Soil compaction occurs when soil particles are pressed together, reducing pore space between them (see below). Heavily compacted soils contain few large pores and have a reduced rate of both water infiltration and drainage from the compacted layer. This occurs because large pores are the most effective in moving water through the soil when it is saturated. In addition, the exchange of gases slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength-the ability of soil to resist being moved by an applied force-a compacted soil also means that roots must exert greater force to penetrate the compacted layer.
Imagine the top layers of soil, like a fresh piece of baked bread with all the capillaries containing the gases, nutrients and moisture. If flatten it like a tortilla the nutrients and gases are no longer available for uptake. Compaction causes serious effects on root growth.
Soil compaction changes pore space size, distribution, and soil strength. One way to quantify the change is by measuring the bulk density. As the pore space is decreased within a soil, the bulk density is increased. Soils with a higher percentage of clay and silt, which naturally have more pore space, have a lower bulk density than sandier soils.
Excessive soil compaction impedes root growth and therefore limits the amount of soil explored by roots. This, in turn, can decrease the plant's ability to take up nutrients and water. From this standpoint, the adverse effect of soil compaction on water flow and storage may be more serious than the direct effect of soil compaction on root growth.
In dry years, soil compaction can lead to stunted, drought stressed plants due to decreased root growth. Without timely rains and well-placed fertilizers, yield reductions will occur. Soil compaction in wet years decreases soil aeration. This results in increased denitrification ( loss of nitrate-nitrogen to the atmosphere ). ( See Flood Stress of Trees http://www.800oakwilt.com/floodstrees.html ). There can also be a soil compaction induced nitrogen and potassium deficiency. Plants need to spend energy to take up potassium. Reduced soil aeration affects root metabolism. There can also be increased risk disease. All of these factors result in added stress to the trees and plant life.
Reducing Soil Compaction
Soils that have excessive amounts of fine silt, compacted by heavy traffic, and sodic soils that have excessive amounts of sodium ions are usually compacted. Compaction is enhanced in wet areas that receive lots of traffic. The soil loses structure and forms an impenetrable layer on the surface. In a residential landscape setting there is almost always a place where kids or animals have worn a path in the yard and the grass does not grow anymore. If soils get compacted and the infiltration of water and air into the soil is impeded, the lawn will probably begin to decline. Very hard, empty spots will develop in those areas of compaction and compaction resistant weeds (like Goosegrass and prostrate Knotweed) will become more prominent.
How to Control Soil Compaction
Soil compaction mostly develops in high traffic areas, but is greatly enhanced by excessive soil moisture. That is why we recommends deep and infrequent watering schedules. In some areas, compaction cannot be avoided and steps must be taken to periodically alleviate the problems. Core aerification gives only temporary relief and within a few weeks compaction often returns to pretreatment levels. Air Injection or deep root injection of fertilizer and water can also help reduce compaction. Applying mulch around trees and shrubbery can encourage populations of earthworms and soil insects, is the best method for alleviating soil compaction on a daily basis. Their tunneling activities create passageways through which air and water can infiltrate. Earthworms can generate 120 tons of casting per acre per year and this material contains valuable nutrients and other soil based microbial life that promote healthy stress-resistant trees and shrubs. ( See article Earthworms http://www800oakwilt.com/earthworms.html )
Remember, no amount of fertilizer, soil amendments, gypsum or water applied to your landscape can make any substantial improvements if the most limiting factor is sever soil compaction.
Soils that have high levels of sodium loose structure and have poor water infiltration and air movement. They become rock hard. Sodium mostly accumulates from irrigation water. Build-up occurs when irrigation water evaporates and leaves sodium and other ions behind on the soil surface.
Sodium can be removed from soils by adding Gypsum and deep, infrequent irrigation practices. Gypsum replaces sodium ions with calcium ions and the sodium ions can be leached through the soil profile. The soil regains its structure and loosens up. Water and air infiltration begin again. Combining Gypsum application ( 40 to 90 pounds per thousand square feet ) as needed with core aerification is the best recommended treatment.
