Urea and Slow Release Fertilizers: Importance and Best Use Practices

What is Urea in Agriculture?

Urea, chemically known as CO(NH₂)₂, is a highly efficient nitrogen fertilizer widely used in agriculture to enhance crop productivity. With a nitrogen content of 46%, it is the most concentrated nitrogenous fertilizer available, making it an essential component in the global agricultural landscape. This article explores the importance of urea, its uses, and application methods to optimize its effectiveness in crop production.

Importance of Urea in Agriculture -What is Urea Mainly Used for?

Nitrogen is a crucial nutrient for plant growth, playing a vital role in photosynthesis, protein synthesis, and overall plant development. Urea provides a readily available and highly concentrated source of nitrogen, essential for maintaining soil fertility and supporting high crop yields. Urea can be used for a wide range of crops and soil types.

It is suitable for various application methods, including broadcasting, incorporation, and foliar spraying. Urea is commonly broadcast over the field and then incorporated into the soil either with tillage or the application of water (irrigation or rainfalls). Incorporating into the soil reduces nitrogen losses through volatilization, where nitrogen evaporates into the atmosphere as ammonia gas. This method ensures that nitrogen is available to plants throughout their growth cycle.

At a high local concentration, after a band of urea fertilizer is applied, it may take 4–5 weeks for the nitrite to be completely oxidized to nitrate and become a safe N source for plants. Urea should not be applied very close to plant seedlings and seeds because it can be toxic (due to the free ammonia that has been formed).

Best Practices for Urea Use

To maximize the benefits of urea while minimizing environmental impacts, the following best practices are recommended:

1. Soil Testing: Conduct soil tests to determine nitrogen needs and apply urea accordingly. This helps avoid over-application and reduces the risk of environmental pollution.
2. Precision Agriculture: Utilize precision agriculture techniques to apply urea accurately and efficiently. This includes GPS-guided equipment and variable rate technology to ensure even distribution.
3. Inhibitors and Stabilizers: Use urease inhibitors and nitrogen stabilizers to slow down the conversion of urea to ammonia, reducing nitrogen losses and increasing availability to plants.
4. Integrated Nutrient Management: Combine urea application with other nutrient management practices, such as crop rotation and organic matter addition, to enhance soil fertility and crop productivity.

What are Slow-release Fertilizers, and How Do They Work?

While urea is an efficient nitrogen source, its quick-releasing nature has important downsides. Slow-release fertilizers are an innovation to reduce nitrogen losses due to leaching, runoff, and evaporation and offer a constant long-term supply of N to the crop. Polymer-coated controlled-release fertilizers (CRFs) have received considerable attention.  They refer mainly to mobile elements such as nitrogen, potassium, and boron.

This is the newest and most sophisticated method in plant production. More specifically, methylene urea (MU) is a slow-release fertilizer widely used in the sector. The role of more sustainable fertilization management and improved nutrient use efficiency is pivotal.

A further reason for considering a fertilizer to be slow-release is that it must meet certain conditions:

• under certain conditions and at a temperature of 25 degrees Celsius, it must not release 15 % of its elements within 24 hours
• under certain conditions and at a temperature of 25 degrees Celsius, it must not release 75 % of its elements within 28 days
• under specified conditions and at a temperature of 25 degrees Celsius, release 75% of its components when specified by the manufacturer

Slow-release fertilizers are divided into inorganic and organic. The only thing that distinguishes them is the conditions that affect their breakdown. Inorganic slow-release fertilizers slow down or speed up the release of their elements depending on temperature, pH, and soil moisture. Factors that can be more easily predicted and improved than the action of micro-organisms, an additional reason that influences the breakdown of slow-release organic fertilizers.

Types of Slow-Release Urea Fertilizers

1. Coated Urea: Urea granules are coated with materials like sulfur, polymers, or resins that control the release rate of nitrogen. Examples include sulfur-coated urea (SCU) and polymer-coated urea (PCU).
2. Urea-Formaldehyde: This is a synthetic compound created by reacting urea with formaldehyde. It breaks down slowly in the soil, gradually releasing nitrogen.
3. Nitrification Inhibitors: These chemicals are added to urea to slow the conversion of ammonium to nitrate, thereby extending the availability of nitrogen. Examples include dicyandiamide (DCD) and nitrapyrin.
4. Urease Inhibitors: These inhibitors slow the hydrolysis of urea, reducing the loss of ammonia to the atmosphere. An example is N-(n-butyl) thiophosphoric triamide (NBPT).

