Yes, you can certainly use urea on onions, but it's crucial to adjust the application rate due to its high nitrogen concentration compared to other fertilizers.
Onions are heavy feeders, especially when it comes to nitrogen, which is essential for vigorous leafy growth that supports bulb development. Nitrogen promotes the lush green tops, which in turn gather energy for the developing onion bulb underground.
Understanding Nitrogen Sources for Onions
Different nitrogen fertilizers contain varying percentages of nitrogen. This percentage is the first number in the N-P-K (Nitrogen-Phosphorus-Potassium) analysis found on fertilizer bags.
For example:
- Ammonium Sulfate (21-0-0) contains 21% nitrogen.
- Urea (40-0-0) contains 40% nitrogen.
The provided reference indicates that "Applying Ammonium Sulfate (21-0-0) or a similar source of nitrogen at a rate of 1 cup per 10 feet will be adequate to generate a good response in your onion crop." It further clarifies, "If you use another source of nitrogen such as Urea (40-0-0), adjust the application based on the first number of the analysis."
How to Adjust Urea Application for Onions
Since urea (40-0-0) has significantly more nitrogen per unit of weight than ammonium sulfate (21-0-0), you will need to apply less of it to provide the same amount of nitrogen.
Here's a comparison to illustrate the adjustment:
| Fertilizer Type | N-P-K Analysis | Nitrogen Content | Recommended Rate (for 10 feet) |
|---|---|---|---|
| Ammonium Sulfate | 21-0-0 | 21% | 1 cup |
| Urea | 40-0-0 | 40% | Approximately 0.5 cups |
To achieve a similar nitrogen input as 1 cup of Ammonium Sulfate (21% N), you would need roughly half a cup of Urea (40% N) for the same 10-foot section. This is because 40% is nearly double 21%, meaning you need roughly half the amount of urea for the equivalent nitrogen.
Practical Tips for Applying Urea to Onions
When using urea on your onion crop, follow these practical insights to ensure optimal growth and prevent damage:
- Calculate Carefully: Always calculate the required amount based on the nitrogen percentage. Over-application of nitrogen can lead to excessive leafy growth at the expense of bulb development, or even burn the plants.
- Split Applications: Instead of applying all the nitrogen at once, consider splitting the application into two or three doses throughout the growing season.
- First application: After transplanting or when seedlings are established (around 6-8 inches tall).
- Second application: About 3-4 weeks later, or when bulbs begin to swell.
- Avoid late applications: Cease nitrogen fertilization about 4-6 weeks before harvest to encourage bulb curing and prevent green necking.
- Water Thoroughly: Always water immediately after applying urea to dissolve the granules and wash them into the soil. This prevents leaf burn and helps the roots absorb the nutrients.
- Even Distribution: Broadcast the urea evenly around the base of the plants, avoiding direct contact with the foliage. For precise application, you can use a hand spreader or carefully sprinkle it.
- Soil Testing: For best results, conduct a soil test before applying any fertilizer. This will give you precise information on your soil's nutrient levels and pH, allowing for a more tailored fertilization plan.
Important Considerations
While urea is an effective nitrogen source, it's essential to use it responsibly. Urea is quickly converted to ammonium and then nitrate in the soil, which can be prone to leaching if not properly managed or if applied in sandy soils with excessive rainfall. Always follow product-specific instructions and local agricultural recommendations for the best outcomes.