( See Gypsum Article http://www.800oakwilt.com/gypsum.html )
Other benefits of mulch include contribution of nitrogen, better water infiltration improved plant vigor, disease and weed suppression and increased root production. The decomposition of the mulch and the return organic nitrogen stimulates biological activity that cycles valuable carbon dioxide back to plant leaves for carbohydrate production, which increases root growth
Damage to soil structure and loss of soil fertility threatens the viability of many landscapes. Although the addition of fertilizers and trace elements can be used to offset losses in soil nutrients, the fertility of soil also depends on its organic matter content and structure. Inadequate levels of soil organic matter and the beneficial soil organism it supports can lead to a wide range of problems and symptoms.
Possibly the most valuable contribution trees can make to soil fertility is through the transfer of nutrients from the sub-soil to the surface soil. This can occur through Leaf litter or as a result of fine root turnover ( the growth and death of the fine feeder roots in the surface soil ). Research suggests that a large proportion of the nitrogen, phosphorous and potassium, and 100% of calcium taken up by trees will be dropped on the surface soil as fine litter ( leaves and branches ). Because most tree species actively extract nutrients from the leaves before they shed, pruning or harvesting of green branches can further increase the cycling of nutrients. Nutrient cycling may be an important means of recovering leached bases ( including calcium ) and nitrates from under landscapes.
Under ideal conditions organic matter binds with soil nutrients, stores water and enhances soil structure and drainage. Maintaining or building soil organic matter levels also contributes to the control of greenhouse gases.
Destruction of forests and woodlands to create urban neighborhoods and landscapes commonly leads to a reduction in soil organic matter content. This is further reduced if the top soils are scrapes away for the sake of progress.
Although this would suggest that re-establishing a urban forest setting lead to a build up of soil organic matter, this may take many years. In fact, if cultivation is used to establish the trees, the soil organic matter content might actually drop before the trees are able to begin turning over sufficient amounts of litter to rebuild the soil.
Organic Soil Structure
Around plant roots, bacteria form a slimy layer. They produce waste products that glue soil particles and organic matter together in small, loose clumps called aggregates. Threading between these aggregates and binding them together are fine ribbon-like strands of fungal hyphae, which further define and stabilize the soil into macro aggregates. It is this aggregated soil structure, which looks a bit like spongy chocolate cake that effectively resists compaction and erosion and promotes optimal plant and microbial growth. Water and air are also stored in the aggregate pores until needed.
Mycorrhizal fungi are especially effective in providing nutrients to plant roots. These are certain types of fungi that actually colonize the outer cells of plant roots, but also extend long fungal threads, or hyphae, far out into the rhizosphere, forming a critical link between the plant roots and the soil. Mycorrhizae produce enzymes that decompose organic matter, solubilize phosphorus and other nutrients from inorganic rock, and convert nitrogen into plant available forms. They also greatly expand the soil area from which the plant can absorb water. In return for this activity, mycorrhizae obtain valuable carbon and other nutrients from the plant roots. This is a win-win mutualism between both partners, with the plant providing food for the fungus and the fungus providing both nutrients and water to the plant. The importance of mycorrhizae in plant productivity and health has often been overlooked.
EXAMPLE Pines are not native to Puerto Rico and therefore the appropriate mycorrhizal fungi were absent in the soil. For years, people unsuccessfully tried to establish pines on the island. The pine seeds would germinate well and grow to heights of 8 to 10 cm but then would rapidly decline. In 1955, soil was taken from North Carolina pine forests, and the Puerto Rico plantings were inoculated. Within one year, all inoculated seedlings were thriving, while the un-inoculated control plants were dead. Microscopic analysis showed that the healthy seedlings were well colonized by a vigorous mycorrhizal population. While the benefits of mycorrhizae is not always as dramatic, it has been well documented that mycorrhizal plants are often more competitive and better able to tolerate environmental stress.
Protecting Trees During Construction