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Sustainable Horticulture

Soil Compaction

Soil compaction is a serious problem for landscape plants, particularly in urbanized environments. Compaction frequently occurs when an area is developed, leading to root disturbance and other problems. Machinery and heavy equipment create great pressure on the soil underneath them. This pressure squeezes the soil particles together, leading to the loss of pores that once held air and/or water. Sidewalks, driveways, and other impervious surfaces also can compact the soil beneath them, harming the roots and microorganisms that live in the soil. Finally, frequent tilling of agricultural land can lead to compaction by destroying soil aggregation and structure, creating dense soils and crusted layers, called plow pans, that prevent water and air movement through the soil. Tillage problems are further aggravated by the use of heavy farming equipment on sensitive soils.

An ideal soil has roughly 25% air space, 25% water, and 50% soil particles (minerals and organic matter), by volume. When compaction occurs, soil particles, particularly those in the uppermost 4 to 8 inches (10 to 20 cm), are compressed into a dense mat, damaging plant roots and reducing pore space, along with the air and water that normally fill the pores, thereby altering the soil composition. Finer textured soils (silts and clays), which have smaller pore spaces, are typically more heavily impacted than sandy soils. When compressed, the fine soil particles can easily fill the pores, destroying air, water, and root channels and creating a more continuous soil layer. Bare soils, or those without leaf litter or mulch, are more easily and commonly damaged than are soils that have protective layers that absorb some of the impact and reduce the compaction damage.

Compaction leads to a number of problems in landscape plants. For instance, soil compaction compresses the macropores that hold air in drier periods, leaving soils less aerated and decreasing soil porosity. When plants have less oxygen available to them, they must use anaerobic respiration. This process generates far less energy for plant growth and maintenance than does normal aerobic respiration. In addition, toxic byproducts can be produced by anaerobic respiration, potentially injuring the plants. The hypoxic (little oxygen) or anoxic (no oxygen) environments that result from compaction also tend to reduce rates of photosynthesis, further lowering energy production.

Compaction may create barriers or abrupt changes in soil layers that restrict plant growth and air, nutrient, and water movement through a soil. The higher bulk density (or weight of soil per unit volume) of compacted soils makes it difficult for roots to grow through these soils by creating a heavy, nearly impenetrable soil barrier that roots must grow around or attempt to grow through. Roots also have less pore space in which to grow and less oxygen to fuel growth and respiration. This decreased root growth inhibits the roots' ability to take up nutrients and water. It also inhibits the roots' ability to stabilize the plant by growing all throughout the soil, making the plants more susceptible to wind throw.

Compacted soils may retain water, rather than air, in their pores, becoming saturated and poorly drained. Conversely, hydrophobic (water-repelling) surface layers may inhibit water infiltration, creating drought-prone soils. This can lead to signs of drought stress, such as wilting leaves, stunted growth, or marginal leaf death, even when a soil is saturated or has standing water on it.

Beneficial organisms, including mycorrhizae, are less likely to be found in compacted soils. These organisms require oxygen to survive and uptake or fix nutrients for transfer to the host plant. Without the assistance of these organisms, host plants may be unable to take up sufficient nutrients, which can be seen in such symptoms as off-colored (yellow, red, etc.), curled, or otherwise deformed leaves. Finally, compaction may escalate the occurrence of root rot damage by stressing and weakening roots, making them susceptible to infection by soil pathogens as well as limiting root regeneration and replacement.

Diagnosing soil compaction can be fairly easy. Perhaps the easiest way to tell if soil is compacted is through a shovel test. A shovel or soil probe can be dug into the soil to see how deeply it can be inserted before hitting highly resistant layers. If a shovel or probe cannot easily penetrate the soil, it is likely too compacted, and plant roots will have as difficult a time penetrating the soil as did the shovel. Other soil related diagnostic clues include soil crusts, standing water, increased water runoff, and generally poor drainage. Plants may exhibit reduced seedling emergence and survival, chlorosis, wilting, stunted or delayed growth, and increased susceptibility to wind throw.

Once compaction exists in a site, it may be difficult to remove. However, mulch (e.g. wood chips, straw, or organic matter) can work wonders on compacted soils. A 3 to 4-inch thick layer applied liberally atop a compacted soil can begin working immediately. Mulch acts to mediate soil temperatures, reduce weed growth, retain soil moisture, and increase soil fertility, providing a good substrate for soil microorganisms. Worms (which may need to be added to the site along with the mulch) and other microorganisms will take leaves and organic matter from the soil surface and move it into the soil where it is broken down. Nutrients from this material help to feed the worms, soil microorganisms, and plants. In addition, the worms create holes throughout the soil that allow oxygen to enter and move throughout the soil space. Deeply rooted plants may also help to break up compaction layers. However, to be of any benefit, the plants must have large, strong root systems and must either already be present on a site (and protected from compaction) or be able to establish in compacted soils.

Since compaction can be quite difficult and expensive to fix once it has occurred, prevention is crucial. Fortunately, much compaction can be avoided through proper care. When construction or major site disturbance will occur, barriers should be created around existing trees and shrubs, or around sites where new plants will be installed. The barriers should be, at a bare minimum, around the drip line of existing trees (the area beneath the canopy), though greater distances are better, as root systems extend far beyond drip lines. A good rule of thumb is to protect a circle with a radius approximately equal to the height of the tree. Construction and farming equipment and humans should be kept to a limited portion of any site and away from protected areas to avoid widespread soil compaction. The number of times a piece of equipment is run over a soil should be kept to a bare minimum. If machinery must be repeatedly moved through an area, wooden planks, a 4-6 inch layer of gravel or mulch, or similar material should be laid down in the machinery's pathway. If gravel is used is should not contain limestone or dolomite, both of which act to increase the pH of soil, harming acid-loving plants. Gravel or a similar protective layer will act to spread the force of the equipment throughout a greater area, lessening the pressure and its accompanying compaction in any given area. Such protection is especially important in areas with existing vegetation or where plantings are planned. Some machinery can be equipped with wide wheels that spread the weight over larger areas, again lessening the compaction pressure in a given area. Wet soils are especially compaction-prone, since the water acts as a lubricant, helping soil particles to stick together and remove pore spaces. For this reason, construction activity or other disturbances should never take place on wet soils and equipment should only be used on dry soil. Following these preventative measures could protect existing and newly installed plants from great stress, reduce the need to fix the compaction or replace plants, and save both time and money.

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