies we can determine ideal growing densities for soil.

Our research since 2013 has been focused on Ontario Ministry of Transportation and Alberta Transportation Right-of-Way sites and has demonstrated that soil bulk density is typically high (compacted) compared to ideal growing conditions (< 1.40 g cm-3). Of the 11 research sites in Alberta and Ontario we have tested to-date, most of the soils fall into ranges that restrict root growth e.g. densities above 1.55 g cm-3.  Based on our research sites, we have found that tree establishment and survival is strongly correlated to bulk density. To improve the physical structure of the soil, it is recommended to increase organic matter content for these soils. Addition of organic matter has been shown to lower soil bulk density more effectively and with longer lasting effects compared to mechanical de-compaction alone. Incorporation of organic matter has also been shown to improve soil structure, water holding capacity, biological activity and nutrient concentrations in amended soils.

With Vineland’s recommended practices there are three stages that comprise site preparation. The first step is to lay out the planting polygon or trench. We recommend creating planting beds; the planting bed treatments in our trials performed significantly better than back-filled planting pits. 

 

Step 1: Subsoiling

*Before carrying out any soil restoration work, acquire locates on your site to ensure you know the whereabouts of any below-ground infrastructure.*

Use a single tine bucket-mounted excavating ripper for the entire planting area (e.g. for our other sites we used a subsoiling Hitachi 200 or a CAT 33DL; operating weight approx. 30,000 kg). Smaller agricultural subsoiling implements do not have the horsepower required to effectively rip these soils to the required depth.

  • Tine width is usually 10 - 25 cm width
  • Deep ripping depth of 90 cm is recommended (36 inches). Make sure before you subsoil to check your soil depth.
  • Ripping spacing is approximately 30 – 60 cm (spacing between tine placement)
  • Soils are ripped perpendicular to the road (and/or with the slope of the site)

Step 2: Organic Amendment

There are several types of organic amendment that can be used to help restore soil function, including composts and mulches. Key considerations when sourcing composts include maturity, soluble salts, organic matter content, pH, nutrient content and Carbon:Nitrogen ratio (C:N). Trace metal content, sharps and presence of soil-borne pathogens are also sometimes tested. In our research we have the most experience working with municipal compost as it is widely available, it's production is regulated and it tends to be cost-effective. We have outlined some important qualities below that we recommend in a composted product:

- Mature compost (assessed based on respiration levels)

- Moisture content between 40% - 50%

- Organic matter content (based on Loss-on-Ignition test) should be a minimum of 30%

- Carbon to Nitrogen ratio should be between 10:1 and 25:1

- Soluble salts (of a saturated paste) should be less than 4 mS/cm 

For more information on compost quality, refer to the following links:

Ontario Compost Quality Standards

Compost Quality Recommendations for Remediating Urban Soils - Heyman et al. 2019

Interpreting Compost Analyses - Sullivan et al. 2018 

Step 3: Organic Amendment Application

Typically the organic amendment is moved within the site using a skid-steer or front-mounted bucket on a tractor. Once the amendment is placed onto the planting bed and is fairly level, a PTO driven rotary soil spader/ rototiller incorporates the amendment into the soil to a depth of 20-30 cm. Skid-steer operators may need to back-blade the amendment to level it out before rototilling or spading can take place. It sometimes takes several passes to have the amendment incorporated homogeneously.  

Completed Planting Beds Ready for Trees!

Ready to Plant