Soil Considerations for Growing Trees and Shrubs
Soil is a complex part of the living landscape. A natural soil takes centuries or millennia to develop. Precipitation, temperature, plants & animals, land forms, and geologic material (bedrock, glacial deposits, river sediments, etc.) all influence soil development over time. Every plant requires different soil conditions to do well, yet we sometimes take them for granted and assume plants will grow in any soil. Soils can vary considerably, however, especially those that have been disturbed by human intervention, and this impacts a plant's ability to survive. What is an optimal landscape soil? This will depend on the plants being grown and the conditions they require to do well. These are some of the functions that soil performs
- Reservoir for plant nutrients and water
- Habitat for bacteria and fungi that break down organic matter
- Habitat for insects and other animals that mix the soil
- Medium for anchoring plant roots
Many tree and shrub problems in urbanized areas can be traced to changes in the physical or chemical aspects of the soil. Grading and associated construction activity can quickly alter the natural soil environment and disturb roots. Understanding the effects of these impacts on soil and plant health, and learning how to improve them, will enable gardeners and landscapers to more wisely maintain the soil and its environment.
SOIL PHYSICAL PROPERTIES
Texture Soil texture refers to the ratio of sand, silt, and clay-size particles in the soil. Sandy soils feel gritty, silty soils feel smooth like flour, and clayey soils feel sticky and form a ribbon when kneaded between your thumb and forefinger. Soil texture is important because it influences several other soil characteristics, including:
- Water holding capacity (the ability of a soil to hold water)
- Water, air, and nutrient availability
Soils rarely consist of only one particle size, but are typically a mix of the three. Medium-textured soils (see table) have a relatively even mixture of sand, silt, and clay. Generally, these soils are most favorable for growing plants.
Soil Texture Classes
|Coarse-textured soils||Sand, loamy sand, sandy loam||
|Medium-textured soils||Loam, sandy clay loam, silt loam||
|Fine-textured soils||Sandy clay, clay loam, silty clay loam, silt,
silty clay, clay
Structure Soil structure refers to the natural bonding of soil particles into larger clumps (shapes) of varying size. Soils with good structure have plentiful natural pore spaces that allow for adequate water infiltration into the soil, good drainage and air movement, and strong root development.
Soil compaction effectively destroys soil structure. Compaction closes soil pores, interfering with the movement of water and air through the soil and limiting root growth. During wet periods, compaction at the soil surface or in the subsoil can cause water to pond on top of the soil or in the subsoil. When dry, these same soils are nearly impossible to penetrate with a trowel or shovel. Planting holes can become compacted during digging, which may allow water to collect in the bottom of the hole, much like a clay pot. In such saturated conditions, trees may drown due to lack of oxygen. Wet soils and those high in clay are more susceptible to compaction; soils higher in sand are less susceptible. Vehicles and foot traffic are responsible for most compaction. During the development of subdivisions, large machinery (bulldozers, trucks, etc.) compact the soil as they move over it. Intense compaction on a small scale can occur in yards where walking, running, or mowing on lawns and around trees is common, particularly when the ground is wet.
Soils are poorly drained if water pools at the surface or in the subsurface for several days or more after a wet period. Compaction promotes wet, saturated zones in the soil, devoid of oxygen. Roots of sensitive plants are rapidly killed in saturated soils and cannot take up water or perform oxygen-requiring respiration. Foliage dies, ironically, from lack of water. Drowning plants show the same symptoms as those suffering from drought: drooping leaves and browning leaf margins.
Dig a 12" x 12" x 12" hole. Be careful to not compact the bottom of the hole. Fill it with water, let it drain, and refill it. Water should drain within 24 hours. If not, there is a good chance that drainage is poor.
Improving Physical Soil
Properties Adding organic matter (mulch and/or compost) is the best way to amend a soil with physical limitations. Organic matter is critical to longterm plant and soil health. It is most beneficial in the upper part of the soil where plant roots concentrate. Over time, it is incorporated into the soil by insects and other animals, and by freezing & thawing. As organic matter decomposes, it provides plant nutrients, improves soil texture, structure, and water holding capacity, and reduces the effects of compaction. These are some of the ways to improve a soil’s physical limitations:
- Plan ahead to minimize drainage, compaction, or other problems. Before landscaping, take time to understand existing soil limitations and develop a plan to amend them. If soil properties cannot be improved significantly in the long-term, select plants that will be more likely to do well in these soils
- Incorporate composted leaf mold, grass clippings, etc., into a planting bed before planting, followed by 3-4 inches of wood chips after planting. Replenish with additional mulch as needed
- To minimize compaction, never work soils when they are wet
- Till the soil to remove compacted layers. This is best done before planting. Tilling after planting will cut plant roots. Gypsum is not an effective amendment for compacted soils except in saline soils, which are only a problem in the Midwest immediately adjacent to heavily salted roadways
- Use a core aerator to improve a compacted soil surface. Core aerators remove small soil plugs to a depth of about 3 inches. In areas without sod, add organic compost or mulch after aerating
- In sites with poor drainage, drain tiles can be effective in removing water quickly. Tiles should slope downward and away from the site. Consider where the tile ends so as not to produce a problem elsewhere.
SOIL CHEMICAL PROPERTIES
The chemical properties of a soil are easily altered by the addition of fertilizers, water runoff (weathering) from concrete or limestone surfaces, road salts or other pollutants, and construction debris, such as mortar, concrete, and brick, mixed in the soil. An excellent tool for assessing soil chemistry is soil pH - the measurement of the acidity or alkalinity - which influences nutrient availability and plant growth.
Acid or Alkaline?
