When you stop to think about it, soil is one of the many hidden factors that influence the overall health and vigor of a tree. Others include the arrangement of the roots (since you can’t see them, it could be a tangled mess down there), columns of internal heartwood decay (you might be able to see them with a sonic tomograph) and the precise cultivar of the tree (sure, it’s a crabapple, but which one?). Each of these can lead to successful, sturdy growth or weak and stunted shoots, roots, flowers and fruits. As you are in the initial stages of problem diagnosis, keep soil (and these others) in mind as a possible source of the malady.

Minimum size and depth

Soils can be classified as various types in a soil textural triangle.

Graphic Courtesy Of University of Nebraska-Lincoln

Though there are no hard and fast rules around the absolute minimum amount of soil that’s required to sustain a tree throughout its life, there are some general guidelines. After planting, as a tree root ball begins to send out lateral roots, it begins to need more and more space for nutrient and water extraction as well as structural support. Over time, the size and depth requirements increase to fill the allotted space in the landscape.

According to forestry consultant Phil Pierce, RCA, though various trees differ in terms of demand, the average, medium- sized tree (oak, pine, maple, honeylocust, buckeye, etc.) needs about 1,000 square feet of surface area, or 3,000 cubic feet of soil volume, for adequate rooting to have enough soil in place to meet its overall requirements. Part of this recommendation is based on Pierce’s 40 years of experience and training, but also on how most trees grow.

The common growth habit for hardwood and evergreen trees is to develop approximately 80 percent of their support roots in the upper 2 feet of soil surrounding the flare, with a few sinker or spiker roots that grow for support as deeply as possible. The bottom lines are:

  • if a tree is growing in a less-than-adequate-sized volume of soil, it’s probably suffering or soon will be, and
  • though it may sound trite, the more soil, the better.

Natural soil layers

In undisturbed sites, Mother Nature drops seed for new trees to grow into the upper soil layers, to be further nurtured by rainfall, sunlight and decaying organic matter. The first layer, usually 3 to 4 inches in depth, is called duff. More technically, it’s called the O horizon, which is comprised of a mix of fallen leaves, fruits, stems and bark. Under the duff is a layer of fertile, well-drained material called the A horizon. Sometimes, this is commonly referred to as topsoil, which is a bit risky, in that it can vary widely from region to region and most certainly is not a standard commodity.

Most A horizons are darker in color because of higher organic matter content and usually have the physical appearance of coffee grounds or grape nuts. The layering process continues with the B horizon, distinctly lesser in terms of fertility and drainage potential. But depending on the depth of this layer, healthy tree growth can still be supported well with duff, A and B horizon soils.

Below the B horizon is a layer called the C horizon. Usually fairly devoid of nutrients and adequate aeration, the C horizon can also be variable in depth and location.

On most sites, the lowest layer is a supporting material referred to as bedrock, or the R horizon.

This material is comprised of rocky, hardened materials that are incapable of supporting tree growth. In disturbed urban landscapes, these well-defined layers are usually not present, either in form or adequate size and depth. In most cases, the duff layer is missing outright, the A horizon has been scraped off and moved to another site and the B and C layers are all that is left to support rooting. On poorer sites, where many apartment complexes, shopping malls and new residential housing construction sites are located, only C horizon and bedrock exists, causing substantial stress to trees.

A common fix on disturbed sites is to incorporate liberal volumes of compost in the entire planting area, not just the planting hole.

Image Courtesy Of John C. Fech, UNL

Sand, silt and clay

Lots of adjectives can be used to describe soils, yet some of them can’t be used in mixed company. The authors often hear phrases such as “that doggone awful sticky gumbo” or “slimy blue crap.” These are funny to toss about, but hardly descriptive.

The ones that matter are sand, silt and clay. The relative percentages of these three mineral components describe soil texture — and all soils are basically some combination of them. Most often, two of the three are used together to describe a particular texture and help provide an idea of how the soils will support tree growth and function in terms of drainage, compaction and nutrition. Terms you may commonly hear are textures like silty-clay or sandy-clay.

A soil textural triangle is a good visual illustration of how the three components work together as they interact with trees. When reading through the results of a soil test report (covered below), a characterization is often made in the notes section, again, usually as a two-word moniker. What you don’t see is the exact percentages of each component. For example, a sandy clay may be actually comprised of 60 percent sand, 30 percent clay and 10 percent silt.

So, what’s in a number? Nothing in a practical sense, except that a given soil takes on the characteristics in each individual component. Sand resists compaction, holds little in the way of nutrients or water, and when it comprises more than 70 percent of the soil particles, encourages drainage of excess water. Clays are just the opposite, as they’re easily compacted, are good holders of water and nutrients and have poor drainage capacity. Silts are somewhere in-between.

The available soil volume must allow for development of woody supporting roots, nonwoody roots and microscopic absorbing and root hairs.

