Soil Nutrient Management
SOIL 4234

Soil Nutrient Management

Ammonia Volatilization from Urea Application Lab

SOIL 4234     Laboratory Exercise
Ammonia Volatilization from Urea Application 

Overview:
            In Oklahoma, and much of the Central Great Plains in the US, the second most popular N-fertilizer sold is urea.  Much of the urea is applied as solid urea in a prill or pellet form; however, increasing amounts are being used as a component to N solution fertilizer formulations such as Solution 28 and 32.  When urea is applied to soil or plant surfaces it is possible for some of the N to be lost as ammonia gas into the atmosphere.  This occurs as a result of urea conversion by urease to ammonium bicarbonate.  When the surface dries the ammonium may be converted to ammonia and lost as a gaseous product.  The degree to which this happens depends on environmental conditions, especially temperature, soil pH, wetting and drying, soil texture and wind.  This exercise examines some of these influencing factors, how much N may be lost, and how fertilizers may be managed to minimize loss. 

Objectives:
1.      To evaluate the influence of temperature, moisture, and residue on N loss from surface applied and incorporated urea. 

Approach:
       Urea reacts with water in a humid environment to produce urea in solution.  When there is sufficient water the urea solution infiltrates and moves downward in the soil just as water alone because the dissolved urea is not and ion (it is uncharged).  Urea reacts with water (is hydrolyzed) by the enzyme urease, present in all soils and plant material (dead or alive) according to the following reaction:
               NH₂-CO-NH₂ + 3 H₂O + urease ====è 2 NH₄⁺ + OH^- + HCO₃^-                  (1)

The ammonium ions (2 NH₄⁺) react with hydroxide (OH^-) to form water and ammonia gas according to the reaction: 
              NH₄⁺ + OH^- ç===è H₂O + NH₃ (gas)                                              (2)

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).

 Materials

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.

 Treatments:

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.