LIMITATIONS OF UREA AS A NITROGEN SOURCE

By Brad Brown, Extension Cereal Specialist, Parma R&E Center

Urea’s high N analysis and easy handling have made it the most popular dry N source for southern Idaho. Our reliance on urea in-creased with 9-11, and the loss of ammonium nitrate from the market-place.

While urea has much to recom-mend it as a dry N source, it has its limitations. There is potential for loss of ammonia N from urea applied as a top-dress to soils, particularly if sub-sequent rainfall is sufficient to dissolve urea but not enough to move it beyond the surface and into the soil. A number of factors affect ammonia loss including soil surface pH, clay content, organic matter, temperature, and soil moisture to name a few. A Montana publication, “Management of Urea Fertilizer to Minimize Volatilization” Montana State University Extension Bulletin EB173 covers many of the principles involved with ammonia volatilization from urea. The publication is available online for downloading at http://msuextension.org/publications/AgandNaturalResources/EB0173.pdf.

In addition to volatile ammonia N losses, ammonia toxicity is another limitation relative to other dry N sources. Urea placed too close to seed can both delay and reduce emergence. Urea toxicity to seed is well known when it is banded with seed. Less known are the affects of broadcast urea on crop emergence. Broadcast urea at low to moderate N rates has little effect on emerging crops. At higher rates, urea can cause significant reductions in plant populations.

A study at the Parma R & E Center was conducted for three years to compare fall broadcast applied urea with other dry fertilizers for onions planted the following spring. Urea was broadcast at rates of 100 and 200 lb urea N/A in mid November and shallowly incorporated just prior to the forming of beds. Onions were planted the following March. Significant reduction in stand resulted in two of the three years of the study. Of course yields were also reduced when stands were reduced as much as shown in the picture. Stands were affected by urea N rates as low as 100 lb urea N per acre under these conditions. Slow release urea did not always fully mitigate the effects of urea at the highest N rate, but they were safer.


The results caught even the researchers by surprise. With the urea applied four months earlier, we did not anticipate the striking effects of the high urea N rates on onion stands. Dry and nearly frozen soils in mid November precluded dry urea dissolution and conversion to ammonia. Little conversion of urea in frozen soils occurred over winter. Warming soils in March provided temperatures that allowed the enzymatic conversion of urea to ammonia. Ammonia release from urea increases soil solution ammonium-N concentrations. Under higher temperatures, microbial nitrification converts the ammonium in solution to nitrate-N which is less phytotoxic to seed. However, in cold soils, as in March, the nitrification process is slower than the conversion of urea to ammonia and ammonia concentra-tions increase to the point of affecting germinating seeds.

Shallow incorporation of urea is not uncommon prior to fall bedding. Soils in the surface two to three inches cool more rapidly than deeper soils in the fall and are likely to be dryer; both conditions can slow urea hydrolysis. Shallow incorporation also concentrates the fertilizer N in the seed planting depth. Earlier or deeper incorporation of fall applied urea would likely have had less influence on onion seed germination and emergence. However, fall applied urea N, even when it does not affect onion stand has not been as effective as spring side-dressed N for onions. A delayed onion planting may also have avoided the effects of urea on final plant populations.

As in our study, the affects of late fall shallowly in-corporated urea N may not occur every year, depending on conditions. Urea can be an effective N fertilizer for onions but high one-time broadcast N rates can be problematic.

For more information, contact Brad Brown at 208-722-6701, or bradb@uidaho.edu.