Urea fertiliser has become more popular than its cousin ammonium nitrate, according to the University of Minnesota Extension. It contains 46 per cent nitrogen and involves almost no explosive hazard, making it easier to handle and transport. It is the most inexpensive form of nitrogen per pound, and it lends itself to a variety of applications and blending. Urea is volatile and failure to use it properly can result in your garden getting much less nitrogen than you think.
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Urea fertiliser provides nitrogen, which promotes green leafy growth that not only makes your garden look lush, but is also important for photosynthesis. The key is that urea provides only nitrogen -- no phosphorus or potassium -- so it is primarily used where the features of other fertilisers, such as bloom growth and vertical height may not be desired.
Urea is most commonly broadcast into the soil in solid form, but care must be taken to keep it at least 2 inches from seedlings to protect them from the initial alkaline reaction. Utah State University Cooperative Extension recommends turning the urea into the soil rather than just placing it on top to minimise the loss of nitrogen to the atmosphere as the fertiliser breaks down. Urea can also be dissolved in water as a 50/50 mix to use as a foliar spray. It should be applied according to the nitrogen fertiliser recommendations for the crop.
Urea is best applied in the spring, but can be applied in fall if subsequently ploughed down. Although cooler temperatures reduce volatility and nitrogen losses in the fall, spring applications generally bring better results. High soil pH and high temperatures maximise the loss of nitrogen, so the more alkaline your soil, the more beneficial it is to fertilise in either early spring or late fall. The exception is pastures, in which case the urea should be applied in the summer when there is a chance of rain.
Urea blends freely with solid mono- and diammonium phosphates, but not superphosphates. The reaction creates a damp mixture that creates problems with accurate spreading. Even if the mixture is dry, urea has a different weight than other granular fertilisers, so the mixture may throw unevenly. Liquid formulas of urea and ammonium nitrate are frequently used for better uniformity, but have different properties than a simply liquid urea mixture and should be studied carefully before use.
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- University of Minnesota Extension; "Fertilizer Urea"; Curtis J. Overdahl, et al.; 1991
- Utah State University Cooperative Extension; "Urea: A Low Cost Nitrogen Fertilizer with Special Management Requirements"; D.W. James; December 2010
- Ohio State University Extension; "Selecting Forms of Nitrogen Fertilizer"; Jay W. Johnson