In
your garden
Twenty nutrients
have been identified that are required by plants.
Of these,
nitrogen, phosphorus, and potassium are required in
relatively large
amounts. Nitrogen is associated with lush vegetative
growth, adequate
phosphorus is required for flowering and fruiting,
and potassium is
necessary for durability and disease resistance.
Calcium, sulfur,
and magnesium are also required in comparatively
large quantities.
These six nutrients are referred to as macro nutrients.
The other
nutrients, referred to as micro nutrients, are required in
very small
amounts. These include such elements as copper, zinc,
iron, and boron.
While both macro and micro nutrients are required
for good plant
growth, over-application can be as detrimental as a
deficiency.
Over-application of plant nutrients not only may impair plant
growth, but may
contaminate groundwater by leaching through the soil
or pollute
surface waters by washing away.
Soil
testing
Testing your soil
for nutrients and pH is important to provide your
plants with the
proper balance of nutrients while avoiding over-
application. If
you are establishing a new garden , a soil test is
strongly
recommended. The cost of soil testing is minor in comparison
to the cost of
plant materials and labor. Correcting a problem before
planting is much
simpler and cheaper than after wards. Once your garden
is established,
continue to take periodic soil samples. While many people routinely lime their
gardens , this can result in raising the pH too high.
However, since
many fertilizers tend to lower the pH, the pH may drop
below desirable
levels after several years, depending on fertilization and
other soil
factors. Home tests for pH, nitrogen, phosphorus, and potassium
are available
from garden centers. While these may give you a general
idea of the
nutrients in your soil, they are not as reliable as tests performed
by the
Cooperative Extension Service at land
grant universities. University
and other
commercial testing services will provide more detail and you can request
special tests for micro nutrients if you suspect a problem. In addition
to the analysis
of nutrients in your soil, they often provide recommendations
for the application
of nutrients or on adjusting the pH. The test for soil pH
is very simple–
pH is a measure of how acidic or alkaline your soil is.
A pH of 7 is
considered neutral. Below 7 is acidic and above 7 is alkaline.
Since pH greatly
influences plant nutrients, adjusting the pH will often
correct a
nutrient problem. At a high pH, several of the micro nutrients
become less
available for plant uptake. Iron deficiency is a common
problem even at a
neutral pH on such plants as rhododendrons and
blueberries. At
very low pH, other micro nutrients may be too available,
resulting in a
plant toxicity. Phosphorus and potassium are tested
regularly by
commercial testing labs. While there are soil tests for
nitrogen, these
may be less reliable. Nitrogen is present in the soil in
several forms and
the forms can change rapidly. Therefore, a precise
analysis of
nitrogen is more difficult to obtain. Most university soil test
labs do not
routinely test for nitrogen. Home testing kits often contain a
test for nitrogen
which may give you a general idea of the presence of
nitrogen, but
again, due to the various transformations of nitrogen, the
reading may not
be reliable. Organic matter is often part of a soil
test. Soil
organic matter is highly desirable. Organic matter has a large
influence on soil
structure. Good soil structure improves aeration and
water movement
and retention. This encourages increased microbial
activity and root
growth, both of which influence the availability of
nutrients for
plant growth. Soil organic matter also affects the availability
of plant
nutrients and how pesticides react in the soil. Soils high
in organic matter
tend to have a greater supply of plant nutrients
compared to many
soils low in organic matter. Organic matter tends
to bind up some
soil pesticides, reducing their effectiveness. Tests for micro nutrients are
usually not performed unless there is reason
to suspect a
problem. Certain plants have greater requirements for
specific
micro nutrients and may show deficiency symptoms. Iron
deficiency is
common on blueberries, unless the soil is quite acidic. On
these plants, the
younger leaves will usually show signs of the deficiency
first. The areas
between the veins will be yellowish while the veins remain
green. Other
plants growing in the same soil will show no signs of a
deficiency. In
this case, altering the pH will often correct the problem.
Taking
a soil test
1. If you intend
to send your sample to the land grant university in your
state, contact
the local Cooperative Extension Service for information
and sample bags.
If you intend to send your sample to a private testing
lab, contact them
for specific details about submitting a sample.
2. Follow the
directions carefully for submitting the sample. The
following are
general guidelines for taking a soil sample.
a. Sample when
the soil is moist but not wet.
b. For each acre
of land to be tested, 10 to 15 sub-samples are
recommended.
Areas that appear different or that have been used
differently
should be sampled separately. For example, a separate
sample should be
submitted for an area that has been in a garden
and one that has
been lawn.
c. Obtain a clean
pail or similar container.
d. Clear away the
surface litter or grass.
e. With a spade
or soil auger, dig a small amount of soil to a depth
of 6 inches.
f. Place the soil
in the clean pail.
g. Repeat steps d
through f until the required number of samples
have been
collected.
h. Mix the
samples together thoroughly.
i. From the
mixture, take the sample that will be sent for analysis.
j. Send
immediately. Do not dry before sending.
