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.

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.

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

Manure Source

N (%)

P (%)

K (%)

Chicken

3.8

1.9

1.8

Duck

2.2

1.1

1.2

Pig

2.8

1.4

1.2

Cattle

1.9

0.6

1.4

Goat

0.6

0.1

0.2

Horse

2.3

0.8

1.3

Rabbit

1.7

1.3

1.1

Guano

0.6

0.4-6.6

-

Table 1: Nutrient composition of selected animal manures.

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.

Stage

Number of Leaves

Amount to Apply per Acre

Total Applied for Season*

Pre Plant

4

20 units

40

Roots Established (2 weeks after planting)

5

20 units

60

Slow Growth (every 2 weeks)

6-8

20 units every 2 weeks for 3 applications (6 weeks)

120

Rapid Growth before bulbing

9-11

20 units every 2 weeks for 2 applications (4 weeks)

160

Bulbing

12 (100 days after planting)

Quit any additional applications

n/a

Table 2: Timing of applications of Nitrogen in onions.
*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.