Volatilization losses of ammonia from urea have been a major concern of producers and agronomists because of the lack of precipitation. The nitrogen (N) cycle is very complex as it includes all forms of matter: solid (fertilizer and manure), liquid (dissolved N as nitrate and ammonium) and gas (ammonia). The cycle is highly influenced by soil micro-organisms and enzymes, pH, moisture content of the soil and atmosphere, and temperature.
Corn and wheat require large amounts of N per acre to attain desired yield goals and therefore N application options are commonly limited to mostly surface broadcast. Surface broadcast N fertilizers require precipitation to incorporate it into the soil for plant root absorption. Depending on the N fertilizer source, the N could be susceptible to volatilization losses. Urea N forms are susceptible to volatilization losses while ammonium and nitrate are not.
Understanding urea N volatization
To better understand urea N volatilization, one needs to understand the urea hydrolysis and other chemical soil process. Urea conversion can only take place when dissolved in soil water. Urea in combination with water and urease enzyme results in the formation of ammonium and bicarbonate (Reaction 1). Soil pH and the hydrolysis rate control the amounts of ammonia present in the soil or near the surface.
If the soil is acidic, enough hydrogen ions exist to convert bicarbonate into water and carbon dioxide (Reaction 2), N will remain as ammonium, a cation attracted and held by the soils cation exchange capacity (Reaction 3). However, if the soil is alkaline (pH>7) or the hydrolysis rate is too extensive for slightly acid soils, hydrogen ions are not readily available and some or all of the N is in the form of ammonia, a volatile gas that can be lost into the atmosphere. Therefore, rate of urea N hydrolysis is the process that controls the amount of ammonia susceptible to volatilization losses.
Factors influencing hydrolysis
The factors that influence hydrolysis rate are: 1) presence and amount of urease enzyme in or on the soil, 2) soil temperature and 3) soil water content. Urease enzyme is present on old plant residues (straw, stover and partially decomposed organic matter). Therefore, in no-till and other conservation tillage systems, an abundance of urease enzyme exists. Urease enzymatic activity increases with temperature. A temperature increase from 44 to 80 degrees Fahrenheit (F), increased urea conversion to ammonium four times. Soil water content is the greatest controlling factor for urea hydrolysis. At soil moisture levels near wilting point to air-dry, urea hydrolysis is practically nothing. The greatest hydrolysis rate would occur on a very warm soil (near 80 degrees F) with higher levels of crop residues and a soil surface that does not dry quickly.
Implications for producers?
The conditions that we experienced when producers spread urea this spring were not conducive to large amounts of urea lost through ammonia volatilization. The soils in South Dakota during this time were very dry and mostly cool. During the warmer days, soil temps did increase, but really provided a mechanism for continued soil drying. While it is almost impossible to determine how much N was lost from urea volatilization due to the complexities of the N cycle, given the dry soil moisture conditions that occurred during urea fertilizer application, losses were hopefully minimal to slight. The agronomic management plan should consist of routine scouting looking for sustained N deficiency symptoms and not short term plant health effects influenced by the environment. While South Dakota has not published recommendations for the pre-side dress N test (PSNT) for corn, it is an important tool that could be used to access the N status for corn.