Ammonia Volatilization from Urea Application Lab
The ammonium ions (2
NH4+) react with hydroxide (OH-) to form
water and ammonia gas according to the reaction:
Loss of NH3 occurs (reaction goes to the right) when the environment dries (water is removed from the equation) and/or there is a continual supply or increase of hydroxide (basic solution is maintained).
In this exercise we will apply urea at a 200 lb N/acre rate to sandy or loam soils. Conditions of the application will include dry or moist soil, with or without plant residue, and surface or incorporated urea. After treatments have been completed a cap will be placed over the end of the tubes to contain any NH3 that may be produced. After a day of reaction time the cap will be replaced by a small coffee maker filter which will be secured over the end of the each tube with a rubber band. Immediately following, about 2 ml of universal indicator will be added to each filter paper to saturate the paper with indicator. After about 30 minutes of reaction time students will be asked to rate the relative loss of NH3 based on color of the indicator-paper (dark blue = 10, green = 7, orange = 4, pink = 1).
1. Sandy soil and loamy soil, about 2 kg of each.
2. Plant residue, such as ground wheat straw.
3. For each soil (lab section): seven 20-cm clear plastic tubes, 4.34-cm diameter with caps for both ends.
4. Balance, to separately weigh:
A. 270 g soil
B. 0.059 g of granular, reagent grade urea (weighed before lab by instructor)
C. 0.400 g ground plant material.
5. Universal indicator solution.
6. Coffee maker filter papers.
Carefully add water (32 mL for sandy soil, 40 mL for loam soil) to the tube, and then add 270 g of soil and fertilizer as indicated in the table below. Label each tube with sec. and treatment.
Table 1. Urea-soil application treatments.
Observations and Discussion:
Using the data provided in the above table and what you have learned about use of urea, complete the following questions.
1. In no-till crop production systems, explain why farmers often try to inject N-fertilizer below the accumulated surface residue instead of spreading urea on the surface.
2. If urea is surface applied to no-till cropping systems (including turf) what considerations must be made to minimize loss?
3. If urea were surface applied to a clean-tilled wheat field in Oklahoma in September, how much N do you think would be lost? Explain you answer.
4. Explain which fertilizer material you expect would result in the least volatilization loss of ammonia if urea and UAN were each surface applied to a no-till cropping system in the middle of the summer.
5. Explain why you believe these results may be higher or lower (loss) than would be experienced in the field.