Nitrogen sources for Topdressing

Using Variable Nitrogen Fertilizers

Why are we worried about managing Nitrogen inputs?

There not worried about Nitrogen!

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

• It is no surprise that Urea is It is no surprise that Urea is widely used here either because widely used here either because •Available everywhereAvailable everywhere•Easy to handleEasy to handle•Not nearly as dangerous as AANot nearly as dangerous as AA

• Urea is the most widely used Urea is the most widely used solid fertilizer N source in the solid fertilizer N source in the world; 23,610,991 Mt used for world; 23,610,991 Mt used for cereal production in 1998 cereal production in 1998 (www.FAO.org).(www.FAO.org).

• How is N from Urea lost? How is N from Urea lost?

• VolatilizationVolatilization losses from losses from surface applications of urea-surface applications of urea-based fertilizers are thought to based fertilizers are thought to occur in sizeable quantities (Ernst occur in sizeable quantities (Ernst and Massey, 1960; Fenn and and Massey, 1960; Fenn and Hossner, 1985).Hossner, 1985).

Urea 46-0-0

Urea fertilizers must be Urea fertilizers must be hydrolyzed by the enzyme urease hydrolyzed by the enzyme urease before urea-N becomes plant before urea-N becomes plant available.available.

This of course takes timeThis of course takes time

Increases in urease activity can Increases in urease activity can result in more N loss due to result in more N loss due to volatilization.volatilization.Urease activity is moisture and Urease activity is moisture and temperature dependent.temperature dependent.

Protecting Urea Investments

•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 Volk (1966) stated that after applying urea not incorporated, 14 days later 80% urea not incorporated, 14 days later 80% of N applied was gone.of N applied was gone.

Conditions that favor N loss

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 Rainfall or irrigation can result in subsurface

incorporation thus minimizing volatilization losses.incorporation thus minimizing volatilization losses. Significant precipitation (>.25 cm) 3 to 6 d after Significant precipitation (>.25 cm) 3 to 6 d after

application can efficiently reduce volatilization application can efficiently reduce volatilization losses (Havlin et al., 1999).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 exchangeCapacity soil (sandy)

High cation exchange capacity

soil (silt or clay-dominated)

Crop residue, perennial forage,

or ash layer from recent burnevent

Urease Inhibitor

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

• 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

• 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

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