How to calculate fertilizer application rates
Calculating fertilizer application rates starts with knowing the following:
- The required nutrient application rate
- The fertilizer grade / composition
- The field’s area
Required nutrient application rate – this is the rate of nutrient to apply, or the nutrient recommendation. For soil applications it is usually expressed in units of lbs/acre or kg/ha.
The nutrient recommendation should take into consideration the crop nutrient uptake, soil analysis, the mineral content of the irrigation water and other parameters.
The guaranteed analysis of the fertilizers refers to the guaranteed nutrient content of the fertilizer and is expressed as a percentage by weight and appears on the fertilizer label.
In addition, it is important to note that both nutrient requirements and fertilizer grade can be expressed either in the oxide form or in the elemental form.
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Nutrients that are usually expressed in the oxide form are:
| Nutrient | Oxide form | Elemental form |
| Phosphorus | P2O5 | P |
| Potassium | K2O | K |
| Calcium | CaO | Ca |
| Magnesium | MgO | Mg |
However, total nitrogen, sulfur and micronutrients are usually expressed in their elemental form.
HOW TO READ THE FERTILIZER LABEL
The grade of a fertilizer usually refers to the percentage of the three major nutrients – nitrogen, phosphorus and potassium. The grade appears as three big numbers on the fertilizer label.
The first number is the total nitrogen concentration in the fertilizer, the second is the phosphorus concentration and the third refers to potassium.
In most countries, the content of phosphorus and potassium is indicated in the oxide form, not in the elemental form. Therefore, the grades refers to N-P2O5-K2O and not to N-P-K.
For example, a fertilizer with a grade of 13-10-27 contains 13% nitrogen, 10% of phosphorus as P2O5 and 27% of potassium as K2O.
A fertilizer with a grade of 46-0-0 contains 46% nitrogen only.
However, the grade alone does not always provide enough information about the fertilizer content.
Fertilizer may contain additional nutrients that are not indicated in the grade and, therefore, it is extremely important to review the fertilizer label.
The information on the fertilizer label includes the complete guaranteed analysis.
For example, the grade of ammonium sulfate is 21-0-0. Looking at the grade alone, one might think that this fertilizer contains only nitrogen. However, as the fertilizer’s name suggests, ammonium sulfate also contains 24% sulfur.
Worth Noting
The nitrogen percentage in the fertilizer grade always refers to total nitrogen, and does not give an indication of the form or forms of nitrogen in the fertilizer. Fertilizers usually contain nitrogen as ammonium (NH4), Nitrate (NO3), urea (CO(NH2)2), or a combination of those. The guaranteed analysis should detail the nitrogen forms available in the fertilizer.
HOW TO CALCULATE FERTILIZER RATES
For solid fertilizers, you can use the following formula :
Fertilizer rate = required nutrient application rate X 100 / % nutrient in the fertilizer
Let’s see a few examples:
Example 1
A grower wants to apply 50 pounds of nitrogen to his 6 acres field, using Urea 46-0-0 fertilizer. How many pounds of urea he needs to apply?
Fertilizer application rate = 50 x 100 / 46 = 108.7 lbs
Example 2
A grower applied 80 kg of Mono Ammonium Phosphate (MAP 12-61-0) to his 5-hectares field. The grade represents the complete guaranteed analysis of the fertilizers.
Which nutrients did he apply? At what rate per hectare?
The first number of the fertilizer grade is the percentage of nitrogen in the fertilizer (12%).
The second number refers to the percentage of phosphorus as P2O5 (61%)
Therefore, nitrogen application rate can be calculated as follows:
80 = N x 100 / 12
N = 80 x 12 / 100 = 9.6 kg per 5 hectares or 9.6/5 = 1.92 kg/ha nitrogen
Calculating the phosphorus application rate:
80 = P2O5 x 100 / 61
P2O5 = 48.8 kg per 5 hectares or 9.76 kg/ha
For liquid fertilizers, the weight of the fertilizer must be accounted for, because the fertilizer nutrient content is given as percentage by weight.
For example, Urea Ammonium Nitrate (UAN), a liquid fertilizer with a grade of 30-0-0 contains 30% nitrogen by weight. The weight of UAN is 10.86 lbs. per gallon. Therefore, if a grower wants to apply 15 lbs. of nitrogen. How much UAN 30-0-0 should he use?
First, we can use the previous formula:
Fertilizer rate = Required nutrient application rate X 100 / % Nutrient in the fertilizer
Fertilizer rate = 15 x 100 / 30 = 50 lbs.
Since 1 gallon of the fertilizer weights 10.86 lbs:
50 lbs. / (10.86 lbs./gallon) = 4.6 gallons.
The formula for calculating liquid fertilizer rates becomes, therefore:
Fertilizer rate = (Required nutrient application rate X 100) / (% nutrient in the fertilizer x W)
Where W is the fertilizer weight.
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