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

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

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

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 46-0-0 UAN 28% /32% Ammonium nitrate 33-0-0

Urea 46-0-0 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 (www.FAO.org). 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).

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.

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.

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).

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

Volatilization

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

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

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

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