Advantages and Disadvantages of Slow-release Fertilizers.

Advantages of Slow-Release Urea Fertilizers:

• As we mentioned above, the biggest advantage of slow-release fertilizers is the higher nitrogen use efficiency and the reduction of the losses of the nutrient (economical loss, pollution, etc). It is a more environmentally friendly solution.
• The controlled release reduces the risk of fertilizer burn, which can occur when high concentrations of nitrogen are applied all at once.
• Suitable for sowing, as they do not overload the seed with salts, as is the case with conventional urea fertilizers.
• They offer multiple benefits to plants, enhancing root development and plant growth and providing a more steady nitrogen supply for longer periods of the growing season.
• The controlled release of nitrogen reduces the need for multiple fertilizer applications, saving time, labor, and costs for the farmers.
• Suitable for all types of plants, crops and soils.
• Suitable for basic fertilization since even a larger amount applied based on studies increases production by 20-30% and reduces subsequent surface fertilization by 20-30%.

Disadvantages of Slow-Release Urea Fertilizers

1. Slow-release urea fertilizers are generally more expensive than conventional urea due to the added materials and technology used to control nutrient release.
2. In some cases, the slow release may not meet the immediate nitrogen demands of fast-growing crops or during critical growth stages, potentially limiting plant growth and yield.
3. Some slow-release urea fertilizers may not provide a balanced supply of all essential nutrients that plants require, necessitating additional fertilizers.
4. Slow-release urea fertilizers may require specialized equipment or techniques to ensure even distribution and proper incorporation into the soil.
5. If these elements are coated with polymers, they leave plastic residues in the soil, which is negative for the environment. However, the scientific community is studying the use of biodegradable polymeric coatings.
6. Because the release of these fertilizer elements is tested in a laboratory and not under real field conditions, the amount of fertilizer indicated on the packaging as dissolving in a certain time may differ or not dissolve at all in the soil. Factors such as soil temperature, moisture, and microbial activity can affect the release rate of nitrogen from slow-release urea fertilizers, leading to variability in nutrient availability.

References:

• Farmacon, “5 things to know before using urea fertilizers”, Farmacon team, Athens 09/11/2017 https://blog.farmacon.gr/katigories/texniki-arthrografia/threpsi-lipansi/item/1780-5-pragmata-pou-prepei-na-gnorizete-otan-xrisimopoieite-lipasmata-ouria
• Farmacon, “Slow release nitrogen fertilizers”, Farmacon team, Athens 19/10/2023 https://blog.farmacon.gr/katigories/texniki-arthrografia/threpsi-lipansi/item/3469-ta-azotoyxa-lipasmata-vradeias-apeleftherosis
• Utah State University, “Urea: A low-cost nitrogen fertilizer with special management requirements”, D.W. James, 08/07/2024 https://extension.usu.edu/yardandgarden/research/urea-a-low-cost-nitrogen-fertilizer-with-special-management-requirements
• The University of Peloponnese, “Application of Slow Release Fertilizers and Chemical Fertilizers in Agricultural Crops”, Bachelor Thesis Liokalos Emmanuel, Kalamata 2022
• https://www.sciencedirect.com/science/article/pii/B9780444529756500046
• https://extension.umn.edu/nitrogen/fertilizer-urea#placing-urea-756361
• https://www.researchgate.net/publication/346313534_Characteristics_and_types_of_slow-_and_controlled-release_fertilizers
• https://www.sciencedirect.com/science/article/abs/pii/S1537511013000639

Further reading

Sustainable Plant Nutrient Management

Sustainable Plant Nutrient Management (SPNM): An overview

When are approaches to Plant Nutrient Management actually Sustainable?

Sustainable Nutrient Management: Introduction to Concept, Strategies, and Principles

Nutrient Mobility in Soil and Plants

Mineral fertilizers (including ash) and sustainability