Urea has several agricultural uses, but homeowners turn to this synthetic fertilizer when lawns and gardens lack the nitrogen needed for healthy plant growth. Unlike balanced fertilizers, which contain all three primary plant nutrients, urea is a nitrogen-only fertilizer. Plants use more nitrogen than any other nutrient, but nitrogen easily leaches away through soil. This leaves plants in need of replenishment. When soil lacks nitrogen, urea fertilizer provides what your plants need.
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When Urea Is Used
Fertilizer recommendations call for urea when plants need high nitrogen levels quickly. Fast-growing turf grasses and vegetable crops need plentiful nitrogen to stay healthy and productive. When soil tests show that phosphorus and potassium -- the other two primary plant nutrients -- are present in good supply, nitrogen may be all that's needed. Adding unnecessary nutrients can be toxic to plants and to soil. Urea fertilizer is 46 percent nitrogen and contains no phosphorus or potassium. Unlike slow-release organic fertilizers, which provide nitrogen through decomposition, urea's nitrogen is immediately available. However, unless applied properly, much of urea's benefit can be lost.
How to Apply Urea
Urea comes in pellets or granular forms to be broadcast on soil and immediately tilled or watered in. If left on the surface, urea gases dissipate into the air, taking nitrogen with them. Sixty percent or more can be lost within a day, depending on soil and weather conditions. The manufacturing process that creates urea combines gases to mimic what happens in mammals when metabolized proteins create nitrogen. When urea hits moist soil, the highly soluble product begins to convert to gas. Working urea into the soil or letting water take it lower counteracts the process. As little as 1/2 inch of water keeps urea in the soil where plants can use it.
Calculating the Right Amounts
Because nitrogen's availability in soil varies, soil tests don't check for nitrogen. Instead, they make recommendations based other nutrient levels, the crops you're growing and past fertilizer use when known. Recommendations generally call for rates of "actual nitrogen" per 1,000 square feet. As a 46-0-0 fertilizer, urea has 46 pounds of actual nitrogen in every 100 pounds of fertilizer. Divide 100 by 46 and you'll find it takes just over 2 pounds of urea to provide 1 pound of actual nitrogen. Follow soil recommendations closely. Too much nitrogen can be detrimental to plants and the environment.
The gases produced in the first few days after a urea application can harm seeds and seedlings. Use urea at least three days before you seed or transplant or wait until plants grow larger. Long-term urea use lowers soil pH, making nitrogen and other nutrients less available to plants. Test your soil pH every three to five years, and adjust your regimen according to current recommendations. Urea isn't combustible, but don't store it near ammonium nitrate fertilizers. They'll draw moisture in high humidity. When applying urea, wear gloves, protective clothing and safety eyewear. Work on a calm day, when lightweight urea won't drift on wind.