What is Granular Urea?
Granular urea is a solid, nitrogen-rich organic compound that is commonly used as a fertilizer in agriculture. With a chemical formula of CH4N2O, granular urea contains 46% nitrogen by weight and makes an excellent source of nitrogen for plant growth and development. Unlike other nitrogen fertilizers like ammonium nitrate or calcium ammonium nitrate, granular urea dissolves slowly and provides nitrogen to soil and plants over an extended period of time.
Physical and Chemical Properties
Granular Urea is produced as small prills or granules that range from 2-4mm in size. The shape and size of urea granules allows for uniform application with conventional fertilizer spreaders. Chemically, granular urea has no electrical charge and is highly soluble in water. When urea dissolves in soil, it hydrolyzes to form ammonium bicarbonate which converts to ammonium and carbonate ions. The ammonium is then nitrified to nitrate which plants can readily absorb through their roots.
Role of Nitrogen in Plant Growth
Nitrogen is an essential macronutrient required by plants in large quantities. It is a key component of amino acids, proteins, enzymes, chlorophyll, and genetic material in plants. Adequate supply of nitrogen helps promote healthy foliage growth and deep green coloration of leaves. Nitrogen deficiency results in stunted and yellowed growth. Application of granular urea fertilizer replenishes soil nitrogen levels to meet the high nitrogen demands of crops during vegetative and reproductive growth stages.
Factors Affecting Urea Hydrolysis
Several soil chemical and physical properties influence the rate of urea hydrolysis in soil. Higher soil moisture and temperature accelerate the conversion of urea to plant-available ammonium. Soils with higher pH (less acidic) and organic matter content also hydrolyze urea faster. Compaction and poor drainage hamper urea movement into soil profile and slow down hydrolysis. Hydrolysis is faster on soil surface versus below due to higher moisture and microbial activity in topsoil layers.
Advantages of Using Granular Urea
Some key advantages of applying granular urea fertilizer include:
Slow Release of Nitrogen
The gradual dissolution and hydrolysis of urea prills in soil provides a steady supply of nitrogen to plants over 4-6 weeks. This reduces nitrogen losses through leaching and volatilization compared to rapidly soluble nitrogen sources.
Higher Nitrogen Content
With 46% nitrogen, granular urea has a higher nitrogen concentration than other solid nitrogen fertilizers. This makes transportation and application more economical on a nitrogen basis.
Compatibility
Granular urea is physically and chemically compatible with most pesticides and fertilizers. It can be blended with phosphorus, potassium and other fertilizer blends for application convenience.
Cost Effectiveness
Urea is widely available and relatively inexpensive to produce. On a dollar per pound of nitrogen basis, urea offers good agricultural value compared to alternatives.
Handling and Storage
As granular prills, urea has good handling characteristics during transportation, storage and application. Care needs to be taken to prevent moisture absorption which can lead to caking of urea masses over time. Ventilation and low moisture storage helps maintain flowability.
Potential Issues with Granular Urea Use
While granular urea has several agronomic advantages, some potential issues to watch out for include:
Leaching Risk
In coarse-textured and sandy soils prone to excessive drainage, urea applied after heavy rains may leach below the root zone before complete hydrolysis and plant uptake.
Volatilization Losses
If left uncovered on high pH soils, ammonia fumes can volatilize from freshly applied urea granules during warm, moist conditions and be lost to the atmosphere.
Application Timing
Early pre-plant applications of urea should be delayed until soil temperature rises above 45-50°F to ensure adequate hydrolysis and availability to germinating seedlings or transplants.
Soil Physical Properties
Compacted, poorly drained, low organic matter soils may restrict downward movement and uniform distribution of urea granules, causing uneven fertilization.
High Rate Applications
Excessive urea applications in single doses can burn sensitive crops. Split applications are preferred along with proper incorporation for high nitrogen demanding crops.
Proper Management Key for Maximizing Benefits
With judicious management of application rates and timings tailored to soil and climatic conditions, granular urea offers an economic source of plant nitrogen that farmers worldwide have relied upon for decades to boost crop productivity in a sustainable way. Continued research also helps overcome weaknesses and enhances granular urea’s role as a vital global plant nutrient.