Volatilization of Urea Affected by Temperatures and Soil pH

Volatilization of Urea Affected by Temperatures and Soil pH

Volatilization is the loss of a gas to the atmosphere. In the case of urea, hydrolysis converts the urea to ammonia, and if the urea is not incorporated, the ammonia is lost to the air. This is the reason urea should be incorporated during or shortly after application. A rainfall of as little as 0.25 inches is sufficient to blend the urea into bare soil to a depth at which ammonia losses will not occur. Soil covered with residue needs at least 1/2 inch.
Volatilization losses of urea are highly dependent on the rate of hydrolysis which in turn is influenced by soil temperature, soil pH, moisture/humidity, and residue of coverage of the soil surface. The effect of soil temperature on the volatilization of ammonia from surface-applied urea is shown in Table 1. Volatilization losses are small (less than 6%) within a 10-day period when soil temperatures are less than 60F. Losses are greater when residue is present, since the concentration of urease is at least 100 times greater on residue than in soil. At surface soil temperatures of 75 to 90, which is very likely in May or early September, volatilization losses of 10 to 20% of the urea-N may occur within 8 to 10 days. High soil pH also stimulates volatilization losses as shown in Table 2. Ten percent of the N in urea left on the soil surface for 4 days may be lost through volatilization when the soil pH is 6.0.  At pH 7.5, volatilization losses double to 20% when urea is left on the soil surface for 4 days. Most soils in North Dakota have a pH greater than 7.5. Quick incorporation becomes a must on these soils. Plant residue remaining on the soil surface stimulates volatilization of urea also because of higher levels of urease associated with the residue. Consequently, urea should be incorporated quickly when applied to fields where conservation tillage is practiced. High humidity and/or wet surface soil conditions also lead to quicker hydrolysis and greater volatilization. Urea readily absorbs moisture from the air at relative humidity greater than 75%. When the surface soil is dry and humidity low, volatilization losses would be expected to be minimal.

Table 1. Percent of surface-added urea volatilized as ammonia as influenced by soil temperature and days urea was left on the soil surface (from Overdahl, et.al., 1987)
Temperate (F) Days	45	60	75	90
	-------------- % of added N volatilized ----------------
0	0	0	0	0
2	0	0	1	2
4	2	2	4	5
6	5	6	7	10
8	5	7	12	19
10	6	10	14	20

 

Table 2. Percent of surface-applied urea volatilized as ammonia as influenced by soil pH and days urea was left on the soil surface (from Overdahl, et.al., 1987)(surface soil temperatures > 75 F)
	Soil pH
Days	5.5	6.0	6.5	7.0	7.5
	-------------------------------- % of added N volatilized -------------------
0	0	0	0	0	0
2	0	0	0	01	5
4	2	5	10	18	20
6	5	7	11	23	30
8	9	12	18	30	33
10	10	13	22	40	44