How to Calculate Fertilizer Application

Fertilizers are defined by law as soil improvement with a guaranteed minimum nutrient content. Fertilizers are supplied with a percentage of nitrogen, potassium, and phosphorus. For example, a 10-5-5 fertilizer consists of 10% nitrogen, 5% potassium and 5% phosphorus. Different plants require various amounts of nutrients, and gardeners can calculate application rates based on plant needs, garden area, and nutrient concentrations. Nitrogen is the essential nutrient for lawns and many other deciduous plants, so gardeners often calculate fertilization rates based on nitrogen content. The rate can also be calculated for nutrients in flowers and fruits, such as potassium and phosphorus.

Fertilizer Content Information

How to Calculate Fertilizer Application

Fertilizer is good for plants. However, too many inadequate nutrients can have harmful effects, and too much fertilizer can burn roots and shoots. Reading the compost contents will give you an idea of the amount of each macro-nutrient in the formula and any other nutrients such as calcium and magnesium. The analysis or remark on the fertilizer label gives a 3-digit ratio (NPK) of each macro element of the product. It is essential to note this if you are trying to nourish the plant’s leaves or promote flowering.

The 3-digit ratio makes it possible to interpret the amounts of nitrogen, phosphorus, and potassium in the way they are written in the report. Nitrogen, the first digit, accompanies leaf growth, while phosphorus helps with bud formation and rooting. Potassium is essential for the general health of plants and increases their protection against adverse conditions and diseases. For example, 10-5-5 is nitrogen fertilizer, and 5-10-5 is a stimulant for flowers.

The product also contains other nutrients and fillers. When fertilizing, it is important to note these figures to balance the needs of the plants. In this case, the application rates must be taken into account. For lazy gardeners like myself, consider releasing the nutrients gradually over 3-6 months. Immediate delivery is possible with fast-acting liquids or granules.

What to Consider When Calculating Fertilizer Application Rates

How to Calculate Fertilizer Application

To calculate the application rates, you must first know the following

  • Nutrient application rate required
  • Fertilizer type/composition
  • Storage area

The required nutrient application rate is the nutrient application rate or nutrient recommendation. When they are applied to the soil, it is usually expressed in lbs/acres or kg/ha.

Nutrient recommendations should consider plant uptake, soil analysis, mineral content in irrigation water, and other parameters.

The guaranteed fertilizer analysis refers to the guaranteed nutrient content of the fertilizer and is expressed as a percentage by weight and indicated on the fertilizer label.

It is important to note that the nutrient and quality requirements for the fertilizer can be expressed as an oxide or elemental form.

Fertilizer Rates and Applications

How to Calculate Fertilizer Application

Read the soil analysis or recommendation to ensure that you have read the recommended standards correctly. Sometimes suggestions are given in quantities of 100 square meters. Too much fertilizer can damage the garden and the lawn.

Calculate the square footage of a lawn or garden by measuring two sides and multiplying the numbers. For example, if your yard is 50 feet long and 100 feet wide, multiply 50 by 100. In this example, your lawn covers 5000 square feet.

If your lawn is irregular or oddly shaped, mark it in squares, calculate each square area, and fold it up.

Divide the recommended amount of nitrogen per 1,000 square feet by the amount of nitrogen in the fertilizer mix. For example, if the recommended amount is 3 pounds per 1,000 square feet and the nitrogen in the fertilizer mix is 21 percent, divide three by 0.21 or 3/.21 = 14.28 (21 percent of the nitrogen required per 1,000 square feet).

Multiply the amount needed by 1,000 square feet by the number of square feet your garden or lawn needs. In our example, 14.28 (pounds per 1,000 square feet) x 5 (1,000 square feet per 5,000 square feet) = 71.42 pounds of fertilizer with 21 percent nitrogen for application.


Never add fertilizers containing ingredients other than those recommended in the soil test. For example, if the use of nitrogen is approved, the fertilizer described in 21-12-15 contains different elements that may not be necessary and may damage plants or the soil.

Wrapping Up

It is essential to know how much fertilizer to give in your garden or lawn because excess fertilizer is toxic to the soil and plants. Most application guidelines are given in pounds per 1000 square meters. For soil analysis, you can recommend adding 3 pounds of nitrogen per 1,000 square meters in spring. Because pure nitrogen is difficult to obtain, a fertilizer such as nitrogen sulfate is generally considered 21-0-0, which means that each pound is 21% nitrogen. You need to know how to calculate the amount of 21-0-0 products to meet the need for 3 pounds of nitrogen per 1000 square meters.

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