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  MG Manual Reference
Ch. 2, pp. 24 - 27
[Fertilizers: fertilizers | analysis | types | organic | applying | application | improving | compost ]


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

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.

Fertilizer Formulation Amount
Triple super phosphate
Muriate of Potash
100 lbs.
100 lbs.
100 lbs.

Equivalent to
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 FertilizersTop
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 FertilizersTop
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 soil’s 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.

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