The pH scale is 1 to 14, with 7 being neutral, below 7 being acid, and above 7 being alkaline (see pH chart below). A change of one pH unit is a 10-fold change in acidity. For example, a pH of 6 is ten times more acid than a pH of 7. Most naturally occuring soils have a pH of 5 to 8. A general measurement of soil acidity can be done with a garden pH kit, available at most garden stores. Commercial soil laboratories can provide more detailed analyses (see “Soil Testing & Interpretation” below).
Some factors to consider regarding soil pH:
- Slightly acid soils (pH of 6 - 6.5) provide the most favorable environment for nutrient availability, thus most plants perform well in these soils (see nutrient chart below)
- Urban soils are typically alkaline, a result of soil alterations already mentioned
- Although alkaline soils may actually contain adequate nutrients to support most plants, the chemistry (pH) of these soils may be such that some nutrients are not available for uptake by roots of affected plants, resulting in nutrient deficiencies. Deficiencies of some nutrients lead to reduced chlorophyll production (chlorosis) or other maladies, including stunted growth and decline. Nitrogen is the nutrient that is depleted from the soil most quickly and needs regular replenishment.
Modifying Soil pH The addition of agricultural lime to increase pH is rarely needed for urban soils, unless for some reason the soil is too acid for the plants being grown. If alkaline-induced plant problems are suspected (chlorosis symptoms), it’s likely that the soil needs to be acidified for greater nutrient availability in conjunction with other improvements to the rooting environment. Here are some suggestions to reduce alkaline-induced plant problems:
- Add organic compost (acidic leaf mold) topped with composted woodchip mulch
- Don’t fertilize chlorotic plants with potassium or phosphorus, unless a soil test indicates a deficiency, and avoid nitrate-containing fertilizers
- Once a year, either in early spring or late fall, add granular sulfur or aluminum sulfate at a rate of no more than 3 pounds/100 square feet onto the soil beneath plants where acidification is needed. Or, break this into 1.5 pounds/100 square feet in early spring and in fall. Do not apply onto lawn or onto plant foliage. Water thoroughly after application
- Acid-loving plants should be kept away from concrete and other limestone materials that have alkaline runoff.
Reducing soil pH to an acceptable level can take many years and much patience, so planting alkaline-tolerant trees, shrubs, and other plants is another approach.
Generally, soil salts are not a problem in Midwestern soils because the region receives adequate precipitation to leach salts through the soil system. Sometimes, however, there are concerns. Two primary sources of soil salts are fertilizers and roadway deicers. Although different in composition, their effects on plants are the same. Fertilizer salts become a problem when too much fertilizer is used. The application of any fertilizer should be minimal in summer because hot, dry spells reduce moisture available to plants and subsequent fertilizer uptake can increase salt concentrations in plant tissues, leading to desiccation or burn of these plant parts. Desiccation or “drying out” of leaves is seen as browning of leaf edges or the entire leaf during the growing season. Roadway deicers, such as sodium chloride, can accumulate in soils immediately adjacent to heavily salted roads or sidewalks. If concentrated enough in the soil, the salts can draw moisture out of plant roots, causing desiccation of the entire plant. Excessive uptake of sodium or chloride into the plant may also occur, resulting in toxicity and desiccation. (Note, however, that most deicing salt damage to plants occurs as a result of salt-laden splash and spray being deposited directly onto twigs and evergreen needles.)
Minimizing Damage From Soil Salts
- Watering plants, in addition to normal precipitation, will help flush salts through the soil and below the root zone if a soil is adequately drained. Do not over water
- Fertilize plants only when necessary. If a nutrient deficiency is suspected from off-color foliage or abnormal growth, perform a soil test to help determine what nutrients or fertilization routine might be needed. A layer of organic mulch and compost around plants will provide a natural “slow-release” fertilizer
- In areas where roadway deicing salt use is high, plant only salt-tolerant species
- To minimize plant damage from aerial salt spray from roadways, place a temporary burlap fence next to the plants to intercept the spray, or plant salt-tolerant species.
SOIL TESTING & INTERPRETATION
Assessing the Problem Some soil problems can be assessed and interpreted by the homeowner. Many of the physical properties of the soil can be examined on-site using two procedures:
- Use a soil probe or shovel to take surface samples (0-4 inch depth) and subsurface samples (12-16 inch depth). Examine these for texture, color, structure, compaction, or other physical qualities
- Use a home pH kit to get a general indication of soil acidity. For larger planting beds, take samples from three or more sites for a more complete assessment. Under established trees, take samples from 3 sites beneath the tree crowns (which is where most roots are concentrated). Sometimes more expertise may be needed. The Morton Arboretum does not conduct soil tests. Some regional university Cooperative Extension Service offices can determine pH, potassium, and phosphorus levels, but many problems go beyond these basic tests. Soil scientists, listed in phone directories, may also be hired. Testing laboratories can perform a range of soil tests, including pH, nutrient levels, and texture. Regional laboratories include:
- A&L Great Lakes Laboratories in Fort Wayne, IN (260-483-4759)
- Alvey Laboratories in Belleville, IL (618-233-0445)
- Kane County Farm Bureau in St. Charles, IL (630-584-8660)
- University of Wisconsin Cooperative Extension Service in Madison, WI (608-262-4364) Contact the laboratory for specific sample requirements.
Interpretation of Tests
When using a commercial laboratory for testing, it is best to use their expertise to interpret the data. Interpretation requires knowledge of “normal” soil conditions suited to particular plants. Only then can proper recommendations be made. This interpretation will normally require an additional fee. “At home” testing will require some homework to understand “normal” soil conditions and plant requirements. The Morton Arboretum’s Sterling Morton Library offers an array of books, periodicals, and other horticulture references to learn more about plant and soil requirements. Be aware, however, that the nutrient requirements of many landscape plants have not been well documented.