Apply only the nutrients your plant can use
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 macronutrients.
The other nutrients, referred to as micronutrients, are required in
very small amounts. These include such elements as copper, zinc,
iron, and boron. While both macro and micronutrients 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.
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 afterwards. 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 micronutrients 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 micronutrients
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 micronutrients 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 micronutrients are usually not performed unless there is reason
to suspect a problem. Certain plants have greater requirements for
specific micronutrients 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, rototilling 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.
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.