Urease inhibitors.
This is the second blog on slow-release fertilizers: nitrogen stabilizers in turf. The first is on nitrification inhibitors and this covers the stabilized nitrogen urease inhibitors. Both of these fertilizer technologies are in common use in Australia.
Urease inhibitors are part of a ‘tool box’ that can save you both time and money. This ‘tool box’ includes Sirflor 38 and products like these can help you to produce high quality surfaces at minimal cost. There are now in fact, several options available for turf grass.
At the end of this article is a free ammonia loss calculator, which can demonstrate potential losses from your site.
How do Urease Inhibitors stop Volatilization?
When you apply urea or ammonium fertilizers to a soil surface, they can lose nitrogen (N) due to ammonia volatilization. This often happens with urea and liquids like urea ammonium nitrate (UAN).
Urease inhibitors stop or slow down the transition of urea to ammonium hydroxide and ammonium. As they slow down the rate of urea hydrolysis in the soil, they reduce ammonia losses.
Factors Effecting Urea Volatilization.
- Evaporation Rate.
- Soil Moisture and Rainfall.
- High soil pH.
- Temperature.
- Soil texture and finally
- Thatch levels.
Urease Inhibitors and Evaporation Rate.
Factors that increase evaporation tend to increase volatilization losses. As ammonia moves to the soil surface, it increases the concentration in the soil solution. Windy conditions increase moisture loss and volatilization.
Soil Moisture and Rainfall
Volatilization of surface-applied urea increases as soil water content increases. This continues until the soil becomes saturated. If you apply a granular fertilizer to dry soil, volatilization doesn’t occur. Fertilizer granules must dissolve before losses occur.
Once the granules dissolve, volatilization increases as water evaporates from the soil. The moisture loss increases the concentration of ammonia at the soil surface. Be aware that volatilization losses are high if you apply urea to moist soil and the soil dries out. Significant rainfall or irrigation after you apply fertiliser reduces volatilization. Water dilutes the ammonia, and lowers the ammonia concentration at the soil surface.
You need 6mm or more of water to leach urea deep enough into the soil, to prevent volatilization.
Critical Relative Humidity.
The humidity within a turf canopy may be greater than in the atmosphere, so urea dissolves.
The solubility of urea is high. At 20°C, 1079 g of urea dissolves in a litre of water. When you spread granular urea it comes into contact with wet soil. It is also subjected to dew, humidity, rainfall and irrigation.
Urea is very hygroscopic, and absorbs moisture from the air. It dissolves if the critical relative humidity (CRH) is exceeded. This is another reason to consider using a nitrogen stabilizer.
The CRH is the relative humidity of the atmosphere (at a specific temperature). This is when fertilizer starts to absorb atmospheric moisture. Below this temperature it doesn’t absorb atmospheric moisture. The critical relative humidity of most fertilizers decreases with increasing temperature.
-
Mixtures of salts usually have lower critical humidities than individual fertilizers.
-
Fertilizers that contain urea exhibit a much lower CRH than fertilizers without Urea.
At a temperature of 30°C, the critical relative humidity for urea to dissolve is 72.5%.
Urease Inhibitors and Soil pH.
High soil pH and high temperatures increase volatilization.
- They increase soil water ammonia concentrations and
- The warmer soil water gets the less ammonia gas it can hold.
Urea-based fertilizers increase soil pH in a 25mm diameter of the urea granule. This causes an increase in volatilization. This pH increase is higher in high sand content rootzones due to a lack of buffering capacity.
The rise in pH increases the conversion rate of ammonium to dissolved ammonia. This increases the amount of ammonia that can volatilize. Although this is a temporary increase, it results in volatilization loss from soils with a pH as low as 5.5. An increase in soil pH from 6.5 to 7.5, doubles volatilization from 10 to 20% if you leave urea on the surface for four days.
Urea fertilizers plus a urease inhibitor, slow the conversion of urea to ammonium. They also minimise any pH increase.
Temperature
Warm temperatures increase volatilisation. Increasing temperature does two things. It increases the rate of urea hydrolysis and also the conversion of ammonium to ammonia.
An increase in temperature from 7°C to 16°C doubles volatilization loss when moisture is the same.
Soil Texture.
Ammonia losses tend to be higher from coarser than fine-textured soils. This is because fine-textured soils have a high CEC and so hold more ammonium.
Soil factors that reduce volatilization include:
- High clay content.
- High CEC soils.
- Soil organic matter, and
- Bicarbonate content.
Volatilization from urea is almost 3X higher in sandy loam soil with a CEC of 7, than in silt loam soil with a CEC of 12.1.1Keller GD. and Mengel DB. 1986. Ammonia volatilization from nitrogen fertilizers surface applied to no-till corn. Soil Science Society of America Journal. 50:1060-1063.
Urease Inhibitors and Thatch Levels
High levels of thatch increase volatilization.
