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Applying Fertilizer Computing the amount of fertilizer needed for a given area is rather
tricky at first, but after a few times, this becomes second
nature. Following are some examples of fertilizer determinations
for lawns and gardens. |
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Example 1. |
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Determine the amount of ammonium sulfate needed by a 5000
square-foot lawn if 1 pound of nitrogen per 1000 square feet
is required. |
Lawn: |
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5000 square feet |
Fertilizer: |
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ammonium sulfate (21-0-0) |
Rate: |
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1 pound of nitrogen per 1000 square feet. |
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- Since we need 1 pound of nitrogen for every 1000 square
feet and we have 5000 square feet, we need 5 pounds of
nitrogen.
- Ammonium sulfate is 21 percent nitrogen (round to 20
percent).
- 20 percent is the same as 0.20 or 1/5. This means that we
need 5 pounds of fertilizer to get 1 pound of nitrogen.
- Since we need 5 pounds of nitrogen, 5 x 5 = 25 pounds of
fertilizer.
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Total fertilizer needed = |
N application
rate
(lbs/1000 sq.ft.) N content of fertilizer expressed as a decimal |
x |
lawn size (sq. ft.) 1000 |
= |
1 0.20 |
x x |
5000 1000 |
= |
25 lb. fertilizer |
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Example 2. |
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Determine how much 20-10-5 needs to be applied to ensure
2 pounds of phosphorus per thousand square feet in a garden
that measures 20 x 10 feet. |
Garden: |
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20 x 10 = 200 square feet |
Fertilizer: |
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20-10-5 = 10 percent phosphorus |
Rate: |
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2 pounds of phosphorus per 1000 square feet. |
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Total fertilizer needed = |
2
lb. Phosphorous 0.10 |
x x |
200 1000 |
= |
2 0.10 |
x |
200 1000 |
= |
4 lb. fertilizer |
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Recommendations for fertilizing vegetables are usually
stated: "Apply 3 to 4 pounds of 5-10-10 fertilizer per 100
square feet of garden space." This is fine, as long as you
are using a 5-10-10 formula. If the fertilizer you want to use has
a different formula, for example, one with a higher nitrogen
content as indicated by the first number in the formula, the rate
of application should be reduced to avoid nitrogen burn. A high
phosphorus fertilizer such as 6-18-6 is often recommended for
vegetables when transplants are set out. However, the amount of
fertilizer applied is determined by the amount of nitrogen because
it is the nutrient most easily lost from the soil. In the
following chart, you can see how the amount to be applied
decreases as the percentage of nitrogen increases: |
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Formula |
Fertilizer
applied per 1000 square feet |
5-10-10 6-18-6 8-12-4 12-6-6 16-16-16 |
3.5 lbs. 2.8 lbs. 2.0 lbs. 1.4 lbs. 1.0 lbs. |
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Nitrogen fertilizers do not burn or damage plants if
they are applied correctly. Fertilizers are salts, much like our
familiar table salt, except that they contain various plant
nutrients. When a fertilizer is applied to a soil, nearby water
begins to move very gradually towards the area where the
fertilizer has been applied. Salts in the fertilizer begin to
diffuse or move away from the place where they had been applied.
This dilutes the fertilizer and distributes it through a much
larger area. If tender plant roots are close to the area where the
fertilizer is placed, water will be drawn from these roots and
from the surrounding soil. The more salt or fertilizer applied,
the more water will be drawn from nearby roots. As water is drawn
from the roots, plant cells begin to dehydrate and collapse, and
the plant roots burn or dehydrate to a point from which they
cannot recover. If soil moisture is limited, most of the water
drawn towards the salt will come from plant roots and the damage
will be severe. |
Material |
Nutrient level |
Relative
Saltiness |
Ammonium nitrate Ammonium sulfate Potassium nitrate Natural organic fertilizer Urea formaldehyde Urea Superphosphate Potassium chloride Potassium sulfate Dolomite Gypsum Epsom salts |
33% Nitrogen 21% Nitrogen 14% Nitrogen 5% Nitrogen 38% Nitrogen 45% Nitrogen 20% Phosphorus 60% Potash 50% Potash 30% Calcium 20% Magnesium 33% Calcium 16% Magnesium |
1.49 1.63 2.67 0.41 0.41 0.81 0.21 0.87 0.43 0.12 1.38 |
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Two rules should be kept in mind when applying a
fertilizer during hot weather when soil moisture is limited: 1) do
not over-apply nitrogen fertilizers; and 2) make sure adequate
moisture is present after applying fertilizers high in salts. The
table above is a chart of commonly used garden fertilizers. The
last column is the practical measure of relative saltiness. A
higher number indicates greater saltiness. Soluble salts will
accumulate on top of the soil in a container and form a
yellow-to-white crust. A ring of salt deposits may form around the
pot at the soil line or around the drainage hole. Salts will also
build up on the outside of clay pots as water evaporates through
the clay. |
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Soluble salts accumulate when fertilizer is applied
repeatedly without sufficient water to leach or wash the old
fertilizers salts through the soil. It also occurs when
water evaporates from the soil, but salts having dissolved behind.
As the salts in the soil become more concentrated, plants find it
harder to take up water. If salts build up to an extremely high
level, water can be taken out of the root tips, causing them to
die. |
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Soluble-salt problems commonly occur with plants in
containers and when irrigation water high in dissolved salt
content is used. The best way to prevent soluble salt injury is to
stop the salts from building up. Water correctly. When water is
applied to containers, allow water to drain through the bottom
holes and then empty the drip plate. Water equal to one-tenth the
volume of the pot should drain through each time you water. Do not
allow the pot to sit in water. If you let the drained water be
absorbed by the soil, the salts that were washed out are taken
back into the soil. Salts can be reabsorbed through the drainage
hole or directly through a clay pot. Likewise, in garden soil,
periodically apply extra irrigation water that will move downward
in the soil profile to leach away any excess soluble salts that
have accumulated. |
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Potted plants should be leached every 4 to 6 months and
garden soils at least once per year. Leach a pot before
fertilizing to avoid washing away all newly added fertilizer.
Leaching is done by pouring water on the soil and letting it drain
completely. The amount of water used for leaching should equal
twice the volume of the pot. For example, a 6-inch pot will hold
10 cups of water, so 20 cups of water are used in leaching. Keep
the water running through the soil to wash the salts out. If a
layer of salts has formed a crust on top of the soil, you should
remove the salt crust before you begin to leach. Do not remove
more than 1 inch of soil. It is best not to add more soil to the
top of the pot. If the soluble salt level is extremely high or the
pot has no drainage, repot the plant. |
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The level of salts that will cause injury varies with
the type of plant and how it is being grown. A plant grown in the
home may be injured by salts at a concentration of 200 ppm. The
same plant growing in a greenhouse where the light and drainage
are good will grow well until salts reach concentrations 10 times
that level, or 2000 ppm. Some nurseries and florist shops leach
plants to remove excess salts before the plant is sold. If you are
not sure that has been done, leach a newly purchased plant the
first time you water it. |