Calculating Fertilizer Rates from Soil Test Recommendations
Extension Agronomist
Phosphorus and potassium fertilizer recommendations are based on the amount (lbs) of nutrients (P2O5, K2O) required at a given soil test level to maintain optimum growth, yet be environmentally safe. Nitrogen recommendations are not based on a soil analysis per se. This is due to the many climatic, chemical and biological factors that influence the amount of nitrogen present in a soil at any given time. Instead of a soil analysis, nitrogen recommendations are based on research results from field experiments to determine the best application rate to attain selected turf properties.
Development of fertilizer recommendations and conveying the information to homeowners in a format that is easily understood is a very comprehensive process. In order to assist homeowners with fertilization programs the University of Georgia Soil Testing Laboratory fertilizer recommendations are given in pounds (lbs.) of a fertilizer grade (e.g. 12-4-8) to apply to a specified area. Lawn and turf recommendations are given in pounds per 1000 sq. ft., ornamental and flower recommendations in pounds per 100 sq. ft., and home garden in both pounds per 1000 sq. ft. and lbs. per 100 linear ft. This alleviates the homeowner from having to make countless calculations. However, occasions will arise where one fertilizer grade will have to be substituted for another or perhaps one will have to calculate the amount of fertilizer needed to supply a specified amount of nutrient per given area (e.g. amount of ammonium nitrate needed to supply 1 lb. of N per 1000 sq. ft). Hopefully the following information will offer some assistance in these matters.
Fertilizers are sold in many grades. Complete fertilizers, such as 10-10-10 or 16-4-8, contain all three primary nutrients. Single nutrient fertilizers contain only one, but they generally are a high analysis, and economical
source of that nutrient (e.g., Urea [46-0-0], Muriate of Potash [0-0-60]). The hyphenated numbers on the fertilizer bag indicate the minimum percentages of nutrients by weight. These percentages are guaranteed by Georgia law to
protect the consumer. For example
a 50 lb. bag of a 16-4-8 fertilizer
contains a minimum of 8 lbs. of nitrogen (N), 2 lbs. of phosphorus (P2O5) and 4 lbs. of potassium (K2O). Most lawn and garden fertilizers are complete fertilizers. They are
convenient to use, but it may be difficult to find one that exactly matches the fertilizer recommendation given on a soil test report. Therefore, sometimes homeowners have to
substitute another fertilizer for the one recommended. When making substitutions for complete fertilizers, select one with a similar ratio of N-P2O5-K2O to that
recommended. For example, 10-10-10, 16-4-8, and 12-4-8 analysis fertilizers have ratios of 1-1-1, 4-1-2, and 3-1-2, respectively. The ratios are calculated by dividing
the numbers in the fertilizer grade by the lowest divisible number. For the three grades (10-10-10, 16-4-8, and 12-4-8) the lowest divisible numbers are 10 and 4, respectively. The ratios are calculated as follows:
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Suppose a recommendation was for 8 lbs of 12-4-8 per 1000 square feet. The garden center had no 12-4-8 available, but an 8-8-8, 27-3-3, 25-5-11, and 16-4-8 were available. Which would be the best grade to substitute for the 12-4-8? Calculating the ratios as illustrated above the fertilizers offered by the garden center have ratios of 1-1-1, 9-1-1, 5-1-2.2, and 4-1-2. So the 16-4-8 would be the closest fit to the recommendation and the 25-5-11 would be the second best fit. It is most important to accurately match the N requirement when selecting the grade and calculating the fertilizer rate and then compromise somewhat if necessary for phosphorus (P2O5) and potassium (K2O). For most lawns and gardens the greatest compromise can be made with phosphorus as many soils used for these purposes test HIGH or VERY HIGH in phosphorus.
To calculate fertilizer rates, the following example is provided:
A soil test report for a bermudagrass lawn recommends the application of 10 lbs. of 10-10-10 per 1000 square ft. when spring growth begins and 8 lbs. of 12-4-8 or 6 lbs. of 16-4-8 per 1000 square ft in June, July, August, and September. The dimensions of the area to be fertilized are 200 feet by 50 feet.
To determine how much fertilizer is required for a given area:
Step 1: If the dimensions of the area are not known, measure the area to be fertilized in feet (e.g., 200 ft. long x 50 feet wide)and then calculate the number of square feet in the area.
Example:
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Step 2: Calculate the amount of fertilizer that would be needed as prescribed on the soil test report. The following equation can be used to calculate the needs.
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So for the spring application a total of 100 lbs of 10-10-10 would need to be purchased.
Step 3: Calculate the amount of fertilizer that would need to be purchased for the 4 summer applications. The same equation can be used.
For the summer applications, a total of 320 lbs of 12 - 4 - 8 would be required for the 4 applications.
Note: Sometimes a soil test report may only recommend that nitrogen be applied. Suppose the recommendation was to apply ammonium nitrate at a rate equivalent to 1 lb of N per 1000 ft2. How much ammonium nitrate (34% N) would be required for the area?
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Also areas to be fertilized are often encountered that are not always square or rectangular in shape. Several approaches can be used to calculate the area. One approach is to mark the area off in as large a rectangle or square as possible. Then mark the remaining area off in smaller squares, or triangles or right triangles. Calculate the areas for each marked off area and sum the values. The formula given above for the area of a rectangle can also be used for a square. The area of a right triangle and triangle can be calculated using the following formulas:
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To calculate the area of a trapezoid (only two sides parallel) use the following formula:
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