Answering Questions On Urea Application

Reprint From June 2003 Ward Letter.

Ward Laboratories professionals often get questions about surface urea application and UAN solutions. Urea is a dry fertilizer that dissolves with water after application creating urease enzyme activity that is naturally present in all soils at some level. Urease enzyme converts urea to ammonia (NH3) and bicarbonate (HCO3) in the presence of water.

How Urea Affects the Soils

If there are hydrogen ions present (as in an acid soil – soil pH below 7.0) bicarbonate combines with another hydrogen ion to form water and carbon dioxide (C02) The CO2 escapes as a gas. The hydrogen ions are used forming water causing soil pH to increase. Very little ammonia (NH3) exists at pH of 7 and below. However, at higher pH, the NH3 increases especially above 8.0. If NH3 is present in the soil it will transform into NH4 and be held on the soil exchange complex. If the NH3 is held on organic residue there is a greater chance of ammonia volatilization as the crop residue dries.

Urease activity and rate of urea hydrolysis is faster in warm temperatures. So urea and UAN solutions applied during the cool part of the season have less chance of ammonia volatilization. For example, a rise in temperature from 44 F to 80 F increases the rate of conversion to ammonium four times faster. Soil moisture is also very important for conversion of urea to ammonium. Urease activity is very low in dry soil and dry residue. The better the moisture the faster the conversion.

So, what is the best method for urea application?

First, it should be noted that UAN solutions contain about 1/2 urea and 1/2 ammonium nitrate, so the liquid N solutions are only 50% urea. Dry urea, conversely, is all urea. Research shows that it is best to incorporate the N fertilizers by tillage or with 1/2 inch of rain. The second best method of application would be to apply UAN in surface strips spaced 12 to 20 inches apart depending an which crop is being fertilized. Spreading dry urea has about the same effect as UAN surface stripping. The least effective method of applying UAN is surface broadcasting. If rainfall or irrigation occurs within 3-5 days after broadcast application the effect is generally the same as incorporation. All methods of application are much better of course, than not applying nitrogen that is needed for the crop.

For conventional farming where a considerable amount of crop residue is incorporated by tillage any method of application of nitrogen fertilizer performs well. The discussion above is written for the no-till and residue management operations.

Another reason that surface application of UAN solutions may not perform quite as well is crop residue tie-up. When UAN solution is sprayed on the residue soil microbes begin using the nitrogen to break down the residues. The more time the fertilizer is on the residue, the more N fertilizer used by the microbes. Once the N fertilizer is used by the microbes it becomes part of the organic matter of the soil and then the N is released slowly over a period of years. If the N fertilizer is knifed-in or washed in, then the N fertilizer is available for the intended crop. The point is that sometimes there may not be any ammonia volatilization, but may be N immobilization.

Late Spring Nitrate Test or PSNT

Have you wondered if the manure or past legume crop is supplying enough nitrogen for the crop? A late spring nitrate test or PSNT may help supply the answer to that question. PSNT is the soil test for nitrate taken when corn is 6 to 12 inches tall or as a pre-side dress nitrate test. Iowa State University has evaluated the method of N recommendations. Take the soil sample from 0 to 12 inches deep using 15 composite soil cores from the sampling area. The area could be up to 80 acres per composite sample. Be sure to ship to Ward Laboratories for analysis ASAP. After nitrate-N analysis, estimate N fertilizer needs by subtracting the concentration of soil test nitrate from 25 ppm N. Twenty-five ppm is the critical nitrate-N concentration where no additional N is needed.

Example:

A soil nitrate test is 16 ppm nitrate-N.
25 ppm N-16 ppm N = 9 ppm N difference (Threshold) (Actual)
The difference is multiplied by 8 lb of N/A/ppm N
Therefore the N recommendation is :

9 ppm N * 8 lb of N/A/ppm N or 72 lb of N/A.