This tip was provided by:
Ronnie Schnell, Cropping Systems – Statewide, College Station, ronschnell@tamu.eduStatewide
Statewide
Wet Weather and Nitrogen Losses from Soil
Nitrogen losses from soil following fertilizer applications to crops can be difficult to quantify after periods of wet weather. How much nitrogen was lost and how much does it vary spatially? If significant amounts are lost, yield reductions are likely. Answering these questions is critical when considering supplemental or “rescue” applications of nitrogen. Understanding nitrogen loss pathways will help to estimate nitrogen loss.
Nitrogen is lost from soil by four main pathways: denitrification, runoff, leaching and volatilization. When excessive rainfall and saturated soil conditions occur, denitrification and leaching are the greatest concerns. Many factors can affect denitrification and leaching losses, including soil texture, fertilizer type and rate, placement and timing, soil temperatures and the amount of rainfall received (duration of saturation).
Nitrate nitrogen (NO3-N) is required for nitrogen to be lost from soil by these two processes (leaching and denitrification). Understanding how much nitrate-nitrogen is in the soil is the first step to estimating potential losses. Fertilizer products may contain nitrogen in ammonium (NH4) and nitrate (NH3) forms. UAN (32-0-0) has about 25% of the total nitrogen as nitrate. Ammonium forms of nitrogen are rapidly converted (oxidized) to nitrate by soil bacteria in a process known as nitrification. The process is faster with warm soils. Most of the ammonium may be nitrified within several weeks under warm conditions. Nitrification inhibitors can be used to delay this process. Saturated soils (oxygen depleted) will also halt the nitrification process. Therefore, timing and source of nitrogen fertilizer plus the use of nitrification inhibitors will affect how much nitrogen is nitrate form. Nitrogen fertilizer applied several weeks or more before excessive rainfall without nitrification inhibitors is likely largely in nitrate form. This does mean it is lost but has the potential to be lost.
Coarse textured soils (sandy) are much more susceptible to leaching of nitrate below the rooting zone of the crop. Finer textured soils (clay) are susceptible to leaching and denitrification, although leaching potential is substantially lower compared to sandy soils. Low infiltration rates and ponding can result in extended periods of saturation. This will increase the potential for denitrification losses. As soil oxygen is depleted, some soil microbes will switch to nitrate for survival, releasing the nitrogen in gaseous forms that escape into the atmosphere. Some estimates suggest 2 to 5% of the soil nitrate-nitrogen can be lost per day of saturated conditions. The total loss will depend on nitrate available in soil, number of saturated days and temperature. Actual losses can vary widely depending on these factors.
Determining potential nitrogen losses in-season is difficult. Deciding if and what amount of additional nitrogen to apply can be challenging. If you have reason to believe significant nitrogen has been lost, there are several considerations when planning supplemental nitrogen applications. First, ensure that plant stands/populations are adequate for expected yield goals. Next, consider the growth stage of the crop. Applying nitrogen closer to growing point differentiation will improve yield response (see previous tips). However, if panicle initiation has passed but conditions were favorable during this period, significant yield potential may exist yet. Some yield loss can be expected with later applications (pre flowering) but it is important to capture yield potential that does exist. Finally, consider grain price and nitrogen cost in combination with expected yield response. Applying 100 lbs/acre of urea (46 units of N) will cost about $19/acre ($0.42/ lb of N) plus application cost. With grain at $6.50/cwt, you need about 300 lbs/acre of grain to cover the fertilizer cost. Yield response to N fertilizer under ideal conditions is 100 lbs of grain for every 2 lbs of available nitrogen per acre (over 2,000 lbs of grain per acre in this example). With late applied fertilizer, yield response will likely be lower but may be profitable yet.
If plants have been severely affected by wet conditions, this can result in damaged root systems, reduced or delayed tillering, lower leaf area and reduced yield potential. Response to additional nitrogen is unlikely. Extended periods of stress (saturated soils) at earlier growth stages (3-5 leaf) will have greater impact compared to later growth stages (post flowering). Carefully evaluate the crops recovery once soil conditions improve before considering supplemental nitrogen applications.
Figure 1. Sorghum showing signs of nitrogen stress following excessive rainfall.
