Nutrients are elements that the plant needs to carry out its life processes. To
fertilize a plant properly is to give it these life sustaining nutrients. The
following section will introduce you to the important factors necessary for good
soil nutrition and plant growth.
Having a soil analysis prior to planting is the first step to any fertilization
program. Thereafter, periodic soil tests every 2-3 years keeps your soil at peak
nutritional levels for optimum plant growth. It is much more efficient and inexpensive
to correct any nutrient deficiencies prior to planting, than to wait until problems
show up in your landscape. Keep in mind that fertilizer is food, not medicine.
Choosing a Fertilizer
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Fertilizers can either be organic or chemical, liquid or dry, complete, simple or
specialized. Then there are the time-released and fertilizers combined with
insecticides, fungicides and weed killers. The choices are enough to send the average
person into a state of confusion. A simple explanation of some basic types of
fertilizer will help you determine what is best for you. Always consult your local
Home Depot nursery professional for specific advice for your area.
- Organic Fertilizer is derived from a living plant or animal source (for example,
manure, blood meal, cottonseed meal). The nutrients in organic fertilizers must
undergo gradual chemical processes to make them available to the plant. This means
that generally speaking, organic fertilizers release their nutrients slowly over a
fairly long period of time. Since they rely on soil organisms to release the
nutrients, most organic fertilizers work very effectively in moist soil, when the
soil temperature is warm enough for the organisms to be active.
- Inorganic or Chemical Fertilizers (such as superphosphate) are either mixed or
manufactured from inorganic minerals. They have the advantage of lower cost, easy
availability to the consumer and rapid availability of the nutrients to the plant's
root system.
- Water Soluble Fertilizers have many advantages. They are easy to use, have a
lower risk of burning the plant if used according to label instructions, are great
for container plants and the nutrients are immediately available to the roots. They
come in concentrated form and must be properly diluted with water.
- Dry Fertilizers are most commonly used by consumers. You sprinkle or spread them
over the soil around the plants and then work them into the soil, taking care not to
damage the roots. Then they must be well watered into the soil. When they start to
dissolve with water, they begin their fertilizing action. This can last up to several
months with time-released varieties.
- Time-Released Fertilizers are ball-like granules of a complete fertilizer coated
with a permeable substance. When these granules come in contact with water, some of
the fertilizer is released into the soil, a little bit at a time, until it is all
used up. The fertilizer can last for many months after the initial application. This
type of fertilizer is great for container plants which need extra feeding due to
leaching of nutrients from frequent watering. It is also ideal for those who want a
more low maintenance garden.
- Special Purpose Fertilizers are packaged to meet the needs of specific types of
plants. You may find "camellia food", "rhododendron and azalea food", "rose food",
for example.
- Complete Fertilizers contain all three of the major plant nutrients - nitrogen (N),
phosphorus (P) and potassium (K). Simple fertilizers contain only one of the three
major nutrients.
- Combination Fertilizers are mixed with insecticides, fungicides or weed killers.
They work well when you need the extra ingredient each time you fertilize.
When and Where to Fertilize
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Location, location, location. This is certainly true in knowing where to fertilize.
To be effective, fertilizer must be placed where the plant needs and can use it. If
timing is everything, then knowing when to fertilize is an important factor in
successful plant growth and maintenance.
Applying fertilizer once a year may not be adequate for some nutrients, and too much
for others. Young seedlings may be injured by large amounts of highly soluble
fertilizers because of the salt content.
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Container plants may need more frequent
fertilizing due to leaching of the minerals. Leaching is when minerals are drained
from the soil during watering. Heavy blooming shrubs like roses may also need
frequent feedings because of their long blooming season.
TIP: It is very important to read and follow label directions when applying
fertilizers. This is true both in application to the plant and in disposal after
use. Substantial damage to the plant can occur by over fertilizing. Sometimes less
is more.
Many plants are helped by foliar feeding. This is done by spraying water soluble
fertilizer directly onto the foliage. Generally, actively growing plants show the
best response to this method. However, be aware that this is a quick fix, and not
meant to replace good soil feeding.
TIP: Make sure plants are well watered prior to foliar feeding to avoid burning the
foliage. Do not apply if temperatures will rise above 85F/29C.
Mixing dry or slow release fertilizer into the planting hole when installing the
plants is one of the most effective methods of fertilizing. It gets the fertilizer
directly into the area where the new feeder roots will grow and take up the nutrients.
