Inhibiting natural urease activity

Urease inhibitors are chemical substances witch inhibit the natural urease activity in the soil.

Urease inhibitors thus allow more time for urea to penetrate the soil. Thereby, the ammonia concentration peak and volatilization risk upon spreading are reduced. What are the characteristics of urease inhibitors?

What is NBPT?

Molecular formula of NBPT

Many compounds have been evaluated as urease inhibitors since the early 70s, however, few meet the requirements of being effective at low concentrations, non-toxic, stable, inexpensive and compatible with urea. N-(n-butyl) thiophosphoric triamide (NBPT), is used in the US since the early 80s. NBPT has been authorized by European legislation in 2008 and is the active ingredient of commercially available urea fertilizers with urease inhibitors.

How does NBPT act?

Transformation of urea to ammonium requires humidity and the activity of urease enzymes in the soil. Ureases are found in numerous bacteria, fungi, algae, plants and some invertebrates, as well as a free enzyme in soil. 

Urease inhibitors are chemical substances which temporarily block the natural activity of these soil enzymes. They thus curb the transformation of urea to ammonium. 

Cumulative volatilization losses, from urea, urea + NBPT and nitrate fertilizers on acid soils (pH 5,8) [7].

Slowing down the transformation of urea to ammonium results in lower pH levels and lower ammonium peaks building up in the soil because of gaining time for urea molecules to move deeper into the soil. Therefore less volatilization occurs. Trials have demonstrated that urease inhibitors can mitigate about 60 to 70 % of volatilization losses from urea and 40 to 50 % from UAN [4].

Concentration minimum nécessaire

For efficient inhibition of urease activity, a minimum product concentration of 0.04% is required. To avoid overdosing of a chemical like NBPT the maximum content may not exceed 0.10% [6].

To be efficient, urease inhibitors need to be available at a minimum concentration. For NBPT, the European fertilizer regulation authority has set 0.04 % and 0.10 % as respective lower and upper limits for urea fertilizers allowed to be declared as “urea with urease inhibitors”. These concentrations ensure roughly 90 % inhibition efficiency.

Stability is an issue

The concentration of NBPT in the fertilizer diminishes over time [5][8]

Urease inhibitors have limited stability and degrade over time. Decomposition is temperature dependent. The typical half-time is less than 6 months. Fertilizers containing inhibitors therefore shall be used quickly upon manufacturing. NBPT decomposes very fast when it gets in contact with sulfate and is therefore incompatible with fertilizers containing sulfate.

Safety and security 

Effects on humans 

NBPT is provisionally classified “dangerous” under European regulation. The pure product is irritating to the eye and is at risk of impairing fertility. Protective clothing, gloves and glasses need to be weared when handling pure NBPT. No restrictions apply for finished urea products containing NBPT. No cases of NBPT traces in the nutrition chain have been demonstrated so far. 

Effects on the soil 

Applied NBPT is rapidly mineralized in the soil and converted to CO2, with a half-time of one to two weeks. There are currently no known harmful effects of NBPT to soil organisms and animals. 

NBPT in plants 

Urease is present in crops and required for conversion of externally or internally generated urea. Urease inhibitors, upon application, can be taken-up by the roots and transferred to the leaves where they diminish endogenous urease activity. As a result, amino acid contents are reduced, indicating alteration of nitrogen metabolism. Chlorosis and necrosis of leaf margins have been observed for various crops upon application of urea with NBPT. These symptoms are transient and seem to be mainly caused by urea accumulation in the leaves.