Above a Toxic Burrito: Soil Amendments Including Biochar and Wood Mulch Can Improve Plant Survival on Urban Fill Soils

Soil health is a neglected aspect of many construction projects involving restoration. Poor soils coupled with global warming exacerbates heat stress in plants. We examined a recent project completed by Environmental Services. In fill soils a year after construction in 2021, most of the planted 13,000 shrubs succumbed to excessive heat. Subsequent soil testing showed characteristics that can exacerbate water and heat stress, including sandy loam texture with big chunks of concrete and “urban fill”. Low soil organic matter also decreased water holding capacity. To improve soil conditions and plant growth as well as monitor outcomes, we set up a trial by amending the soil with 2 different compost and biochar products at 2 different rates. Imported soil areas were also de-compacted using machinery and about ½ of plants were mulched with wood chips. Each treatment had 4 replicate zones and Thimbleberry (Rubus parviflorus) was planted in 5 clusters of 10 plants each. Each plant was monitored annually for 3 years. One “Biotope”/biochar product increased survival by 16 % (+/- 12) (30% average survival) over the control (13% survival), and the “standard compost/biochar” product had a marginal increase in survival (18%). Wood chip mulch plus Biotope increased survival to 51%. Heights and widths of plants didn’t differ across treatments. One compost product shows promise on improving survival outcomes, but it is unclear if it was due to the biochar or other site conditions. Soil conditions should take priority when designing restoration projects.