Nitrogen management in most row crops can be a significant proportion of the overall production cost and can impact the bottom line. Keeping tabs on factors that can reduce the efficiency of N applications and deploying appropriate management strategies is one of the several ways to ensure a profitable production in this season. Nitrogen loss is one of the several factors that can reduce N application efficiency. The most common loss from broadcast or dribbled, unincorporated urea-containing fertilizers is ammonia volatilization losses, especially in no-till production systems. Ammonia volatilization occurs when ammonium ion from the breakdown of urea or fertilizer is subsequently converted to ammonia gas, which escapes to the atmosphere. The striking thing about ammonia volatilization losses is that nearly 90% of the total losses occurs within 3-5 days after fertilizer application. The potential and extent of ammonia loss is dictated by three key factors including soil properties, fertilizer management practices, and environmental conditions. This article focuses on how soil pH, clay content, soil organic matter, and soil moisture levels can influence ammonia loss from urea ammonium nitrate (UAN) applied onto the surface without incorporation.
Soil pH
Soil pH can severely affect efficiency of broadcast UAN fertilizer applications. In soils with pH greater than 7, the ammonia loss from broadcast UAN is very high and sometimes greater than losses from urea applied at the same rate. This is because soils with pH > 7 are dominated by hydroxide ions which favor the rapid conversion of ammonium (UAN contains approximately 25% ammonium) to ammonia gas. This is not the case for soils with pH less than 7 since these soils have higher concentrations of hydrogen ions favoring the conversion of ammonia to ammonium ions resulting in lower ammonia losses. If UAN is applied onto a soil surface with pH greater than 7, the fertilizer should be treated with a proven N stabilizer. Information on N stabilizers evaluated in TN are available in the UT Publication PB1888, Enhanced Efficiency Fertilizers as a Tool to Control Nitrogen Loss in Row Crops Production and UT Publication W1221, Performance of Enhanced Efficiency Nitrogen Fertilizers and Sidedress Nitrogen Placement Methods in Dryland Corn.
Soil organic matter (SOM)
Soil organic matter refers to the decomposed stable component of organic matter occurring in the soil. As with clay particles, SOM has a large surface area and is negatively charged. Soil organic matter can also bind ammonium, thereby reducing the concentration of ammonium available for conversion to ammonia losses. Not surprisingly, therefore, soils with higher organic matter content were less susceptible to ammonia losses. Adopting management practices such as cover cropping and no-tillage can improve soil organic matter content and reduce N losses.
Soil moisture content
Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for 24 to 48 hours. Ammonia loss from surface-applied, unincorporated UAN at or near field capacity decreases as the soil moisture increases. In contrast, when UAN is applied onto a saturated or wet soil surface, ammonia loss increases significantly. Research shows that ammonia loss from broadcast UAN (treated with N stabilizer) on a saturated soil can be 10 times more than application at or near field capacity (Figure below). This is because in saturated soils several of the soil pores spaces are filled with water and thus the ammonia easily escapes to the atmosphere. Urea-based fertilizers should never be surface-applied on wet or muddy soils even if it’s treated with a nitrogen stabilizer.
Clay content
Clay content is the proportion of mineral soil particles with diameters less than 0.002 mm. Clays have large surface area and are negatively charged, so their surfaces can adsorb positively charged cations such as ammonium. Soils with higher clay contents are less susceptible to ammonia loss because of greater negatively charged surfaces, which retain ammonium. In contrast, soils with lower clay content have fewer negatively charged surfaces, so retain less ammonium, so much of the ammonium is available for conversion to ammonia. Since clay content is an inherent soil property and cannot be easily influenced by management practices such as split application of N fertilizers can reduce ammonia volatilization on sandy soils, which due to their low clay content would be susceptible to this.
In summary, it’s important to keep an eye on the soil moisture at application, initial soil pH, soil organic matter, and clay content for broadcast application without any incorporation. Soils with high pH (>7) have greater ammonia loss potential compared with soils of pH 6.0 to 6.2. Lower clay content soils, with lower SOM, and wet or muddy soils results in greater ammonia loss. To reduce ammonia loss in soils that are susceptible to it, UAN should be treated with N stabilizer such as urease inhibitor.
