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.
Undesirable Effects
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.
Sodic Soils
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
Nutrient Decline
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
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
http://www.800oakwilt.com/protecttrees.html