- Soil microorganisms needed to produce the urea enzyme for hydrolysis. They are 40 times more active in surface organic material than in a mineral soil2Torello W and Wehner DJ. 1983. Urease Activity in a Kentucky Bluegrass Turf. Agronomy Journal 75(4) DOI:10.2134/agronj1983.00021962007500040018x .
- Thatch tends to hold water. This increases ammonia in the solution and that can volatilize and
- Surface thatch prevents N from moving into the soil.
-
Use a slow release fertilizers like UF38 in the summer and IBDU in the autumn. This reduces N losses due to volatilization.
-
Thatch does not just effect moisture movement into the soil profile. Excessive thatch costs you money as any nitrogen fertilizer you apply isn’t used efficiently.
Urease inhibitors on the market
Common urease inhibitor in Australia, include NBPT, which is the active in Agrotain. This nitrogen stabilizer inhibits the urease enzyme but does not always result in an increase turf growth and N recovery.
Growth only increases If you reduce volatilization losses from surface-applied urea when:
a) If you don’t use NBPT the amount of N present limits growth and
b) Losses from volatilization are so high that they impact turf growth.
You will always get the best results, when existing N levels are low, and conditions are going to lead to losses from volatilization.
You don’t need to use a urease inhibitor:
a) If you apply at least 6mm of water straight after you apply a fertilizer. In this case volatilization losses are minimal.
b) If it’s going to rain straight after you fertilize.
Use of these products can significantly reduce nitrogen losses from volatilization4Reducing N Fertilizer Loss to Air, Montana State University, https://landresources.montana.edu/soilfertility/soilscoop/ss_UVol.html.
Dual Inhibited products
This nitrogen stabilizer technology combines urea plus a nitrification and urease inhibitor. There are two commercially available options on the market.
Alzon® Neo-N Fertilizer.
Alzon® Neo N is a dual-inhibited N turf fertilizer that contains 46% N. It lasts for up to 10 weeks.
- It is very soluble, and you can use this as a dry granule or dissolve it in a spray tank. As a soluble rates are as low as 25Kg/Ha (2.5g/m2);
-
Alzon Neo N isn’t a coated fertilizer, and the inhibitor is present through the entire granule. This means the inhibitors don’t ‘rub off’ during transit or blending;
-
This increase in N efficiency, means you can use low rates of fertilizer with no loss in turf quality;
-
When you use Alzon as a soluble, it doesn’t leave any tank residues and
-
Research shows that the nitrification inhibitor in Alzon Neo-N reduces leaching by up to 50%. This saves you money, whilst also gives great looking turf.
Umaxx and Uflexx sports turf Fertilizer.
Umaxx and Uflexx combine a urease inhibitor plus an NI, with urea. These stabilizers are NBPT, and “Didin”.
NBPT stops nitrogen losses from volatilization, and the result is it keeps nitrogen stable for longer.
Umaxx is the premium turf-grade nitrogen stabilizer. It is a granular stabilized urea and comes in two particle sizes: SGN 150 and 237. It claims to have a longevity of 12 to 16 weeks.
The other turf grade product is Uflexx, which comes in the same sizes as Umaxx, but contains less of the nitrogen stabilizer. It claims to last 6 to 8 weeks.
Umaxx and Uflexx contain the same amount of NBPT, and it is the NBPT that stops volatilisation for up to 14 days.
You can use these in dry fertilizer blends, as stand-alone applications or as a solubles.
When to use Nitrogen Stabilizers.
Nitrogen stabilizers are not for everyone, but they can save a lot of money and improve the results you get when you fertilize.
As with most things be aware that in many cases you can pay for something that you don’t need. Nitrogen Stabilizers are not for all situations. The flip side is that in some cases, nitrification or a urease inhibitors are a good option to save money and give yourself a good chance to present a better quality surface.
When to use an N stabilizer | |
Urease Inhibitor (NBPT) | Nitrification Inhibitors (DMPP, DCD) |
Neutral and alkaline soils >pH 7 | Highly saturated/waterlogged soils |
Excessive thatch | Heavy, poorly aerated and compact soils |
Soils with low cation exchange capacity (CEC)/ Poorly buffered soils (low organic matter, content, high sand content) | Sandy soils that are prone to leaching |
Heavy, compacted and poorly aerated soils | |
Applications where conversion of Ammonium to Nitrite can still take place. (temperatures of +4°C or warmer) | |
References
Graduated from Newcastle University with an Hons Degree in Soil Science in 1988, Jerry then worked for the Sports Turf Research Institute (STRI) as a turf agronomist before emigrating to Australia in 1993.
He followed this by gaining a Grad Dip in Business Management from UTS. He has worked in a number of management roles for companies as diverse as Samsung Australia, Arthur Yates and Paton Fertilizers.
He has always had a strong affinity with the Australian sports turf industry and as a result he established Gilba Solutions as an independent sports turf consultancy in 1993. Jerry has written over 100 articles and two books on a wide range of topics such as Turf Pesticides and Nutrition which have been published in Australia and overseas.