Because soil-applied fertilizer is intended for root absorption by plants, growers should manage fertilizer nitrogen (N) to keep as much of it as possible in the root zone to maximize crop N uptake and yield, as well as protect the environment. To keep urea fertilizer N in the root zone, incorporate urea into the soil with water or cultivation within a day or two of application, and don’t over-irrigate when incorporating urea using water.
Inject liquid fertilizers containing urea (for example, UAN32) into irrigation systems in the middle third of the irrigation set. This delivers urea N evenly through the root zone, avoiding leaching that can occur when urea is injected too early in the set and limited root zone distribution when injected too late in the set.
Urea is the most commonly used dry nitrogen (N) fertilizer in the U.S. It provides half of the nitrogen in UAN (Urea Ammonium Nitrate) 28 or 32 liquid fertilizers. Dry and liquid fertilizers that contain urea have several advantages — relatively high N content (28% to 46% N), ease of handling, and reasonable price relative to other N sources.
However, nitrogen from applied urea can be lost from the root zone when used improperly, wasting money, reducing plant available N, and risking reduced crop growth and yield. The lost N can also be an environmental contaminant. Growers and pest control advisers should be aware of how to avoid N losses and get the most from urea fertilizer dollars.
Two Ways To Lose
Within days of application, urea N can be lost from the crop root zone in two ways — through ammonia volatilization or urea leaching. The uncharged urea molecule (H2N-CO-NH2) breaks down in or on the soil into two ammonium molecules (NH4+) and a bicarbonate molecule (HCO3) within days of application. Urease, a naturally occurring enzyme in soil and on plant surfaces, drives this reaction. Ammonium produced by urea breakdown (urea hydrolysis) has many potential fates. It can shift form to ammonia (NH3; a gas), a process accelerated by high temperatures (more than 70°F) and high pH. It can be held by the cation exchange capacity of clay or organic matter, absorbed by soil microorganisms or plants, or changed into nitrate (NO3) by certain soil bacteria (nitrification). Where urea transformation occurs has a major impact on whether the N applied actually enters and stays in the root zone.
Ammonia Volatilization: Urea fertilizer — dry or liquid — applied to the soil surface and left there for days to weeks can lose more than 50% of N content into the air through ammonia volatilization. High soil pH, high soil temps (more than 70°F), sandy soils with low cation exchange capacity (CEC), weeds or turf, and moist soils/heavy dew are all factors that increase the ammonia losses from unincorporated urea. Incorporate urea into the soil within a day or two of application to avoid significant N loss.
Urea Leaching: Dissolved urea moves with water. Why? Urea hydrolysis takes several days to complete. Until hydrolysis occurs, the uncharged urea molecule won’t bind to soil particles. This helps with water incorporation, but can result in leaching of urea below the root zone during irrigation if excess water is applied. The most efficient use of urea fertilizer requires good irrigation management. Don’t over-irrigate when incorporating surface applications or injecting urea-containing fertilizers through irrigation systems.
“The Middle Third” Rule
When injecting urea fertilizer in a micro-irrigations system, a good general rule is to add the fertilizer in the middle third of an efficient irrigation set, a set that won’t push water down below the bottom of the root zone.
For example, in a 12-hour irrigation set, add the urea in hours 4 to 8. This reduces the chances of pushing urea below the root zone or at least deeper in the root zone where there are fewer roots.
On the other end of the spectrum, urea-containing fertilizer added late in the set is concentrated near the water source and not evenly distributed in the root zone.
How you use urea can go a long way to helping you get the most out of your fertilizer dollar.