3. If you are
using a home soil testing kit, follow the above steps for
taking your
sample. Follow the directions in the test kit carefully.
Fertilizers
and soil amendments
Once you have the
results of the soil test, you can add nutrients or soil
amendments such
as lime, as needed. If you need to raise the pH, use
lime. Lime is
most effective when it is mixed into the soil, therefore it is
best to apply
before planting. For large areas, using rototiller is most effective.
For small areas
or around plants, working the lime into the soil with a
spade or
cultivator is preferable. When working around plants, be
careful not to
dig too deeply or so roughly that you damage plant roots.
Depending on the
form of lime and the soil conditions, the change in pH
may be gradual.
It may take several months before a significant change
is noted. Soils
high in organic matter and clay tend to take larger amounts
of lime to change
the pH than do sandy soils. If you need to lower the pH significantly, , you
can use aluminum sulfate. Other commercially available fertilizers will also
help lower the pH. In all cases, follow the soil test or manufacturer’s
recommended rates of application. Again, mixing well into
the soil is
recommended. There are numerous choices for providing
nitrogen,
phosphorus, and potassium. If your soil is of adequate fertility,
applying compost
may be the best method of applying additional nutrients.
While compost is
relatively low in nutrients compared to commercial
fertilizers, it is especially beneficial in improving the
condition of the soil.
By keeping the
soil loose, compost allows plant roots to grow well throughout
the soil,
allowing them to extract nutrients from a larger area. A loose soil
enriched with
compost is also an excellent habitat for earthworms and
other beneficial
soil microorganisms that are essential for releasing
nutrients for
plant use. The nutrients from compost are also released slowly
so there is no
concern for "burning" the plant with an over-application.
Manure is also an
excellent source of plant nutrients and organic matter.
Manure should be
composted before applying. Fresh manure may be too
strong and can
injure plants. Be careful when composting manure. If left
in the open,
exposed to rain, nutrients may leach out of the manure
and the runoff
can contaminate waterways. Make sure the manure is
stored in a
location away from wells and any waterways, and that any
runoff is
confined or slowly released into a vegetated area. Improperly
applied manure
also can be a source of pollution. For best results, work
composted manure
into the soil. If preparing a bed before planting,
compost and
manure may be worked into the soil to a depth of 8 to 12
inches. If adding
to existing plants, work carefully around plants.
Green manures are
another source of organic matter and plant nutrients.
Green manures are
crops that are grown and then tilled into the soil.
As they break
down, nitrogen and other plant nutrients become available.
Green manures may
also provide additional benefits of reducing soil
erosion. Green
manures such as rye and oats are often planted in the
fall after the
crops have been harvested. In the spring, these are tilled
under before
planting. With all organic sources of nitrogen, whether
compost or
manure, the nitrogen must be changed to an inorganic
form before the
plants can use it. Therefore, it is important to have
well-drained,
aerated soils that provide the favorable habitat for the soil
microorganisms
responsible for these conversions. There are numerous
sources of
commercial fertilizers that supply nitrogen, phosphorus, and potassium. The
first number on the fertilizer analysis is
the percentage
of nitrogen, the
second number is phosphorus, and the third number is
the potassium
content. A fertilizer like 10-20-10 has twice as much of each
of the nutrients
as a 5-10-5. How much of each nutrient you need depends
on your soil test
results and the plants you are fertilizing. As was mentioned
before, nitrogen
stimulates vegetative growth while phosphorus stimulates
flowering. Too
much nitrogen can inhibit flowering and fruit production.
For many
vegetables, a fertilizer higher in phosphorus than nitrogen is
preferred such as
a 5-10-5.
Fertilizer
application
Commercial
fertilizers are normally applied as a dry granular material, or
mixed with water
and watered onto the garden. If using granular materials,
avoid spilling on
sidewalks and driveways. These materials are water
soluble and can
cause pollution problems if rinsed into storm sewers.
Granular
fertilizers are a type of salt, and if applied too heavily on
plants, they can
burn the plants. If using a liquid fertilizer, apply directly
to or around the
base of the plant. For the most efficient use and to
decrease the
potential for pollution, fertilizer should be applied when
the plants have
the greatest need for the nutrients. Plants that are not
actively growing
do not have a high requirement for nutrients. Therefore,
applications of
nutrients to dormant plants, or plants growing slowly due
to cool
temperatures, are more likely to be wasted.
Generally, nitrogen
fertilizers
should not be applied to most plants in the fall in regions of the
country that
experience cold winters. Since nitrogen encourages vegetative growth, if it is
applied in the fall it may reduce the plant’s ability to harden
for winter. In
some gardens, fertilizer use can be reduced by applying it
around the
individual plants rather than broadcasting across the entire
garden. In the
case of phosphorus, much of the fertilizer phosphorus
becomes
unavailable to the plants once spread on the soil. For better
plant uptake,
apply the fertilizer in a band near the plant. Do not apply
directly to the plant
or in contact with the roots
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