Urea is the workhorse of global nitrogen fertilization. It carries more nitrogen per bag than almost anything else on the market, around 46% N, and it is inexpensive, which is why it dominates fertilizer use worldwide. The problem is that a surprising share of the nitrogen a farmer pays for never reaches the crop. Some escapes into the air, some drains below the root zone, and some is converted by soil microbes into gases that simply disappear. Understanding where that nitrogen goes is the first step to keeping more of it and more of your money in the field.
What happens to urea after it hits the soil
Urea is not the form plants actually take up. Once applied, a soil enzyme called urease rapidly converts urea into ammonium (NH₄⁺). Soil microbes then oxidise that ammonium into nitrate (NO₃⁻), the form most crops absorb most easily. Every one of these transformations is also an opportunity for loss and the losses can be substantial. Under controlled laboratory conditions, researchers have measured up to 37% of applied nitrogen disappearing from surface-applied urea through ammonia volatilisation alone (Soares, Cantarella & Menegale, 2012).
The four main ways nitrogen escapes
1. Ammonia volatilisation
When urea sits on the soil surface, especially in warm, dry, or alkaline conditions, much of its nitrogen can be lost to the atmosphere as ammonia gas (NH₃). Globally, fertilisation with mineral nitrogen urea in particular is responsible for roughly 19–20% of all ammonia emissions from agriculture (Krajewska, 2021). The factors that drive volatilisation are predictable: high temperatures, wind, surface application without incorporation, alkaline soils, and a lack of rain or irrigation to wash the urea into the soil. Field measurements vary enormously anywhere from a few percent to well over half of applied nitrogen but the pattern is consistent: the longer urea stays exposed on a warm surface, the more nitrogen is lost (UW–Madison Extension).
2. Nitrate leaching
Once urea has been converted to nitrate, that nitrate becomes highly mobile in water. Heavy rainfall or over-irrigation can carry it straight down past the roots, where the crop can no longer reach it. Leaching is most severe on sandy soils, in high-rainfall regions, and in fields that are over-watered. Beyond the lost fertiliser, leached nitrate is a recognised contaminant of groundwater.
3. Denitrification in waterlogged soils
When soils become saturated and oxygen runs low, microbes switch to using nitrate instead of oxygen and convert it to gaseous forms nitrous oxide (N₂O) and inert nitrogen gas (N₂). This process, called denitrification, both removes nitrogen from the soil and releases nitrous oxide, a potent greenhouse gas.
4. Soil acidification from heavy, unbalanced nitrogen use
Over many seasons, applying nitrogen fertilisers without balancing the rest of the nutrient picture gradually lowers soil pH. Acidic soils tie up nutrients, slow microbial activity, hinder root growth, and ultimately cap productivity. Regular soil testing is the only reliable way to catch this drift before it costs you yield.
Practical ways to improve urea efficiency
The good news is that the same processes that cause losses also point to the fix. A handful of well-established practices consistently keep more nitrogen in the crop.
Incorporate urea into the soil. Getting urea below the surface through tillage, banding, or simply timing it before rain or irrigation is the single most effective way to cut ammonia volatilisation. Once the urea moves into the soil, the ammonia it forms has nowhere to escape to.
Use a urease inhibitor or coated urea. Products such as NBPT-coated urea, and neem-coated urea, slow the conversion of urea to ammonium, buying time for the fertiliser to be washed or worked into the soil. It is worth knowing the limitation: the protective effect of urease inhibitors is strongest only in the first one to two weeks after application, so the goal is to get the urea incorporated within that window (Soares et al., 2012).
Split your applications. Rather than applying a full season's nitrogen at once, divide it across the periods when the crop is actively growing. Matching supply to demand reduces both volatilisation and leaching losses.
Test your soil. Soil testing tells you what the crop actually needs and lets you monitor pH over time, replacing guesswork with a number.
The bottom line
Urea remains one of the most effective and affordable nitrogen sources available the inefficiency lies in how it is applied, not in the product itself. Ammonia volatilisation, nitrate leaching, denitrification, and gradual acidification are all well understood, which means they are all manageable. By incorporating urea, using enhanced-efficiency formulations, splitting applications, and testing soil regularly, farmers can raise yields while cutting both costs and environmental impact.