Use of coarse substrate to increase the rate of water infiltration and the bearing capacity in tree plantings

This work assesses the suitability of recycled crushed bricks and compost, used as a mixed growing substrate, in urban roadside plantings of Tilia cordata Mill. Coarse recycled materials can prevent compaction, improving air and water flows in the rhizosphere, and thus reducing tree mortality or poor growth. In addition, compost is a source of organic matter, contributing to enhance soil quality. Six substrates were tested: 1) soil; 2) crushed bricks 0–30 mm; 3) crushed bricks 6–30 mm; 4) soil + compost (15% v/v); 5) crushed bricks 0–30 mm + compost; and 6) crushed bricks 6–30 mm + compost. Trees were planted in 0.6 × 0.6 × 0.6 m pits, filled with the six different substrates. The water infiltration rate (Kf) and load bearing capacity (Evd) of the substrate were monitored along with the shoot length, and chlorophyll content of the trees. At the end of the trial, the root biomass and total root length were measured. Crushed bricks positively affected Kf, and Evd; the presence of compost decreased the value of bearing capacity. 0–30 crushed brick substrates (with or without compost) increased the linden shoot length and enabled to develop higher root biomass, compared to the soil (+72%) and 6–30 (+37%). The best trade-off between resistance, compaction and plant growth was 0–30 mm crushed bricks. This research provides evidence that a coarse matrix substrate made of crushed bricks is efficient in preventing compaction and allows plant growth.