Nitrogen is a critical macronutrient for plant growth and development. Nitrogen fertilizers used in agriculture contain nitrogen in one or more of the following forms: nitrate, ammonia, ammonium and urea. Each nitrogen form has unique characteristics and is suited to different conditions. Optimizing nitrogen use requires matching the appropriate form to specific soil, crop and environmental factors.
Nitrate Nitrogen
Nitrate (NO3-) is water soluble and moves freely with soil water. Heavy rains can leach nitrates down through the soil profile into drainage channels, representing a major loss pathway in sandy soils. This process is called leaching.
During drought, nitrates can accumulate near the soil surface as water evaporates. However, once leached below the root zone, upward movement of substantial nitrate is unlikely, rendering it unavailable to crops.
In waterlogged soils, denitrification occurs when soil organisms strip oxygen from nitrates, releasing nitrogen gas into the air. This microbial process causes significant nitrogen losses in fine clay soils.
Ammonia and Ammonium Nitrogen
Anhydrous ammonia gas (NH3) can be compressed into a liquid, like in ammonia fertilizers. When applied, it reacts with soil moisture to form ammonium (NH4+). Liquid ammonia is prone to volatilization loss and must be incorporated into soil.
Despite solubility, ammonium attaches to clay particles and organic matter, reducing leaching. Soil bacteria then convert ammonium into nitrate, the primary plant-available form. Optimal conditions for this nitrification process include a pH of 7, humidity is at 50% of the soil water holding capacity, and soil temperature around 80°F.
Urea Nitrogen
Urea (CO(NH2)2) undergoes a three-step transformation before crop utilization. First, enzymes in soil or crop residues convert urea into ammonia gas. Next, the ammonia quickly reacts with soil moisture to form ammonium. And finally, through the activity of soil microorganisms, ammonium is converted into nitrate.
Like nitrate, urea is water-soluble and can be lost through leaching if not rapidly converted to ammonia and then ammonium in soil. This urea-to-ammonia conversion typically takes 2-4 days under warm, moist soil conditions ideal for crop growth. Cooler temperatures may slow but not stop this transformation. As a result, significant urea leaching losses are uncommon in field conditions.
Some ammonia volatilization can occur when urea is surface-applied, depending on the soil environment. Losses maximize at higher pH above 7, hotter temperatures, and lower moisture. However, when urea is incorporated into the soil, the quick ammonium formation minimizes volatilization loss while also preventing leaching.
By understanding the science behind urea conversion kinetics, growers can better manage urea applications and minimize nitrogen losses through leaching or volatilization in varied field conditions.
Matching Nitrogen Fertilizers to Conditions
At Funo, we are dedicated plant nutrition experts. For over 5 years, our agronomists have helped growers optimize nitrogen fertility programs using research-backed best practices and site-specific recommendations. Our consultants know selecting the ideal nitrogen form depends on the crop, soil properties, irrigation water, and seasonal conditions on your farm. We often advise pairing affordable ammonium or urea with higher nitrate percentages to balance economics and plant nutrition needs.
Excess ammonium can increase risks of toxicity and pH fluctuations during cooler periods. In our experience, exceeding 30% ammonium during colder seasons can stress plants. We advise working closely with our team or local experts to determine the ideal nitrogen percentages and forms based on your crops, conditions and water type. There’s no one-size-fits-all fertilizer recipe!
Optimizing nitrogen use efficiency requires integrating the right source, rate, timing and placement for each unique production scenario. Understanding the behavior of different nitrogen fertilizer forms allows growers to make informed choices.