SOIL 4234   Laboratory Exercise

Ammonia Volatilization from Simulated Anhydrous Ammonia Application 

Overview:

            In Oklahoma, and much of the Central Great Plains in the US, the most popular N-fertilizer sold is anhydrous ammonia.  When N is added to the soil as ammonia (NH₃), such as when anhydrous ammonia is injected into the soil, or when N sources become NH₄⁺ near or at the surface of the soil, it is possible for some of the N to be lost as ammonia gas into the atmosphere.  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.      Evaluate the influence injection depth on N loss from anhydrous ammonia (simulated).

2.      Evaluate the influence of soil texture on ammonia loss from simulated anhydrous ammonia injections. 

Approach:

A.      Simulated anhydrous ammonia injection

Ammonium chloride (NH₄Cl) will be reacted with sodium hydroxide (NaOH) at the base of a capped clear plastic tube.  Immediately after adding 1.0 mL of each solution, a layer of pure quartz sand will be added, followed by addition of soil.  The quartz sand is used to provide a non-influencing environment (no cation exchange capacity, organic matter, etc.) and pore-space within which the two chemicals can react.  The reaction is immediate, but slow, according to the following equations: 

NH₄Cl + 2NaOH  == Na⁺ + Cl^- + NH₄⁺ + OH^- + Na⁺ + OH^-                        (1) 

Na⁺  +  NH₄⁺  +  2OH^-  === H₂O  + NH₃  (gas)                           (2) 

Excess NaOH will be added to assure volatilization of NH₃.  Loss of NH₃ depends upon available soil pores for the NH₃ to expand into without reaching the soil surface, and whether the soil contains moisture to absorb the NH₃ and form NH₄⁺ by the reverse of reaction (2). Acidic soils, and those which have a high cation exchange capacity, will better be able to keep the N in the form of NH₄⁺ on the left side of equation (2). 

Materials

1.      Soil, about 4 kg

A.  Small amounts of the soil must be pre-moistened.

2.      Clear plastic tubes:

3.      1.0 mL pipettes (2).

4.      Solutions of NH₄Cl

A. for 100 lb anhydrous ammonia N-rate:

I. 50-mL NH₄Cl solution (2.76 g NH₄Cl/50 mL)

II. 50-mL NaOH solution (4 g NaOH/50 mL).

B. For 200 lb anhydrous ammonia N-rate:

I. 50-mL NH₄Cl solution (5.51 g NH₄Cl/50 mL)

II. 50-mL NaOH solution (8 g NaOH/50 mL).

5.      1.0 M KCl solution.

6.      Pure quartz sand, about 1 kg. 

Table 1.  Simulated anhydrous ammonia injection treatments.

 *Weigh 90 g sandy soil or loam soil for each 5-cm of depth required by the treatment. 

 Procedure for simulated anhydrous ammonia injection treatments.

I. Weigh 40 grams of sand (pure quartz or other inert sand) into separate small solo cups, one for each of the treatments in Table 1 (each group is assigned a set of two treatments)

II. Weigh the appropriate amount of the designated soil (either sandy or loam) into a second set of small solo cups.

III. Select clear plastic tubes appropriate for your treatments (tubes are 4-cm longer than the indicated depth of soil) that have capped ends, and label them.

IV. Have the instructor adds 1.0 mL of either the 100 or 200 lb N/acre NH₄Cl solution to the bottom of the empty tube.

V. Have the instructor similarly add 1.0 mL of the NaOH solution, then immediately

a)      Pour the 40 g of sand over the solution in the bottom of the tube, and

b)      Pour the designated amount of soil into the tube, on top of the sand.

VI. Store the tube on a counter top.

VII. After two weeks,

a)      Transfer the soil from the tube to a 1-L plastic beaker, add enough 1M KCl solution to obtain a 2.5 to 1 ratio of solution to soil, mix well and filter. 

b)   Transfer 25 ml of the KCl extractant to a small plastic solo cup, label and give to instructor for submittal to SWFAL for analysis.

c)      Instructions on data interpretation will be provided when results of the lab analysis are completed.