Adoption of no-till corn production presents some special fertilizer management challenges. One of the most commonly cited problems involving increased residue and nitrogen (N) fertilizers is ammonia volatilization. When urea is applied to the soil it is hydrolyzed (broken down by chemical reaction with water) to ammonium (NH4+) by the enzyme urease. If conditions are right (poor soil contact and little or no rainfall for several days after the fertilizer application), the NH4+ may be rapidly converted to ammonia (NH3) which can be volatilized (lost, lost, lost) at the soil surface.
Increased soil microbial populations often exist with a surface residue (trash?) layer. Large numbers of soil microbes posses urease. It is not surprising, therefore, that no-till conditions favor high levels of urease activity in surface soil and that broadcast applications of N as urea to no-till fields will be particularly vulnerable to loss. In Early Preplant Nitrogen Application in Corn (P&C Newsletter 3/17/96, No. 2), I provided the evidence (a 14 bu/acre yield increase) to promote N fert ilizer injection at planting over a preemergence broadcast application.
But... what if you want to or have to broadcast urea or UAN at planting? Is there no alternative to just praying for rain within two days of application? Enter the urease inhibitor! A urease inhibitor is a compound that, by one mechanism or another, interferes with urease activity and reduces urea hydrolysis in soils. But... does it work?
In 1989, Phillips, Mengel, and Walker evaluated the ability of urease inhibitor N (n-butyl) thiophosphoric triamide (NBPT, sold under the trade name Agrotain) to enhance the utilization efficiency of surface applied N (Table 1). They found that NBPT generally increased yields of broadcast treatments, especially when application was not followed by timely rainfall. The NBPT effectiveness was maximized when applied with urea but the UAN application, containing less N as urea, also responded favorably to NBP T. The results indicate that the use of a urease inhibitor such as NBPT is a viable additive to surface applied fertilizers.
|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|
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