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Dealing with WET Soil
Compacted and/or clayey soils cause numerous watering problems. The
most obvious is surface flooding caused by slow water penetration into
the ground. When downward water movement is limited by dense or high-clay
layers, soil becomes saturated and oxygen (which moves very slowly through
water) is kept away from plant roots. If the saturated condition persists,
roots will die from oxygen starvation. Highly compacted soils, even when
dry, cause the same problem. Extended periods of water saturation also
lead to increased availability of heavy metals such as iron and manganese,
which in some soils may actually poison plants. Saturated conditions accelerate
soil nitrogen losses, particularly in cool climates and seasons.
There are a number of ways to manage saturation problems in soil. One
is to increase internal water movement by improving aggregation and pore
space. There are several ways to do this:
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Increasing and maintaining organic material levels, changing or
keeping pH in the range between 5.5 and 6.5, adding a soil conditioner
such as very coarse sand, cultivating only when moisture levels are
ideal, and avoiding compaction. But the addition of organic material
is probably the single most effective action you can take.
- Another
way to increase internal water movement in wet soils is to shatter subsoil
pans. If just a few, deep passages for water are made down through the
soil, large amounts of water will flow through them (assuming the underlying
layers will accept the water). Or subsurface drainage can be installed
beneath the soil to carry away excess water. This is usually expensive,
but may be the only alternative in many situations. Still another approach
is to limit the amount of water entering the soil by diverting surface
water away from the poorly drained area, or by digging interceptor trenches
just uphill from it. Plastic mulch can also be used to decrease total water
penetration.
- The best way to avoid saturation problems, however, is to learn how to recognize
sites and soils that are prone to them. First of all, look at their position
in the landscape. Sites that lie low, particularly at the base of long
slopes, receive considerable amounts of subsurface water flow and are almost
always wet. Large flat areas with little surface drainage are also likely
to have subsurface water problems, regardless of how high or low the large
flat areas are. Gently sloping areas are always the best-drained, without
being dry. Take care not to alter surface drainage patterns to ensure that
storm run-off doesn't flood the site. But this is often already a problem
on many suburban and urban sites where natural water drainage has been
changed by construction. Areas with dense subsoil pans or bedrock-perched
water should be avoided whenever possible. Home building and other construction
activities often result in severe compaction or the exposure of clay subsoil
material.
A number of soil properties are indicative of both poor drainage and
water saturation; even a novice soil scientist (that's you) can learn to
recognize them. Look at the soil horizon, or the way a soil is layered.
Both permanent and fluctuating water tables turn soil gray. Solid gray
colors down a soil horizon indicate almost permanent saturation. Fluctuating
saturation will cause mottled gray within the background soil color. Free-draining
soils will have bright brown, yellow, or red in their subsoils.
See also How to Make a Rain Garden...
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