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Fertilization Requirements for Onions

Fertilization of onions is a process that involves calculating current levels of your soil, providing additional fertilizer when needed, and knowing when to stop adding more fertilizer to the crop. The recommended total amount of fertilization in onions is 160-80-140 lbs per acre.

Calculating current levels involves sending some soil off to a testing facility that will return a report with the exact amounts of N (Nitrogen), P (Phosphorous), and K (Potassium) as well as pH of the soil and secondary and minor nutrients. Once the initial amounts are determined, the amounts that need to be added to supplement the soil are formulated. This is accomplished by subtracting the soil sample amounts from 160-80-140 units/acre, respectively. The key ingredient is the Nitrogen, since both Phosphorous and Potassium tend to have high levels in soil testing but are not in a form that is readily available to the plant. Adding additional Phosphorous or Potassium beyond the desired amount will rarely harm the onions.

To determine the amount of Nitrogen to apply:

Step 1: Take 160 lbs (desired amount) ÷ 43,560 (square feet in an acre) = 0.0037.

Step 2: Multiply the square feet of your row per square feet by 0.0037.

Example: A 100′ row that is 3 feet wide is 300 square ft X 0.0037 = 1.11 lbs of Nitrogen needed.

Step 3: Divide the amount of Nitrogen needed by the N formulation of the fertilizer you are using. This is the amount of fertilizer that would need to be applied to each 300 sq ft of onion area if there was no N in the soil analysis.

Example: 1.11 ÷ .21 (21% ammonium sulfate) = 5.3 lbs.
Fertilizer packages are labeled with three numbers that are percentages. The first indicates the percent actual nitrogen (N), the second, the oxide form of phosphorus (P2O5), and the third, the oxide form of potassium (K2O). Ammonium Sulfate is labeled as a 21-0-0 fertilizer, so to get 1 lb of Nitrogen in any space you will need to apply 5 lbs of dry product since 5 lbs X 21% = 1.05 units of Nitrogen. If you are using the 10-20-10, twice the amount of fertilizer would be required since 10 lbs X 10%N = 1 lb of Nitrogen.

Should you want to apply some manure to your garden, refer to the table below for approximate composition of various animal manures:

Table 1: Nutrient composition of selected animal manures.

Manure SourceN (%)P (%)K (%)
This means that you would need to apply approximately 5.5 times the amount of chicken manure to provide the same amount of Nitrogen as Ammonium Sulfate. For 300 sq ft, 27.5 lbs of chicken manure would have to be spread. This requirement is for the total amount of N that is needed, and it should not be placed all at one time since that much Nitrogen could burn the roots of the plant. The table below indicates the timing of these applications.

Table 2: Timing of applications of Nitrogen in onions.

StageNumber of LeavesAmount to Apply per AcreTotal Applied for Season*
Pre Plant420 units40
Roots Established (2 weeks after planting)520 units60
Slow Growth (every 2 weeks)6-820 units every 2 weeks for 3 applications (6 weeks)120
Rapid Growth before bulbing9-11

20 units every 2 weeks for 2 applications (4 weeks)
Bulbing12 (100 days after planting)Quit any additional applicationsn/a
*Assuming 20 units of N in soil testing
For home gardeners, the rate of Ammonium Sulfate to apply 20 units per acre is 1 cup per 20′. If using manure, the rate will be 10 cups per 20′ in most cases. Blood meal’s formulation is usually 13-0-0, indicating a rate of 1.6 cups per 20′ in your garden.
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