What Is Urea Fertilizer?

When gardens need lots of nitrogen fast, urea fertilizer provides it. At 46 percent nitrogen, urea packs more punch per pound than other nitrogen-only fertilizers. Because nitrogen moves easily through soil -- and plants need it in larger quantities than any other nutrient -- it regularly requires replacement, especially in gardens supporting vegetables or other crops. Urea provides ready nitrogen, but unless handled properly, much of the nitrogen can be lost.

A manufacturing process creates urea fertilizer by combining ammonia gas, known as anhydrous ammonia, with carbon dioxide. High temperatures and pressure aid the chemical reaction that ultimately produces urea. The product is synthetic, but the process imitates the cycle that naturally occurs when mammals metabolize nitrogen in their bodies. The natural process creates ammonia that the liver converts to nitrogen-carrying urea, which the body then excretes. By using similar chemical reactions, manufacturers create the 46-0-0 product that is urea fertilizer.

Most gardeners apply urea fertilizer directly to the soil in small pellets, known as prills. The fertilizer dissolves quickly in water, and the slightest soil moisture begins a reaction that converts urea into a compound called ammonium bicarbonate within a few days. It happens fastest at high temperatures or high pH and in sandy soil or soil with low organic matter. In a nod to urea's beginnings, the ammonium converts back to ammonia gas. Unless the gas is conserved, urea can lose 50 percent to 90 percent of its nitrogen to the air.

If urea's conversion to ammonium bicarbonate happens in soil instead of air, the ammonia gas and its nitrogen stay in the soil, where plants can use it. To conserve nitrogen and your fertilizer investment, get urea down into the soil as soon as it's applied. The easiest way is to let water do the job. Whether by rainfall or irrigation, as little as 1/4 to 1/2 inch of water moves urea into soil far enough to protect the ammonia gas from air. Lightly tilling urea into your soil has the same effect.

With chemical changes under way, soil pH directly around urea rises quickly along with soil ammonia. Until a few days pass, the zone is toxic to germinating seeds and seedlings. Wait to seed, or keep a 2-inch buffer between urea and your seeds. Corn (Zea mays) is particularly sensitive. Though urea pushes pH high at first, it lowers soil pH over time. Like all ammonium-based nitrogen fertilizers, acidity can accumulate in topsoil, especially in no-till gardens. Soil tests keep pH on track. Lightweight, urea catches easily in the wind. Wear gloves, protective clothing and safety eyewear when working with urea fertilizer.