Side dressing in rows next to the plants is another way to get fertilizer into
the areas where the roots can use it. As the roots spread into the adjacent rows,
they will feed on the nutrients.
With established plants, the active feeder roots are most concentrated near the "drip
line" of the plant. This is the circular area around the plant directly under the
outermost branches. Rainfall tends to drip off of the plant in this area. When
applying liquid or dry fertilizer, it is most effective to apply the fertilizer in
this area. And it is always important to thoroughly water in dry or granular
fertilizer after it is applied.
The three nutrients used most in commercial fertilizers are Nitrogen (N), Phosphorous
(P) and Potassium (K). It is common to see the percentage of each nutrient in a
fertilizer listed in bold numbers on the manufacturer's label, right under the
product's name. For example, 15-30-15 tells you that the fertilizer contains 15%
Nitrogen, 30% phosphorus and 15% potassium.
- Nitrogen is the first of the nutrients listed on the fertilizer label and is
essential to plant growth. It is not a mineral, so it is not found in the minute soil
particles where plants obtain other nutrients. All nitrogen must come from sources
other than soil, such as air, water, organic matter or fertilizer. Plants use large
amounts of nitrogen to form proteins, chlorophyll and enzymes needed to live and
reproduce. In proper amounts it is responsible for leaf and stem growth as well as
the overall size and vigor of the plant. It is the nutrient most likely to be
deficient in any type plant.
TIP: If nitrogen is deficient, the leaves yellow from the tips back to the stems, the
plant itself yellows from the bottom upward and growth is stunted.
- Phosphorus is the second nutrient on the fertilizer label. Phosphorus helps build
strong roots and is essential for flower and seed production.
The availability of phosphorus in most soils is very low and it does not move readily
through the soil for roots to absorb. So when surface applications of phosphorus
fertilizer are used, it only moves through the top inch or two of soil. Only the
surface roots are fed. It is important to concentrate the phosphorus where the roots
can get to it. One effective way to do this is to mix the fertilizer into the planting
hole when introducing the plant into the landscape.
Lots of phosphorous is great for bulbs, perennials, and newly planted trees and
shrubs. So pick a fertilizer with a high middle number on the nutrient label when
introducing this type of plants into the landscape.
TIP: A phosphorus deficiency will first show up in older leaves. They will lose their
sheen and appear dull and dark. Red, blue or yellow color will show through the green
along the veins on the undersides of the leaf. The tips of the leaves may turn brown
and new growth will be very small.
- Potassium is the third nutrient on the fertilizer label, and is sometimes referred
to as 'potash'. It exists naturally in the soil in many forms but only about one
percent of it can be used by plants. Potassium increases chlorophyll in foliage and
helps plants make better use of light and air. Potassium encourages strong root
growth, water uptake and triggers enzymes that fight disease.
Potassium is essential for the healthy growth of annuals and perennials. It is
especially important in the development of fruit. Deficiency often occurs in acidic,
sandy soils, where there is little organic matter content and low nutrient holding
ability.
TIP: Potassium, like phosphorus, must be placed near the roots, in the path of their
anticipated growth path. When there is a potassium deficiency, older leaves mottle and
yellow between veins, followed by whole leaves that turn dark yellow and die. Flower
and fruit drop are also common problems associated with potassium deficiency.
Some fertilizers also supply other important nutrients such as calcium, magnesium and
sulfur. They are usually found in the soil in sufficient amounts, although some parts
of the country where there is high rainfall, may suffer sulfur deficiencies. Like
nitrogen, sulfur is easily leached from the soil. Calcium plays a key role in cell
manufacture and growth and roots need calcium right at the growing tips. Magnesium is
important in its relationship to the chlorophyll in green leaves.
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Micronutrients (or trace elements) such as iron, zinc or manganese are used in smaller
quantities than macronutrients, but they are important to proper plant growth. Iron
is important in chlorophyll formation. Zinc and manganese help the plant process other
nutrients. However, the plants may not be able to absorb enough of these nutrients if
the soil is highly alkaline.
TIP: Iron and copper also play an important role in plant development. An iron
deficient plant has leaves that turn pale yellow or white while the veins remain
green. A shortage of copper makes new growth wilt and causes irregular growth.
However, if there is an excess of copper, it can cause sudden plant death. Because
of this property, copper is also used as a fungicide and as a tool to fight off
insects and diseases.
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