Graphic Courtesy Of Tivon Feely, Missouri NRD

The good stuff

When we talk about soils in general, our attention usually goes right to the minerals therein. However, at least in a decent soil, half of it is nonmineral, in that air and water comprise approximately half of what’s in the ground; 45 percent is the combination of sand, silt and clay; and 5 percent is often referred to as the good stuff — organic matter. So, why is it so good?

First of all, organic matter and the materials released from organic matter are what help improve the physical characteristics of our soils, primarily the A horizon. These released materials help glue soil particles together and help these larger aggregates to resist compaction and improve air and water movement through the soil. The humates and humic acids found in organic matter are beneficial as buffering agents. Secondly, the humic substances are relatively stable in soils and are slowly degraded by microbes, so they remain for longer periods of time to benefit the soil physically and chemically. It just takes a while for humate levels to build up, so an overall long-term organic matter management plan is needed.

Though not optimal, a small amount of turf/mulch/flower growth covering the soil and paving can sometimes allow for decent tree growth.

Soil testing

In order to determine the existing levels of the essential elements, organic matter content, pH, cation exchange capacity and other characteristics, soil testing is necessary. In addition to the specifics of the test results, the important considerations are how, where and when to test.

Where to test is the easiest, as it’s a simple matter of matching the location of the roots to the site of the testing. Small, newly planted trees should be tested in the immediate area of 3 to 5 feet outside of the root ball. Medium and large trees should be tested at the base, in-between the base and the drip line, at the drip line and twice the distance of the base to the drip line.

How to test isn’t really all that difficult either, keeping the zone of active rooting in mind. Begin by removing a chunk of turf and thatch, then using a post hole digger, a sod spade or a bulb planter (a golf cup cutter works great) to extract a core of soil from the 4- to 8-inch level. A handful of soil from each hole is a good target to shoot for. A dozen or so subsamples per tree is adequate; if the tree has nutrient deficiency symptoms, increase the number of samples so that all sides of the tree are represented.

When to test is mostly a matter of avoiding winter in colder climates and evaluating and comparing in the same month in subsequent years. If spring results in one year are compared to fall results in another, the differences may be due to temperature and nutrient release rather than actual nutrient content.

Some other factors to keep in mind when soil testing include:

Testing labs: There are several factors to consider when choosing a soil analytical testing lab. Most of these labs are not regulated by state and federal agencies, so it’s important for each grower or manager to find a lab that tests soils and provides the most applicable data.

Test methods and laboratory proficiency: Make sure the lab’s methods are sound and provide the most accurate and precise data possible.

Other clients: Ask the lab to provide a list of customers to talk with about their methods, customer service, results data and if their recommendations are consistent with your objectives.

Prices and turnaround time: Ask the lab and other customers how long it takes from the lab to process your soil and provide you with the results and recommendations. As a grower, turnaround time and the overall cost of testing can be critical.

Customer support: It’s also important to find out if the lab provides information or field personnel to help you with sampling correctly and providing the best sample possible. If you have questions about where to go for soil testing, talk with your local extension educator or other professionals in the field about their experiences with soil testing and their results.

Trees with limited root expansion capacity are destined to perform poorly.

Image Courtesy Of John C. Fech, UNL

Soil modification

Following testing, soil modification is often called for. Visiting with the soil testing lab before the testing is important, so that they create recommendations in a green industry sense, with the suggested amounts in terms of pounds per 1,000 square feet (or even pounds per 100 square feet) instead of tons per acre for an agronomic application.

Soil modification is best addressed in three categories. The most flexibility and capacity to modify exists before tree planting, especially if large volumes of material are needed to be incorporated. A common fix for trees that are to be planted on low organic matter sites is to have liberal volumes of compost suggested for incorporation. When this is the case, it’s best to check the specifics of the landscape design, modifying entire tree planting areas, not just the planting holes. As discussed above, tree roots tend to grow laterally rather quickly, so modifying only in the immediate planting holes, usually results in root entanglement and/or severe limitation on lateral root expansion.

When modifying soils one to three years after planting small trees, aerating the sod and applying nutrients — or other amendments — over the turf is a good approach. After aerating and applying the desired material, try to rake or drag it into the aeration holes to help ensure the best success. If this method doesn’t prove to be effective, vertical mulching, where a slender auger is used to create holes for modification on a grid pattern under and outside the drip line, is another method that can be utilized.

When modifying soils under large trees, more complications are at hand. After all, tree care isn’t turf management, where the superintendent or field manager strips off the sod and works nutrients and other materials into the soil with a rototiller. In these situations, changes in the landscape are in order; best results are achieved by working with a landscape designer or architect to remove turf, and apply mulch to the entire area under the foliage canopy of the tree. When the mulch is in place, the recommended amendments can be applied lightly and frequently to improve the health and vigor of the tree. Doing so will benefit the tree by removing the turf as a competitor as well, allowing the tree to be watered and fertilized for its improvement rather than for the requirements of the grass.