If you've wondered why farmers apply fertilizer and still get disappointing yields, the answer is often in the soil. Specifically, in what happens to urea in the hours, days, and weeks after it's spread.
Urea is the world's most widely used nitrogen fertilizer. In Bangladesh, it covers the bulk of nitrogen inputs for rice, wheat, and vegetable crops. It's cheap, concentrated, and easy to handle. It's also surprisingly easy to lose.
Companion post: This article explains the problem — the three nitrogen loss pathways. For solutions, see What is Slow-Release Fertilizer and Why Does Bangladesh Need It?
Figure 1. Nitrogen loss pathways from applied urea. Three pathways compete with plant uptake: volatilisation (red, upward), leaching (blue, downward), and denitrification (purple dashed, rising from the anaerobic zone). Chemical reactions at each transformation step.
What urea is
Urea is a synthetic compound, two nitrogen atoms bound to a carbon atom, written as CO(NH₂)₂. It contains 46% nitrogen by weight, more than any other solid nitrogen fertilizer. A 50 kg bag holds roughly 23 kg of actual nitrogen.
It dissolves quickly in water, which is part of why it's popular. Within hours of application, it starts to change form.
Step 1 Hydrolysis
The first thing that happens after urea is applied is hydrolysis. Soil bacteria produce an enzyme called urease, which converts urea into ammonium and carbon dioxide. This happens fast, often within 24 to 48 hours in warm, moist soil.
Ammonium (NH₄⁺) is positively charged. Soil particles, clay and organic matter in particular, carry a negative charge, so ammonium sticks. It's relatively stable and can wait in the soil until plants or microbes pick it up.
That stability is short-lived.
① Loss Pathway Volatilisation
As urea hydrolyzes, pH rises sharply around each granule. In alkaline conditions, ammonium converts to ammonia gas and escapes into the air. This is volatilisation, and it's the biggest single source of urea nitrogen loss.
Studies in South Asian conditions report losses of 20 to 40% of applied urea through volatilisation alone. On flooded rice paddies, the dominant cropping system in Bangladesh, losses can exceed 50% when urea is broadcast onto the water surface.
Temperature, soil pH, and application timing all determine how much escapes. Broadcasting urea in the heat of a Bangladesh summer, onto an alkaline paddy without incorporating it, is close to the worst possible combination. A significant fraction can be gone before rain even arrives.
Step 2 Nitrification
Ammonium that survives volatilisation doesn't stay as ammonium for long. Soil bacteria convert it first to nitrite and then to nitrate (NO₃⁻). This two-step process is nitrification, and it's what makes nitrogen available to most crops.
Nitrate is the form most plants prefer. It also comes with a problem. Nitrate carries a negative charge. Soil particles also carry a negative charge. So nitrate doesn't stick — it moves freely with soil water. Where the water goes, the nitrate goes.
② Loss Pathway Leaching
Rain or irrigation moving water downward through the soil profile carries nitrate with it. Once below the root zone, that nitrogen is gone. The crop can't reach it.
Leaching losses vary by soil texture, rainfall intensity, and how well-timed the application was. Sandy soils lose far more than clay soils. Heavy rain immediately after fertilizer application is a common problem in Bangladesh, particularly during the kharif season.
Leached nitrate doesn't just disappear. It enters groundwater. In areas with years of high nitrogen inputs, this shows up as elevated nitrate in tube wells, a drinking water problem that communities downstream rarely connect back to fertilizer applied upstream.
③ Loss Pathway Denitrification
Under waterlogged conditions, a different group of bacteria take over. These anaerobic bacteria use nitrate as an oxygen source, converting it stepwise into gases that escape into the atmosphere.
It's particularly significant for rice cultivation. When urea is applied to a flooded paddy, the sequence runs like this: urea hydrolyzes to ammonium, some ammonium volatilises, some nitrifies to nitrate in the thin oxygenated surface layer, and that nitrate diffuses down into the anaerobic zone where denitrification strips it out.
Nitrous oxide (N₂O) is also a potent greenhouse gas, roughly 270 times more warming than CO₂ over a 100-year period. So denitrification is both an economic problem and a climate one.
Step 3 Plant uptake
Of the nitrogen that survives all three loss pathways, crops take up ammonium and nitrate through their roots. In rice, ammonium is the preferred form because paddy conditions suppress nitrification. In upland crops like wheat and vegetables, nitrate dominates uptake.
The plant uses nitrogen to build proteins, enzymes, and chlorophyll. Without enough of it, leaves yellow (starting at the tips and working inward), growth stalls, and yields drop.
Why any of this matters for farmers
Nitrogen Use Efficiency (NUE) is the fraction of applied nitrogen that actually ends up in the crop. In Bangladeshi rice cultivation, measured NUE values typically run 30 to 50%. Half or more of every bag of urea is gone before the crop can use it.
It costs farmers money they don't get back. It puts nitrogen into rivers, groundwater, and the atmosphere. And beyond some threshold, applying more urea doesn't close the gap. Loss pathways absorb the excess before the plant does.
What farmers and researchers are doing about it
Urea deep placement (UDP) involves pushing large urea tablets, called briquettes, into the soil between rice hills at a depth of 7 to 10 cm. Placing nitrogen directly in the root zone avoids surface volatilisation and has cut losses by 30 to 40% in Bangladeshi field trials.
Split application divides the total nitrogen dose into two or three smaller applications timed to crop growth stages. Less nitrogen is present when the crop doesn't need it, which is exactly when loss pathways are most active.
Slow-release fertilizers use coatings or chemical inhibitors to slow the conversion of urea to ammonium, extending the window for plant uptake. The companion post covers how these work and which materials are being tested in Bangladesh.
Urease inhibitors and nitrification inhibitors can be blended with standard urea. Urease inhibitors slow hydrolysis; nitrification inhibitors keep nitrogen in the ammonium form longer, where it's less mobile. Neither is yet widely used in Bangladesh, but both are gaining traction in research trials.
The actual goal
Spreading urea on a field isn't a single act. It's the start of a competition between the crop and the environment. Volatilisation, leaching, and denitrification all run simultaneously, each claiming a share.
Understanding which pathway dominates under which conditions is the starting point for managing nitrogen better. The goal isn't to apply more. It's to get more of what's already being applied into the plant.