Mixing Urea and Ammonium Sulfate: Pros and Cons

Fertilization is one of the most critical factors in successful plant growth, and selecting the right nutrient sources can significantly impact yield, soil health, and overall plant performance. Two commonly used nitrogen fertilizers, urea (46-0-0) and ammonium sulfate (21-0-0-24S), provide essential nutrients that plants need for strong vegetative growth. Many growers and gardeners consider mixing urea and ammonium sulfate to take advantage of their benefits.

But is this combination effective? In this guide, we will explore the pros and cons of mixing urea and ammonium sulfate, the best practices for application, and when this fertilizer mix is most beneficial.

Understanding Urea and Ammonium Sulfate

Urea (46-0-0): A High-Nitrogen Fertilizer

Urea is a widely used nitrogen fertilizer that provides a quick and efficient source of nitrogen. Once applied, urea hydrolyzes into ammonium, which plants can absorb. However, urea is prone to nitrogen loss through volatilization (ammonia gas escaping into the air), especially if left on the soil surface.

Pros of Urea:

• Highest nitrogen content (46%) – More nitrogen per pound compared to other fertilizers.
• Cost-effective – Provides an economical nitrogen source.
• Quick nutrient availability – Encourages rapid vegetative growth.

Cons of Urea:

• High volatilization risk – Requires incorporation into soil or irrigation.
• Lacks sulfur and other essential nutrients – Needs supplementation for balanced nutrition.
• May lower soil pH over time – Can increase soil acidity when over-applied.

Ammonium Sulfate (21-0-0-24S): A Dual-Nutrient Fertilizer

Ammonium sulfate contains both nitrogen (N) and sulfur (S), making it an excellent choice for sulfur-deficient soils. It has an acidifying effect, which is useful for neutralizing alkaline soils.

Pros of Ammonium Sulfate:

• Contains sulfur (24%) – Essential for amino acid and protein synthesis in plants.
• Improves nitrogen efficiency – Sulfur enhances nitrogen uptake.
• More stable nitrogen source – Less prone to volatilization than urea.
• Ideal for alkaline soils – Helps lower soil pH and improve nutrient availability.

Cons of Ammonium Sulfate:

• Lower nitrogen content (21%) – Requires higher application rates compared to urea.
• Potential soil acidification – May reduce soil pH over time.
• Can lead to salt buildup – Improper use may cause salt accumulation in the soil.

Pros of Mixing Urea and Ammonium Sulfate

1. Balanced Nutrient Supply

Mixing urea and ammonium sulfate creates a more balanced fertilizer that provides both nitrogen and sulfur, two essential nutrients for plant growth. Sulfur improves nitrogen uptake efficiency, making this blend highly effective.

2. Reduced Volatilization and Improved Nitrogen Retention

One of urea’s biggest drawbacks is volatilization loss, especially in warm conditions. Ammonium sulfate stabilizes urea, reducing nitrogen loss by preventing excessive ammonia gas escape.

3. Improved Soil pH Management

For growers with alkaline soil, adding ammonium sulfate helps lower soil pH, which improves nutrient availability. Urea alone can make soil pH fluctuate, while ammonium sulfate buffers pH changes more effectively.

4. Versatility for Different Crops

This fertilizer mix is beneficial for various plants, including cereals, vegetables, turfgrass, and fruit trees. Crops requiring both nitrogen and sulfur, like onions, canola, and legumes, benefit greatly from this combination.

5. Cost-Effective Fertilization

Blending urea with ammonium sulfate lowers overall fertilizer costs while still providing a rich nitrogen source. Less urea is needed per application, making it a more economical choice for large-scale farming.

Cons of Mixing Urea and Ammonium Sulfate

1. Potential for Over-Acidification

Since both fertilizers can reduce soil pH, continuous use may lead to excessively acidic soils, which can negatively impact plant health. Regular soil testing is necessary to monitor pH levels.

2. Higher Salt Accumulation Risk

Both urea and ammonium sulfate contribute to salt buildup in the soil. In poorly drained areas or in arid climates, this can harm root development and decrease water absorption.

3. Different Solubility Rates

Urea and ammonium sulfate have different solubility rates, which can cause uneven nutrient distribution if not mixed properly. Pre-dissolving the mixture in water before application can help mitigate this issue.

4. Risk of Nitrogen Leaching in Sandy Soils

In light-textured (sandy) soils, nitrogen can leach beyond the root zone when mixed fertilizers are applied improperly. Split applications and proper irrigation can help minimize this loss.

Best Practices for Mixing and Applying Urea and Ammonium Sulfate

1. Use the Correct Mixing Ratio

The ideal mix ratio depends on your soil’s nitrogen and sulfur needs. A common blend is 75% urea and 25% ammonium sulfate, but this can be adjusted based on soil test results.

2. Incorporate into the Soil

To prevent volatilization, incorporate the mixture into the soil using light tilling or irrigation immediately after application.

3. Apply at the Right Growth Stage

• Early spring – Supports initial vegetative growth.
• Mid-season – Enhances plant development and flowering.
• Late-season (if needed) – Maintains steady nitrogen supply for extended growth.

4. Use Split Applications

Applying all nitrogen at once can lead to leaching and inefficiency. Instead, split the application into two or three doses throughout the growing season.

5. Monitor Soil pH and Adjust as Needed

If soil pH drops below 6.0, apply lime or gypsum to prevent excessive acidification. Regular testing helps maintain optimal conditions.

Mixing urea and ammonium sulfate offers a cost-effective, balanced nitrogen and sulfur fertilizer solution that enhances plant growth and soil fertility. The combination provides quick nitrogen availability, improves sulfur uptake, and reduces nitrogen loss while being adaptable for different crops and soil types.

However, it is crucial to manage soil pH, prevent over-fertilization, and apply the mix properly to maximize benefits. By following best practices such as incorporating into soil, splitting applications, and monitoring soil conditions, you can harness the full potential of urea and ammonium sulfate for healthier plants and higher yields.