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Amendments - Home
Site preparation
prior to the construction of residential units typically involves
removing or stock piling the existing vegetation and topsoil. This has an immediate
hydrologic impact because of the reduction in soil structure, pore
space, organic content and biological activity. After construction, a
thin layer of topsoil is usually spread on the now very compacted
subsoil and then the area is seeded or sodded.
Magnified view of
native soil
(Source:
King County
DNR
Solid Waste
Division)
Magnified view of
disturbed soil
(Source:
King County
DNR
Solid Waste
Division)
The combination of
soil compaction and loss of organic matter has several undesirable
consequences:
·
With
the infiltration capacity of the site significantly reduced, rainwater
more quickly runs off into local streams. This, in turn, tends to
increase erosion, scouring and the sediment load.
·
The
rate of groundwater recharge decreases.
·
Due to
the soil compaction and the loss of organic matter, the availability of
subsurface water to plants is reduced.
·
The
increased volume and frequency of runoff carries pollutants with it that
include pesticides, fertilizers, animal wastes and chemicals such as
phosphorous and nitrogen.
·
Homeowners now have to apply pesticides, fertilizers and irrigation
water in increasing amounts in order to maintain their landscapes.
1
Soil additives,
or amendments, can be used to minimize development impacts on native
soils by restoring their infiltration capacity and chemical
characteristics. After soils have been amended their improved physical,
biological and hydrological characteristics will make them more
effective agents of stormwater management.
Soil Amendment Component
Properties / Quantities
Soil amendments
can include not only compost and mulch but also top soil, lime and
gypsum. These additional components help offset any nutritional
deficiencies and control acidity.
A thorough soil
analysis of the native soil is required to determine the optimum
quantity for each component in order to obtain the maximum benefit
from compost amending. Soil amendment components should generally
be mixed and applied in the following manner.
2
Uniform mixing of
compost using a rototiller
(Source:
US
Composting Council)
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Compost.
The amount of compost to be applied depends upon the organic
content of the existing soil as well as the targeted amount of the
proposed soil amendment. Compost typically has an organic content
of 45-60% and is often used as the sole means of providing organic
material to the soil profile. In soils that have organic contents
of less than one percent, 8 to 13 percent by soil weight is a
typical target of a proposed soil amendment with compost. As a
general rule, a 2-to-1 ratio of existing soil to compost, by loose
volume, will achieve the desired organics level. Locally available
compost may be utilized if it is of high enough quality and
available at a cost effective price.
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Nutrients and
Lime.
If the soil pH
is below 6.0 the addition of pelletized dolomite is recommended,
with application rates in the range of 50 to 100 pounds per 1000
square feet. Nitrogen requirements usually range from 2 to 8
pounds per 1000 square feet, with slow release water-insoluble
forms being the preferred method. Other soil additions may include
sulfur and boron with the amount needed determined by soil
analysis.
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Gypsum.
Hydrated calcium sulfate (CaSO4 ● 2H2O)
is sometimes applied to a soil in order to increase calcium and
sulfur without affecting the pH, as well as to enhance a soil’s
structure in high clay content soils.
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1 Soils for Salmon, 1999: The
Relationship Between Soil and Water, How Soil Amendments and
Compost Can aid in Salmon Recovery.
2 Chollak, T. and P. Rosenfeld, 1998:
Guidelines For Landscaping with Compost-Amended Soils.
University of Washington College of Forest Resources. |
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