1. Nitrogen sources for Topdressing Winter Wheat In Oklahoma
Using Variable Nitrogen Fertilizers

2. Why are we worried
about managing Nitrogen inputs?
There not worried about Nitrogen!

3. The possible consequences of mismanaging Nitrogen at today's volatile prices!

4. What forms of N are we using
What are the safest as far as N loss
What forms are most economical
All these are factors in the decision making process for good reasons
Urea
UAN 28% /32%
Ammonium nitrate

5. Urea
It is no surprise that Urea is widely used here either because
Available everywhere
Easy to handle
Not nearly as dangerous as AA
Urea is the most widely used solid fertilizer N source in the world; 23,610,991 Mt used for cereal production in 1998 (
How is N from Urea lost?
Volatilization losses from surface applications of urea-based fertilizers are thought to occur in sizeable quantities (Ernst and Massey, 1960; Fenn and Hossner, 1985).

6. Protecting Urea Investments
Urea fertilizers must be hydrolyzed by the enzyme urease before urea-N becomes plant available.
This of course takes time
Increases in urease activity can result in more N loss due to volatilization.
Urease activity is moisture and temperature dependent.

7. Conditions that favor N loss
Wind
Increases evaporation
Soil pH exceeding 7.0
Soil Temp exceeding 50F
For Urea, sandy soils with low organic matter will likely favor N loss
Volk (1966) stated that after applying urea not incorporated, 14 days later 80% of N applied was gone.

8. Avoiding loss of N
For urea, apply before an expecting rainfall event. (½ in rain is adequate.
Try to apply on a calm and cool day. (Unlikely in western Oklahoma)
Do not apply Urea on a soil that is damp.
Even a slight dew in the morning’s can cause significant volatilization.
Rainfall or irrigation can result in subsurface incorporation thus minimizing volatilization losses.
Significant precipitation (>.25 cm) 3 to 6 d after application can efficiently reduce volatilization losses (Havlin et al., 1999).

9. Higher risk conditions
Lower risk conditions
High soil temperature (>70oF)
Low soil temperature (<50oF)
Moist soil or heavy dew
Dry soil
High soil pH (>7.0)
Low soil pH (<6.0)
Low cation exchange
Capacity soil (sandy)
High cation exchange capacity
soil (silt or clay-dominated)
Crop residue, perennial forage,
or ash layer from recent burn
event
Urease Inhibitor

10. Volatilization

11. UAN (28, 32-0-0) Advantages Generally available fertilizer
Not as susceptible to N loss as Urea
Can be used as a carrier for herbicides
Contains both Nitrate and Urea, spreads out risk
Disadvantages
Can cause leaf burn
Generally more expensive than Urea or Anhydrous Ammonia
UAN, is still 50% urea so can be somewhat susceptible to volatilization

12. Avoiding Burn do to Fertilization
Avoid application during high wind speed to reduce burn and volatilization
Avoid application on a warm day. (A guy who applied a heavy dose on a 75 degree day lost that whole field)
Use streamer nozzles for improved soil contact and penetration
Avoid application when temps exceed 60F
Many times even a moderate burn from UAN has no detrimental yield hit

13. Urease Inhibitors
Table 1. Mean yields for urease (NBPT*) inhibitor trials conducted in 1989 at the Purdue Agronomy Farm, SEPAC, Pinney Purdue, and the Kosciusko location.
Fertilizer Treatment
Yield (bu/acre)
Control (20 lb N/acre in starter only) 99
Urea broadcast on the surface
130
Urea + NBPT broadcast on the surface
143
UAN broadcast on the surface
135
UAN + NBPT broadcast on the surface
140
UAN dribbled on the surface
139
UAN point (spoke) injected
142
UAN coulter injected
147
UAN knife injected
145
* Urease inhibitor N (n-butyl) thiophosphoric triamide
Source: Phillips, Mengel, and Walker, Purdue University

14. References & Questions
Nitrogen Loss from Urea Fertilizer Applications on Hot, Dry, Soils, R.W. Mullen, G.V. Johnson, K.W.Freeman,K.J. Wynn,
W.E. Thomason, and W.R. Raun
Management of Urea Fertilizers, Montans State University Extension Publication, Paul V. Dixon,
Soil and Fertilizer N, Chapter 5 Lecture Soil Nutrient Management, W.R. Raun