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Incorporating soil amendments
The best amendments for clay soils are pine bark
(less than 1/2 inch in diameter), composted leaf
mold, or small pea gravel (less than 3/8 inch
in diameter). Be careful when selecting leaf mold,
making certain that the material is fully composted
and not merely “aged.” Partially decomposed
materials will compete with plants for nutrients,
especially nitrogen and sulfur, resulting in nutrient
deficiencies and poor plant growth. Peat moss,
sand, hardwood bark, wood chips, and pine straw
are not recommended because they do not adequately
improve the physical properties of a clay soil.
Organic matter content must be increased to 25
percent, by volume, to be effective as a soil
conditioner. For example, to achieve approximately
8 inches of amended soil, a minimum of 2 inches
of material should be incorporated into the top
6 inches of soil. This also raises the bed which
improves drainage and makes bedding plants more
visible. Incorporating over 50 percent organic
matter may have a negative effect on plant growth,
while incorporating less than 25 percent is a
waste of time and material.
Amendments such as pine bark, composted leaf mold,
or peat moss will improve water retention in sandy
soils. These amendments need to be added at a
minimum of 25 percent by volume and a maximum
of 50 percent.
Soil testing and fertilizing
After any amendments to improve aeration and drainage
have been incorporated into the bed, have the
soil analyzed to determine fertilizer needs and
pH. A soil test is the only way to determine if
phosphorus, potassium, calcium, or magnesium must
be added or if pH should be adjusted. Overapplication
or application of unneeded fertilizer could result
in salt injury to plants and cause nutrient imbalances.
It is also environmentally unsound since runoff
of excess nutrients degrades water quality. If
soil test results indicate that nutrient or pH
adjustments are needed, the materials should be
mixed into the soil uniformly since bedding plants
have a very limited root system.
The soil pH for bedding plants should be between
5.5 and 6.5. Lime should be thoroughly tilled
into the soil prior to planting. If the pH must
be lowered, elemental sulfur can be incorporated
into the soil. If only a small decrease in pH
is required, acid-forming fertilizer such as ammonium
nitrate can be used to provide nitrogen and to
slightly lower the soil pH.
If a soil test is not made, incorporate 2 to 3
pounds of 10-10-10 fertilizer per 100 square feet
of bed area. An easy conversion to remember is
that 1 pound of fertilizer is equal to about 2
cups (or 96 teaspoons). Use 2 pounds (4 cups)
of dry fertilizer per 100 square feet or about
2 teaspoons per square foot. An alternative to
10-10-10 is a slow-release fertilizer such as
16-4-8 or 12-4-8.
It is impossible to tell how much calcium and
phosphorus are required without a soil test. However,
because most North Carolina soils are low in phosphorus,
it is usually safe to add 2 to 4 pounds of triple
superphosphate (0-46-0) per 1,000 square feet.
Phosphorus and calcium move very slowly through
the soil profile so they should be incorporated
into the top 6 to 10 inches.
It is not a good idea to guess at the amount of
calcium to apply, first, because it is possible
to get too much calcium in the soil which can
lead to potassium and magnesium deficiency; and
second, because some sources of calcium such as
calcitic and dolomitic limestone raise soil pH
so high that some nutrients become unavailable
to plants. If a soil test shows that calcium is
low but a pH change is undesirable, gypsum can
be incorporated into the soil prior to planting.
Magnesium may be deficient, especially in low
pH soils. If a soil test shows that magnesium
levels and soil pH are low, dolomitic limestone
can be used to raise the pH and to supply the
needed magnesium. To add magnesium without changing
the pH, apply Epsom salts. The general rate for
Epsom salts is 1 pound per 100 square feet.
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