Arkansas Rice: Using Urease Inhibitors

Preflood application of urea+NBPT to rice. Photo: University of Arkansas

This is the time of year we start applying urea fertilizer to the majority of our crops and also a time when the weather doesn’t seem to cooperate nearly as much as we would hope. Urea is a great nitrogen (N) fertilizer source, especially for rice, due to its high N analysis and granular form that aids in both ground and aerial application.

Unfortunately, there is no PERFECT N fertilizer source and for all the great qualities urea possesses it has one fatal flaw – ammonia volatilization loss potential. Urea is technically an “organic” compound as it contains carbon, oxygen, nitrogen, and hydrogen (that’s organic in the chemistry sense – not the farming classification sense).

Urea must be dissolved and then hydrolyzed or converted to ammonium before the plant can take it up. The process of urea hydrolysis (conversion from urea to ammonium) is catalyzed by an enzyme known as urease – which is basically everywhere.

A few key things to understand about ammonia volatilization loss from urea:

  • volatilization is a surface loss mechanism – urea that has been incorporated with tillage, rainfall, or irrigation is not prone to N losses via volatilization;
  • the higher the soil pH the more ammonia loss potential;
  • soils with lower CEC contents (sands and silt loams) are more likely to experience significant ammonia loss via surface applied urea than heavier textured soils such as clay loams and clays;
  • urea hydrolysis and ammonia volatilization losses do take time to occur – the loss is not immediate.

For most soils and environmental conditions, it requires 2-3 days before we see appreciable or measurable N loss via ammonia volatilization.

A quality urease inhibitor that contains NBPT is worth its weight in gold when it comes to mitigating ammonia volatilization losses from urea applied preflood. If you are on clay soils or require 3 days or less to flood then you probably will see no benefit from a urease inhibitor.

If you are on a silt loam soil and conditions are right you can lose as much as 50% of your applied urea-N in as little as 7 days.

Notes to remember when using ammonium sulfate (AMS) – ammonia volatilization losses are seldom an issue when using AMS and there is no need for a urease inhibitor; urease inhibitors are only for urea. If you are blending urea and AMS have the Co-op treat the urea with a urease inhibitor separately before blending.

Do not let them blend the products and then charge you for the cost of treating both the AMS and the urea. Another question that has been asked concerns using a urease inhibitor in standing water.

DO NOT USE A UREASE INHIBIOTR IF YOU ARE APPLYING UREA INTO STANDING WATER! You will get no benefit from a urease inhibitor for either early N or midseason N applied if the permanent flood has already been established.

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If the field is muddy and there are puddles here and there AND you intend to let the field dry before you establish the permanent flood then yes you do need to use a urease inhibitor.

The University of Arkansas System Division of Agriculture does a significant amount of laboratory and field testing to validate the quality of urease inhibitors and the two things that should be considered are active ingredient and concentration – similar to how you would select and use herbicides.

The most consistent and reliable urease inhibitor is NBPT (N-(n-butyl) thiophosphoric triamide), but more recently NPPT has shown to have similar benefits in reducing ammonia volatilization losses from urea.

For more information on Nitrogen Fertilizer Additives such as urease inhibitors please see fact sheet FSA-2169. Table 2  provides information on the products tested and found to be effective at reducing ammonia volatilization losses from urea.

 

 

 

Product Name

  Recommended Volume

qt per ton urea

  NBPT Concentration %
Agrotain Advanced 2.0 30.0
Agrotain Ultra                           3.0                             26.7
ANVOL⁋ 1.5 16
Arborite AG-NT 3.0                             24.0
ContaiN                                     4.0                        unknown‡
Factor                                        3.25                            24.5
Limus   3.0   16.88#
N-Fixx PF                              3.0 4.0                   unknown‡
Nitrain 3.0   26.7
  Nitrain Express                         3.0                             24.8
N-Veil     3.0 4.0   26.7
PinnitMax                                  1.5                             50.0

‡ Unknown, product label does not specify concentration of NBPT.
⁋ ANVOL contains 16% NBPT and 27% duromide which has also been shown to reduce ammonia volatilization loss.
# Limus contains 16.88% NBPT and 5.63% NPPT, which is a proprietary inhibitor owned by BASF.

Currently, there are many urease inhibitors on the market and oftentimes there are too many for us to test. Just because a product is not in our current list does not mean that it cannot be used effectively. There are some rules of thumb to keep in mind.

First off, the product should contain NBPT or another published urease inhibitor. Secondly, the concentration of NBPT will help determine the application rate per ton of urea. For products that contain <20% NBPT you need to use 4 qt/ton urea. For products that contain ~26-30% NBPT you need 3 qt/ton urea.

There are some concentrated formulations that you can apply a lower rates such as 2 qt/ton urea. It’s always best to read and follow labels to ensure that you are getting what you pay for.

There has been some chatter and speculation recently that urease inhibitors “tie up” the N in a way that might delay uptake and slow plant growth because the N cannot be taken up. There are no grounds for this.

The only way that the N can be taken up by the plant is after is has been incorporated into the rootzone which is typically done using rainfall or irrigation.

Typically, as urea is incorporated with water there is a dilution and separation from the urease inhibitor which allows the urea to hydrolyze to ammonium and become plant available. Once the urea is incorporated below the soil surface we do not need to worry about ammonia volatilization losses.

There have been countless field trials with numerous urease inhibitors which have shown the rice total N uptake and yield when using an effective urease inhibitor are equal to or greater than untreated urea. Also, common sense always comes in handy.

An effective urease inhibitor is a great investment if the conditions are present for significant volatilization loss from surface applied urea. Costs for effective urease inhibitors can run from $5-15/acre depending on the urease inhibitor selected and the rate of N being applied.

Rarely do we find a product that works as consistently as an effective urease inhibitor and more often than not if conditions are prone to volatilization losses it will more than pay for itself. Familiarize yourself with when and where urease inhibitors should be used effectively and if all else fails give us a call and let us help!




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