Bulletin 46

What is the most efficient nitrogen combination that is effective when soil and air temperatures are cold in the 40-50°F range? 

In late winter and early Spring when temperatures are in the 40-50°F, there are occasions when immediate green-up of turf is desired due to winter conditions, but difficult to achieve.  Nitrogen is the first primary nutrient that is utilized.  The last thing you want to do is apply a nitrogen fertilizer in the early spring, not see a response, and then reapply.  The turf greens up, but then grows too fast which negatively affects playability and increases disease.  

Turf, like many plants, can utilize nitrogen only when it is converted down to the ammonium (NH₄+) or Nitrate (NO₃-) form.  During these cold time frames, utilizing nitrogen sources that are ammonium and or nitrate will perform the fastest.  Fertilizers that contain a high percentage of nitrogen that are in these forms will be the most effective during cold weather conditions. This applies to both granular and liquid fertilizers.

How much to Apply?

Typically, only .10-.15 lbs./1000 sq.ft. of nitrogen in the 85-100% ammoniacal and nitrate form is needed.  This is a low rate, so make sure you spray liquid fertilizer or utilize 50-90 SGN granular fertilizer.  Uniform coverage at these low rates is critical.

Look at the Fertilizer label.

The fertilizer label will provide valuable insight to guide your nitrogen selection decision.  The fertilizer label will state the total amount of nitrogen that is in the product. This is listed as “Total Nitrogen (N)”.   Directly below the “Total Nitrogen (N)” statement will be a breakdown of the forms of nitrogen utilized in the fertilizer product.  Utilize ammoniacal nitrogen and nitrate nitrogen which are the most effective in the cold temperatures.  Once they are applied to the turf and receive moisture, they can be utilized at the cold temperatures.  The percent of the ammoniacal and/or nitrate nitrogen should be as close as possible to the total nitrogen.  Typically, 85-100% of the total available nitrogen.  In example A, the Total Nitrogen is 10%.   The ammoniacal nitrogen is 4.5%, and the Nitrate Nitrogen is 4.5% for a total of 9%.   9% divided by 10% is 90% of the nitrogen is in the ammoniacal and nitrate form.  90% of the available nitrogen from this product would be available at cold temperatures which makes this a great option.  Examples of ammonium and nitrate nitrogen ingredients would include:

• Ammonium nitrate
• Ammonium sulfate
• Ammonium polyphosphate
• Ammonium thiosulfate
• Calcium nitrate
• Potassium nitrate
• Magnesium nitrate
• Diammonium phosphate
• Mono-ammonium phosphate

What about Urea nitrogen? Urea is an immensely popular and diverse nitrogen source. Due to its high nitrogen analysis, excellent turf safety, high solubility it is a very desirable and effective nitrogen source.  Urea nitrogen is a quickly available nitrogen source.  However, urea nitrogen does have to go through a conversion to be utilized once it is solubilized.  Urea nitrogen will dissolve very quickly in moisture.   It dissolves into the form of ammonium carbonate.  However, an additional step is involved for urea nitrogen to be available.  The natural urease enzyme will synthesis and release the ammonium from the carbonate.  Once in the free ammonium form, the plant will utilize it.  At cold temperatures, the urease enzyme works much slower which in turn can delay the release of urea nitrogen until temperatures increase. (Graph B) How much slower? Generally, 2-7 days slower when temperatures are in the 35-50°F range.   So, in this particular cold weather situation, utilizing a non-urea nitrogen is preferred.  Important: urea nitrogen will become available as temperatures warm up; the response is just delayed during the temperatures less than 50°F.

When to switch to urea and slow-release nitrogen products?  Once you start mowing and are removing new turf leaf tissue, introduce urea and stabilized, organic and slow-release nitrogen sources.

What about micronutrients for fast color?

Two micronutrients that provide excellent color are iron and manganese.   They act as an enzyme that enhance chlorophyll.  Low rates of iron can help, but make sure to include manganese to produce a vibrant, natural green turf blade.  In general, cool-season turf prefers a 3-1 ratio of iron to manganese for a natural, deep vibrant green turf blade. 

Research has also shown that ammonium nitrogen enhances the leaf uptake of manganese.  Adding a little ammonium nitrogen with manganese will provide a slightly faster response. Urea nitrogen facilitates the uptake of iron into the leaf blade.

Finally, to maximize your fertilizer efficiency, make sure you properly manage your soil chemistry.  Soil and salinity test provides this information.

Summary

• When temperature is less than 50°F, utilize a nitrogen fertilizer that is 85-100% ammoniacal and/or nitrate nitrogen.
• Urea nitrogen is quick-release but will be slow to respond when temperatures are less than 50° due to slow urease enzyme activity.
• Do Not Over apply. Generally, only .10-.15 lbs./1000 sq. ft. of actual nitrogen is needed if the nitrogen source is in the ammonium and/or nitrate form.
• Do not forget to manage your soil chemistry.

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