How Much Fertilizer is Needed for One Application?
The release characteristics of a fertilizer and its burn potential determine the amount that can be applied in a single application. Fertilizers with quick-release sources of nutrients can burn the plants if applied at high rates. In addition, applying too much nitrogen in one application is inefficient since nitrogen not used by the plant can leach through the soil and out of the root zone. Consequently, quick-release forms of nitrogen should always be applied at a rate not exceeding 1 pound nitrogen per 1000 square feet in any one application.
It is important to not confuse the numbers on the bag with the amount of actual nutrients, in particular nitrogen (N), required for a given lawn size. For example, if we want to apply one pound of actual nitrogen to each 1000 square feet of lawn area and we have the choice between a 40 pound bag of a 5-0-10 lawn fertilizer or a 40 pound bag of a 25-0-15 lawn fertilizer, which one covers the area at the desired rate with the least amount of product? The answer to this question is outlined in Table 7.8.
Table 7.8. Determining Actual Amount of Fertilizer Nitrogen in Container
If we only consider the amount of nitrogen contained in each bag, then either formulation could provide the one pound of N per 1000 ft2. However, five times more product would be needed to provide that rate of N from the 5-0-10 compared to the 25-0-15. Why? The answer lies in the amount of actual N contained in the respective bags of fertilizer.
Using the 5-0-10 would require 20 pounds of product to apply one pound of N per 1000 ft2:
1 lb N per 1000 ft2 ÷ 0.05 lb N per 1 pound of fertilizer =
20 lbs of fertilizer to apply 1 lb of N per 1000 ft2
On the other hand, if we choose to use the 25-0-15 product we would only need 4 pounds of this fertilizer to apply the same rate of N per 1000 ft2 of lawn area:
1 lb N per 1000 ft2 ÷ 0.25 lb N per pound of fertilizer =
4 lbs of fertilizer to apply 1 lb of N per 1000 ft2
Therefore, 20 pounds of 5-0-10 = 4 pounds of 25-0-15. In other words, it takes 5 times more of the 5-0-10 than the 25-0-15 to apply the same 1 pound N per 1000 ft2 of lawn area.
Another way to look at it is to consider how much area of lawn each 40 pound bag of fertilizer would cover. In this case, the 5-0-10 would cover 2000 square feet of lawn area at the rate of one pound of nitrogen per 1000 square feet while the 25-0-15 would cover 10,000 square feet of lawn area at that same rate. Thus, the higher the concentration or percentage of a nutrient contained in the fertilizer, the LESS of it needs to be applied to achieve a desired rate per application (i.e., pounds of nutrient per 1000 square feet of lawn area.). See Table 7.9 to determine the amount of fertilizer to apply to any given lawn area based on the nutrient percentage and the weight of the bag.
Since slow-release forms of nitrogen are released gradually over a longer period of time, slightly higher rates can be applied. Some fertilizer products will provide suggested application ranges for their product.
Table 7.9. Calculating Fertilizer Application Rates
Using a desired nitrogen application rate, the area to be covered by a bag of fertilizer, can be determined from the information on the bag. Note: the fractional basis means the percentage expressed as a decimal equivalent.
(weight of bag x percent N as a decimal) ÷ desired rate of application =
area to be covered by fertilizer in bag
For example, if you want to apply fertilizer at a rate of 1 pounds N per 1000 ft2 and you are using a 20 pound bag of a fertilizer having an analysis of 23-0-6 (the fertilizer is 23% N), then:
20 lbs of fertilizer per bag x 0.23 lb of N/1 lb of fertilizer = 4.6 lbs N per bag
4.6 lbs N per bag ÷ 1 lb N/1000 ft2 = 4.6 x 1000 ft2 = 4600 ft2 per bag
Therefore, this bag will cover 4600 ft2 at a rate of 1 pound of N per 1000 ft2.
If the lawn area is less than 4600 ft2, only a portion of the bag is needed to supply 1 pound N per 1000 ft2. For example, if your lawn area is 2500 ft2, then 11 pounds of fertilizer should be applied.
(20 lbs of fertilizer/bag x 2500 ft2) ÷ 4600 ft2/bag = 11 lbs of fertilizer
If the lawn area is larger than 4600 ft2, more than one bag is needed to supply 1 pound N per 1000 ft2. For example, if your lawn area is 5500 ft2, then 24 pounds of fertilizer should be applied, therefore, more than one bag of fertilizer is required.
(20 lb of fertilizer/bag x 5500 ft2) ÷ 4600 ft2/bag = 24 lbs of fertilizer