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.
Step 1: Take 160 lbs (desired amount) ÷ 43,560 (square feet in an acre) = 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.
Onion seedlings require proper nutrition at specific times to support healthy growth and maximum bulb yields. Knowing when to fertilize onion seedlings is key to boosting vigor while avoiding fertilizer burn. This comprehensive guide covers ideal timing, signs of nutrient needs, fertilizer options, application methods, and tips to develop a smart fertilization schedule.
Why Fertilizer Timing Matters
Fertilizing at the wrong time can easily damage tender onion seedlings. Their small root systems are highly sensitive to concentrated fertilizers. Applying too much too soon disrupts growth by burning roots and leaves. Alternatively, fertilizing too late reduces vigor, weakens plants, and limits yields. Following optimal timing allows efficient nutrient absorption to energize growth.
Early Growth Stage Needs
Onion seedlings primarily need phosphorus after sprouting. Phosphorus strengthens roots, allowing more efficient uptake of water and nutrients. Robust roots prevent seedlings from toppling over as their foliage expands. As the first true leaves emerge, begin fertilizing with a balanced, starter solution. Dilute to half strength. Repeat every 1-2 weeks.
Ongoing Fertilizer Requirements
Once seedlings establish, continue feeding with a balanced fertilizer like 10-10-10 or 20-20-20. This supplies nitrogen for leaf growth phosphorus for root and shoot development, and potassium for overall plant health. Add micronutrients like magnesium and zinc if soil is deficient. Fertilize lightly every 2-3 weeks during active growth.
Signs Seedlings Need Fertilizer
Watch for these cues signaling it’s time to fertilize onion seedlings:
- Yellowing or purpling leaves
- Leggy, weak stems
- Stunted growth
- Paltry root system
- Wilting despite watering
- Few or no flowers
- Small, malformed new leaves
Organic vs Synthetic Options
Both organic and synthetic fertilizers effectively nourish onion seedlings when used properly,
Organic: Compost tea, worm castings, fish emulsion. Release nutrients slowly. Improve soil.
Synthetic: Balanced, water-soluble mixes. Provide rapid nutrient uptake. Careful dilution is crucial.
Proper Application Methods
- Water seedlings thoroughly before and after fertilizing to avoid burning roots.
- Dilute any liquid fertilizer to half strength or less.
- Apply as a root drench or light foliar spray.
- Spread dry granular fertilizer lightly around seedlings, then water in well.
- Repeat every 2-3 weeks during active growth.
Tips for Fertilizing Onion Seedlings
Follow these tips for success when fertilizing onion seedlings:
- Test soil to determine existing nutrient levels before fertilizing.
- Dilute fertilizer more than recommended for mature plants.
- Fertilize in early morning or late evening to avoid leaf burn.
- Avoid overapplying nitrogen which causes excessive leaf growth.
- Don’t fertilize after bulbs begin swelling.
- Stop fertilizing if plants show signs of burn like brown tips.
- Resume fertilizing after any transplant shock subsides.
- Adjust schedule based on seedling health and response.
- Keep surrounding weeds at bay to reduce competition for nutrients.
Sample Fertilizing Schedule
Use this sample schedule as a starting point for your onion seedlings:
- At sprouting: Dilute phosphorus fertilizer or compost tea.
- 2 weeks after sprouting: Balanced starter fertilizer at half strength.
- 4 weeks after sprouting: Balanced all-purpose fertilizer at quarter strength.
- 6-8 weeks after sprouting: Balanced fertilizer every 2-3 weeks.
- When bulbs start swelling: Discontinue fertilizer.
Proper fertilization scheduling, combined with close observation of plant health and growth, allows you to maximize the productivity of your onion seedlings for bountiful harvests. With a little patience and care, you can enjoy vigorous seedlings that mature into large, flavorful bulbs.
Frequently Asked Questions
What ratio of NPK is best for onion seedlings?
A balanced NPK ratio like 10-10-10 works well. Slightly emphasize phosphorus at first. Once seedlings establish, switch to an evenly balanced fertilizer.
When can fertilizer burn onion seedlings?
Fertilizer burn happens when concentrated salts build up in soil, dehydrating and damaging roots. To avoid burn, dilute fertilizer more than recommended for mature plants and leach soil regularly with water to remove any excess salts.
Should I use organic or chemical fertilizers?
Both can work well. Organic options like compost tea release nutrients slowly. Chemical fertilizers allow precise control but require very dilute application to avoid burning delicate seedlings.
How often should I fertilize outdoor onion seedlings?
After transplanting seedlings, fertilize every 2-3 weeks during active growth with a diluted balanced fertilizer. Discontinue fertilizing once bulbs start swelling.
What are signs my seedlings need more nutrients?
Watch for stunted growth, yellowing leaves, weak stems that flop over, small root systems, or wilting despite regular watering. These signal it’s time to fertilize.
Can I fertilize seedlings with coffee grounds?
Yes, used coffee grounds provide nitrogen and acidify soil. However, don’t use coffee grounds alone. Supplement with a balanced fertilizer and monitor soil pH.
When should I stop fertilizing onion seedlings?
Cease fertilizing when bulbs begin swelling, as excess nutrients at this stage can impede proper bulb formation and maturation.
With smart timing and care, fertilizing your onion seedlings gives them the healthy start they need to yield an abundant onion crop. Pay close attention to their growth and responses to fine-tune your fertilizing approach.
Table 2: Timing of applications of Nitrogen in onions.
| 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 |
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 | – |