Soil Conditions For Trees

Trees need appropriate amounts of loose, moist, well-aerated, and non-compacted soil in order to mature in the urban environment. These conditions enable the tree’s roots to obtain nutrients, oxygen, and water that are all essential for healthy tree growth for the life of the tree.

In addition to the nutrients that trees obtain from soil through their roots, they also need the oxygen and water that occupy the voids between soil particles. These voids are abundant in non-compacted soil. However, the soil in urban areas is usually compacted to provide structural integrity for structures and paved surfaces, thus making void spaces between soil particles minimal to non-existent.

Urban Soil Conditions

By understanding the soil conditions that urban trees need to reach maturity, tree care professionals can take the required steps in specifying the systems and best practice procedures that will ensure the success of the urban tree survival.

Trees planted in city sidewalks are usually surrounded by soil that was compacted during the sidewalk construction. Furthermore, the little bit of moisture that is present in the soil will often migrate toward the surface by capillary action. But this moisture is trapped by the concrete slab. This often leads tree roots to seek out this space where air and water are present. This causes pavement heaving and breakage as the root increases its diameter during growth.   If tree roots can’t grow into any surrounding soil, they continue to grow slowly until they have filled the all the root-friendly space that is available and the heaving and breaking sidewalk becomes a hazard. When a tree’s need for nutrients, water, and air can no longer be met, the health of the tree suffers and begins to decline and eventually it will die. Trees grown in these conditions hardly ever reach maturity and do not provide the many benefits that healthy trees are suppose to offer to the city and its residents.  This challenge creates a fundamental conflict for trees in paved areas. Therefore, before planting a tree, careful consideration needs to be taken regarding the above and below ground space to ensure that each tree has what it requires to reach maturity. The traditional method of providing a tree pit area the size of the pavement opening is clearly insufficient and results in a short lifetime of costly pavement repairs and replacing a soon-to-be-dead tree.

Minimum Soil Area Calculation Options

How much suitable soil do trees need to be healthy and reach maturity? The following are some of the various methods have been developed by researchers to determine the required soil volume necessary for a tree to develop a healthy root system. The choice of method is up to the arborist in charge of the planting: 

Mature Canopy Method – one simple method of calculating soil volume is estimating the projected area of the mature tree canopy calculated from the area of its branch spread (usually provided by nursery catalogs) and multiply it by a depth of 2 ft.

Average Method – allow the following based on the growth of an average city tree. For example:     

Trees that grow less than 30 ft tall or spread need 600 cu ft of soil.

Trees that grow 30 to 50 ft tall or spread need 1,200 cu ft of soil.

Trees that grow larger than 50 ft tall or spread need 2,700 cu ft of soil.

Trees that are to grow in structural soils or under suspended pavements need 1,500 cu ft of soil.

Trees that are to be used as screens, shelter belts, or group plantings need 20 ft of canopy spread  x  3 ft deep or 400 cu ft per tree. (The number for group plantings is low because trees in this growing situation will share the available root space.) 

Mature Trunk Caliper Method – allow the trunk diameter to predict the root spread. (This calculation is appropriate for columnar trees.) Since this calculation will change as the tree grows in size, providing an adequate amount of soil at planting time requires knowledge of the trunk size at maturity. Allow 3 sq ft (0.45 sq meters) of soil area for each square inch (6.5 s. cm.) of trunk diameter.

Minimum Method – the minimum recommended soil volumes are:

             Small Tree: 175 – 550 cu ft (5-15 cu m) of soil

             Medium Tree: 700 – 1500 cu ft (20-40 cu m) of soil

             Large Tree: 1800+ cu ft (50+ cu m) of soil

Parking Lot Standard – A minimum of 1,000 cubic feet (30 cu. meters) of soil is required for any tree to thrive in a parking lot.

In all options, suitable soil should be at least 30 inches (0.75 m.) deep.