Cornell Structural Soil Mix

by Nina Bassuk

From City Trees, The Journal of The Society of Municipal Arborists

Vol 35, Number 1


Space is always an issue when growing trees in a downtown setting. Above ground space is obvious, below ground is more important, We must do what we can to provide root space for tree growth. How do you grow trees in downtown soils? Cornell’s structural soil mix is an excellent answer.

Since sidewalks, curbs and streets need a soil that is solid and compact. Soil particles must be compacted to meet engineering standards for load bearing sidewalks. The space for trees is not ample for proper growth of more than 3 to 7 years.

Soils for proper tree growth contain: 50% solids which provide tree nutrition, 25% micropores which hold soil water and 25% macropores which provide air and allow the water to drain. Compaction results in the loss of the macropores so water does not drain and tree roots all die. Secondly, the roots can not penetrate the compacted soil.

Structional Soil is basically crushed rock varying in size between 1/2 and 1-1/2” diameter. The rock easily compresses and in between the rock are large pores. These pores are filled with soil for the roots. The stone bears the load and the roots penetrate the voids for soil, water and air. The percentage of soil versus stone is critical to prevent 8.11 the pores from being filled completely with soil and no benefit for the trees can occur. The soil is stuck to the rock by using hydrogel as a sticking agent. The ratio is 30 grams of hydrogel per 100 kilograms of soil and 500 kilograms of crushed rock. The material is prepared nearby the sidewalk site. The rock is spread out, the hydrogel and soil is added and mixed into the rock. Then the entire mix is moved to the site so the sidewalk can be built and the trees planted later. The structural mix should go for as long as possible under the sidewalks. The mix is one to three feet deep for proper tree growth and walk stability.

The growth of trees in the structural soil has greatly increased over conventional planting according to measurements taken in the past three years. Up to 2’ of growth has been reported per year. If You would like further information contact Nina Bassuk or Andy Hillman who is the City Forester in Ithaca, New York.

Nina is Professor of Urban Horticulture at Cornell University.