In discussions with consultants and farmers this spring, the use of ammonium thiosulfate (ATS) liquid with urea ammonium nitrate solutions (UAN) has come up more than once. The primary question has been about the potential inhibition properties of ATS when mixed with a UAN solution. 

While we typically suggest ATS as a sulfur source for crops, there has been research showing that ATS has urease and nitrification inhibition properties. However, unlike a true urease inhibitor, ATS does not directly affect the urease enzyme and is only indirectly inhibiting after interacting with soil. As a urease inhibitor, the product inhibits the hydrolysis of urea, which is the process by which products like Agrotain, which contains NBPT, helps reduce the risk of nitrogen loss.

Urease inhibitors are best for situations where urea is applied to the soil surface and the urea is not incorporated or incorporated at a depth of less than two inches, and no rainfall is forecasted within four days of application. If the long-term forecast does not have a significant chance of at least a quarter inch of rainfall within a week, then you may want to consider a product with NBPT to help reduce the risk of N loss.

In order to get any potential inhibition from ATS, the amount of ATS should be at least 8% v/v with UAN (28 or 32%). It’s a good idea to band the UAN/ATS mixture, which concentrates the materials in a smaller volume of soil. Regardless, research has shown inconsistent benefits of ATS as a urease inhibitor, and when it does work, the inhibition is not as long for ATS compared to NBPT. If volatilization loss is a concern, using a product with NBPT is the best alternative. If you are interested in ATS because of the sulfur it supplies in addition to nitrogen, but volatilization loss is also a concern, then a good alternative is ammonium sulfate. Ammonium sulfate also provides both nitrogen and sulfur and is not subject to volatilization loss.

If you’re sidedressing urea, we recommend including NBPT in your surface application since you won’t be incorporating the urea. There are a number of new products which have come on the market recently that contain NBPT. Make sure that 1.3 to 1.8 pounds of NBPT are applied per ton of urea. Application of a lower rate of NBPT will not provide good inhibition. Two new products on the market are Anvol (Koch Industries) and Limus (BASF). Both are urease inhibitors that contain a lower rate of NBPT in addition to another urease inhibitor. Recent research has shown both are effective when applied at labeled rates.

Remember that products containing Nitrapyrin and DCD are nitrification inhibitors and they will not do anything to protect from volatilization loss. Make sure you are educated on the differences between nitrification and urease inhibition before purchasing. Some products have both urease and nitrification inhibitors, but for sidedress time, the potential of N loss by leaching or denitrification (which can happen once nitrogen is nitrified) are minimal. When selecting an inhibitor, the risk of volatilization (if the product is not properly incorporated in the soil) is the only risk to consider, so there is little benefit in having a nitrification inhibitor at this time in the growing season.