Two concerns are sometimes expressed by growers using urea as a N source for crop nutrition. First, when urea remains on the soil surface, a portion of the applied N may be lost through NH3 volatilization…thereby diminishing its fertilizer value. When urea is first applied to soil, it generally reacts quickly with soil enzymes (urease) to convert to NH4 + then to NH3 (Figure 1) which may be lost as a gas. Considerable effort has been made to understand this NH3 loss pathway, resulting in urea coatings (such as controlled-release fertilizers), additives (such as urease inhibitors), and management practices that can substantially reduce these losses.
Many years ago, researchers found that plant growth was reduced or completely eliminated following high applications of biuret to soils, and this growth suppression often persisted for a period of many weeks. Although the ability to degrade biuret is widespread among soil microorganisms, microbial growth is only half as fast with biuret as a N source as it is with urea. The presence of biuret also decreases the rate of nitrification in soil.
When urea with elevated biuret is placed adjacent to seeds, toxicity may result to the geminating plant. Some of this damage is due to the NH3 evolved from the urea during normal hydrolysis, but biuret may make the harsh condition more severe.
Many crops can tolerate large amounts of biuret applied with urea if it is not in direct contact with the seed. A general guideline for safe use of urea applied to soil would permit a maximum 2% biuret in urea. Many crops are not adversely affected until biuret concentrations greatly exceed this level, which is greater than the 1.0% biuret commonly found in most urea currently produced in North America. There are a few plant species (such as citrus and pineapple) that do not tolerate elevated levels of biuret.
Plants are not able to rapidly metabolize biuret. In one experiment, biuret still remained in the leaves of orange trees eight months after foliar application. Soil-applied biuret similarly accumulates in plants for long periods of time. The exact mechanism of biuret damage to plants is still uncertain, but the harmful effects of high concentrations have been well documented.