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Urea is best applied on a cool day, in weather between 32° to 60° F (0°-15.6° C), and with little to no wind.  At colder temperatures, the ground is frozen, making it difficult to incorporate the urea into the soil.  At higher temperatures, and in windy conditions, the urea is broken down faster than it can soak into the soil. Urease is the enzyme that begins the chemical reaction that turns urea into the nitrates plants need. Applying urea fertilizers before planting leads to high amounts of the urea being lost before it can benefit your plants. Using a fertilizer with a urease inhibitor can slow the chemical reaction, and helps retain urea in the soil. Urea is packaged and sold as small, solid pellets or granules. Broadcast the urea with a fertilizer spreader or sprinkle the pellets by hand evenly across your soil. For most plants, you will want to keep the urea close to the plant's roots, or close to where you will plant the seeds. Before urea is turned into the nitrates your plants need, it first becomes ammonia gas. Because gases can easily escape from the surface of the soil, applying fertilizer when the ground is wet will help to incorporate the urea into the soil before the chemical reaction begins. This way, more of the ammonia is trapped within the soil. The top half inch (1.3 cm) of soil should be wet to keep as much ammonia gas in the soil as possible.  You can water the soil yourself, apply the urea before it rains, or apply within 48 hours after the snow on your fields has completely melted. Tilling your fields or garden is a great way to incorporate the urea fertilizer into the soil before any of the ammonia gas can be lost. Harrow, drag, or hoe the field in order to incorporate the urea into the top layer of the soil. Certain varieties of potatoes can handle high levels of nitrogen, while others cannot. Be cautious and treat all potatoes the same way. Avoid giving potato plants large quantities of nitrogen with your urea fertilizer.  Urea fertilizers can be applied directly onto potato plants, or in a solution with other fertilizers, as long as the solution is made up of 30% nitrogen or less. Solutions of urea fertilizer that are more than 30% nitrogen should only be applied to fields before potatoes are planted. Urea can be applied directly onto most cereal grains, but never in temperatures above 60° F (15.6° C). When applied in warmer temperatures, the plants will give off an odor of ammonia. Only apply urea to corn indirectly, by spreading the urea onto soil at least 2 inches (5 cm) away from the corn seeds. Direct exposure to urea is toxic to the seeds, and greatly reduces the corn plant's yield.
Minimize ammonia loss by applying urea on a cool day. Use a urea fertilizer with a urease inhibitor before planting. Spread the urea evenly across the soil. Wet the soil. Till the soil to incorporate the urea. Control the amount of nitrogen you give to potato plants. Fertilize grains with urea on a mild day. Apply urea to corn seeds indirectly.