|
Complete vs Incomplete A fertilizer is said to be complete when it contains nitrogen, phosphorus,
and potassium. The manufacturers of commercial fertilizers are
required by Arizona law to state the amounts of nutrients on the
container as a guaranteed analysis. Examples of commonly used
fertilizers are 10-10-10, 16-16-16, and 20-10-5. An incomplete
fertilizer will be missing at least one of the major components.
Examples of incomplete fertilizers are indicated on the following
chart. |
|
Common Incomplete (Farm-Type) Fertilizers
|
%Nitrogen |
%Phosphorus |
%Potassium |
Ammonium nitrate |
34 |
0 |
0 |
Ammonium sulfate |
21 |
0 |
0 |
Ammonium phosphate sulfate |
16 |
20 |
0 |
Mono-ammonium phosphate |
11 |
48 |
0 |
Superphosphate |
0 |
20 |
0 |
Triple superphosphate |
0 |
45 |
0 |
Urea |
46 |
0 |
0 |
Urea formaldehyde |
38 |
0 |
0 |
Muriate of potash (Potassium chloride) |
0 |
0 |
60 |
|
|
|
|
|
|
|
|
|
|
Incomplete fertilizers can be blended to make complete
fertilizers. As an example, if 100 pounds of urea (46-0-0) were
combined with 100 pounds of triple super phosphate (0-45-0), and
100 pounds of muriate of potash (0-0-60), a fertilizer grade of
15-15-20 would result. |
|
Example:
Fertilizer |
Formulation |
Amount |
Urea Triple super phosphate Muriate of Potash |
46-0-0 0-45-0 0-0-60 |
100 lbs. 100 lbs. 100 lbs. |
|
Yield Equivalent to |
46-45-60 15-15-20 |
300 lbs. 100 lbs. |
|
|
This tells us that 100 pounds of urea fertilizer
contain 46 pounds of nitrogen; 100 pounds of triple super
phosphate contain 45 pounds of phosphate; and 100 pounds of
muriate of potash contain 60 pounds of potash. When these three
quantities are combined, each quantity is diluted by the other
materials to one-third of its original concentration, provided
each bag has equal weight. |
|
The fertilizer ratio indicates the proportion of
nitrogen, phosphate, and potash contained in the fertilizer. The
specific fertilizer ratio you will need depends on the soil
nutrient level. For example, a 1-1-1 ratio (10-10- 10,15-15-15,
20-20-20, etc.) is widely used at the time of lawn establishment,
but established lawns generally respond better to fertilizer
ratios high in nitrogen. Two of the more common complete
fertilizers used by homeowners for flowers and vegetables are
10-10-10 and 5-10-10. |
|
Special-Purpose Fertilizers When fertilizer shopping, you will find fertilizers packaged for
certain uses or types of plants such as Citrus Food, Rose Food or
Azalea Food. The azalea fertilizer has been specifically
formulated to acidify the planting medium. Acid-loving plants such
as azaleas, rhododendrons or camellias generally perform best in
artificial media which can be acidified. Remember that natural
soil containing calcium carbonate is very difficult to acidify. |
|
Packaging of other fertilizers for specific types of
plants is offen based more on marketing then reliable research.
For example, the next time you are shopping, compare the
fertilizer ratios of different brands of rose fertilizers and pick
the least cost product that supplies the correct amount of
nutrients for roses. Plants cannot read labels! |
|
A soil test can be performed before the purchase of any
expensive, special-purpose fertilizers. It is not possible to make
a blanket statement that one fertilizer is best for every area of
the state. It is true that different plants use different
nutrients at different rates. What is unknown is the reserve of
nutrients already in the soil. This changes with every soil type
and location. A word of caution though: it is usually advisable to
spend your limited gardening dollars on appropriate fertilizers
and soil amendments rather than a routine soil test. Generally a
soil test is only recommended if there are severe problems with a
garden or landscaping area or when high-value plantings are being
established. Be sure to contact a reputable soil testing
laboratory in advance to obtain prices and sampling directions.
The Arizona County Extension Agent in your county can provide you
with a list of laboratories currently operating in the state. |
|
Slow-Release Fertilizers Plants can absorb nutrients continuously, so it is beneficial to provide
them with a balance of nutrients throughout their growth. An
efficient way to achieve this is to apply a slow-release
fertilizer, which releases nutrients at a rate that makes them
available to the plants over a long period. Slow-release
fertilizers contain one or more nutrients. Slow-release
fertilizers can be categorized according to their release
mechanism. The three major types of nutrient release mechanisms
are: (1) materials that dissolve slowly, (2) materials which must
be decomposed by soil microorganisms in order to release nitrogen,
and (3) granular materials with coatings made of resin or sulfur
to control the rate of nutrient release into the soil. |
|
Sulfur-coated urea is a slow-release nitrogen
fertilizer with a covering of sulfur around each urea particle.
Different thicknesses of sulfur control the rate of nitrogen
release, which increases with temperature. Watering does not
affect its release rate. Sulfur-coated urea applied to the soils
surface releases nitrogen more slowly than if incorporated into
the soil. This material generally costs less than other
slow-release fertilizers, and it also supplies the second
nutrient, sulfur. |
|
When fertilizer products coated with multiple layers of
resin come into contact with water, the layers swell and increase
the pore size in the resin so that the dissolved fertilizer can
move into the soil. Release rate depends on the coating thickness,
temperature, and water content of the soil. There is often a large
release of fertilizer during the first two or three days after
application. Release timing can be from 0 to 6 months, depending
on the coating. |
|
Slow-release fertilizers need not be applied as
frequently as other fertilizers, and higher amounts can be applied
without danger of burning. Plants may use the nitrogen in
slow-release fertilizers more efficiently than nitrogen in other
forms, since it is released over a longer period of time and in
smaller quantities. Slow-release fertilizers are generally more
expensive than other types. The real benefit, however, is the
frequency of application, which is much lower than conventional
fertilizers. |
|
Urea formaldehyde and sulfur-coated urea have been used
as turf fertilizer, while resin-coated fertilizers are
predominantly used in container growing. |
|
Caution should be used in applying slow-release
fertilizers around trees or shrubs, as they may keep the plant in
growth late in the summer. The late-season growth may not harden
off completely, and excessive winter damage may occur. |