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  MG Manual Reference
Ch. 2, pp. 31 - 35
[Fertilizers: fertilizers | analysis | types | organic | applying | application | improving | compost ]

Applying FertilizerTop
Computing the amount of fertilizer needed for a given area is rather tricky at first, but after a few times, this becomes second nature. Following are some examples of fertilizer determinations for lawns and gardens.
Example 1. Determine the amount of ammonium sulfate needed by a 5000 square-foot lawn if 1 pound of nitrogen per 1000 square feet is required.
Lawn: 5000 square feet
Fertilizer: ammonium sulfate (21-0-0)
Rate: 1 pound of nitrogen per 1000 square feet.
  1. Since we need 1 pound of nitrogen for every 1000 square feet and we have 5000 square feet, we need 5 pounds of nitrogen.
  2. Ammonium sulfate is 21 percent nitrogen (round to 20 percent).
  3. 20 percent is the same as 0.20 or 1/5. This means that we need 5 pounds of fertilizer to get 1 pound of nitrogen.
  4. Since we need 5 pounds of nitrogen, 5 x 5 = 25 pounds of fertilizer.

Total fertilizer needed =
N application rate
(lbs/1000 sq.ft.)
N content of fertilizer
expressed as a decimal
 x lawn size
(sq. ft.)



 = 25 lb.

Example 2. Determine how much 20-10-5 needs to be applied to ensure 2 pounds of phosphorus per thousand square feet in a garden that measures 20 x 10 feet.
Garden: 20 x 10 = 200 square feet
Fertilizer: 20-10-5 = 10 percent phosphorus
Rate: 2 pounds of phosphorus per 1000 square feet.
Total fertilizer needed =
2 lb. Phosphorous

= 2
x 200
 = 4 lb.

Recommendations for fertilizing vegetables are usually stated: "Apply 3 to 4 pounds of 5-10-10 fertilizer per 100 square feet of garden space." This is fine, as long as you are using a 5-10-10 formula. If the fertilizer you want to use has a different formula, for example, one with a higher nitrogen content as indicated by the first number in the formula, the rate of application should be reduced to avoid nitrogen burn. A high phosphorus fertilizer such as 6-18-6 is often recommended for vegetables when transplants are set out. However, the amount of fertilizer applied is determined by the amount of nitrogen because it is the nutrient most easily lost from the soil. In the following chart, you can see how the amount to be applied decreases as the percentage of nitrogen increases:
Formula Fertilizer applied per 1000 square feet
3.5 lbs.
2.8 lbs.
2.0 lbs.
1.4 lbs.
1.0 lbs.
Nitrogen fertilizers do not burn or damage plants if they are applied correctly. Fertilizers are salts, much like our familiar table salt, except that they contain various plant nutrients. When a fertilizer is applied to a soil, nearby water begins to move very gradually towards the area where the fertilizer has been applied. Salts in the fertilizer begin to diffuse or move away from the place where they had been applied. This dilutes the fertilizer and distributes it through a much larger area. If tender plant roots are close to the area where the fertilizer is placed, water will be drawn from these roots and from the surrounding soil. The more salt or fertilizer applied, the more water will be drawn from nearby roots. As water is drawn from the roots, plant cells begin to dehydrate and collapse, and the plant roots burn or dehydrate to a point from which they cannot recover. If soil moisture is limited, most of the water drawn towards the salt will come from plant roots and the damage will be severe.
Material Nutrient level Relative Saltiness
Ammonium nitrate
Ammonium sulfate
Potassium nitrate
Natural organic fertilizer
Urea formaldehyde
Potassium chloride
Potassium sulfate

Epsom salts
33% Nitrogen
21% Nitrogen
14% Nitrogen
5% Nitrogen
38% Nitrogen
45% Nitrogen
20% Phosphorus
60% Potash
50% Potash
30% Calcium
20% Magnesium
33% Calcium
16% Magnesium


Two rules should be kept in mind when applying a fertilizer during hot weather when soil moisture is limited: 1) do not over-apply nitrogen fertilizers; and 2) make sure adequate moisture is present after applying fertilizers high in salts. The table above is a chart of commonly used garden fertilizers. The last column is the practical measure of relative saltiness. A higher number indicates greater saltiness. Soluble salts will accumulate on top of the soil in a container and form a yellow-to-white crust. A ring of salt deposits may form around the pot at the soil line or around the drainage hole. Salts will also build up on the outside of clay pots as water evaporates through the clay.
Soluble salts accumulate when fertilizer is applied repeatedly without sufficient water to leach or wash the old fertilizer’s salts through the soil. It also occurs when water evaporates from the soil, but salts having dissolved behind. As the salts in the soil become more concentrated, plants find it harder to take up water. If salts build up to an extremely high level, water can be taken out of the root tips, causing them to die.
Soluble-salt problems commonly occur with plants in containers and when irrigation water high in dissolved salt content is used. The best way to prevent soluble salt injury is to stop the salts from building up. Water correctly. When water is applied to containers, allow water to drain through the bottom holes and then empty the drip plate. Water equal to one-tenth the volume of the pot should drain through each time you water. Do not allow the pot to sit in water. If you let the drained water be absorbed by the soil, the salts that were washed out are taken back into the soil. Salts can be reabsorbed through the drainage hole or directly through a clay pot. Likewise, in garden soil, periodically apply extra irrigation water that will move downward in the soil profile to leach away any excess soluble salts that have accumulated.
Potted plants should be leached every 4 to 6 months and garden soils at least once per year. Leach a pot before fertilizing to avoid washing away all newly added fertilizer. Leaching is done by pouring water on the soil and letting it drain completely. The amount of water used for leaching should equal twice the volume of the pot. For example, a 6-inch pot will hold 10 cups of water, so 20 cups of water are used in leaching. Keep the water running through the soil to wash the salts out. If a layer of salts has formed a crust on top of the soil, you should remove the salt crust before you begin to leach. Do not remove more than 1 inch of soil. It is best not to add more soil to the top of the pot. If the soluble salt level is extremely high or the pot has no drainage, repot the plant.
The level of salts that will cause injury varies with the type of plant and how it is being grown. A plant grown in the home may be injured by salts at a concentration of 200 ppm. The same plant growing in a greenhouse where the light and drainage are good will grow well until salts reach concentrations 10 times that level, or 2000 ppm. Some nurseries and florist shops leach plants to remove excess salts before the plant is sold. If you are not sure that has been done, leach a newly purchased plant the first time you water it.

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