By Sonia Menegaz and Fabian Fernandez
Pre-plant urea fertilization is a traditional nitrogen (N) practice in much of Minnesota. Split application and using enhanced efficiency N fertilizer sources, such as ESN that is a polymer coated urea (PCU), are a few of the advanced management practices that have been suggested to improve production and reduce N losses to the environment. But how effective are these advanced practices at reducing N losses and keeping N in the soil available to the crop?
This was the focus of a seven-year study (2014-2020) in southwestern Minnesota that is part of a coordinated multi-state project across the Midwest. The study was supported by the Agricultural Fertilizer Research and Promotion Council (AFREC), the Foundation for Agronomic Research (FFAR), and the Fertilizer Institute (TFI). Before we dive into our findings, let’s review a few fundamental concepts.
What are the most common pathways of N loss?Nitrogen fertilizer is costly, yet extremely important for profitable corn production. Nitrogen loss negatively impacts the environment and reduces farmers’ profitability. Thus, N management practices that reduce losses and improve N use are extremely important.
Plants take up N from the soil as nitrate (NO3) and ammonium (NH4). When urea is applied, it converts almost immediately to ammonium and then quickly to nitrate. If we could keep N in the ammonium form longer, it could be beneficial because nitrate is easily lost through leaching if rain exceeds the capacity of the soil to store water.
Nitrogen can also be lost to the atmosphere as gas through denitrification. Denitrification happens when soils become saturated with water and N oxides such as nitrous oxide (N2O) or N2 gas are produced. While N2 gas is not a problem for the environment, N oxides are. This is because N oxides are greenhouse gases. Additionally, ammonia (NH3) volatilization occurs when urea fertilizers are surface-applied and not sufficiently incorporated into the soil. This loss of N can be even greater when urea is broadcast without a urease inhibitor. Ammonia volatilization is higher when soils are moist and warm, there is substantial crop residue on the soil surface, and the soil is alkaline (pH greater than 8). Regardless of the loss pathway, once N leaves the field, it is no longer available to the crop.
Traditional vs advanced N practices
In our recent study, we evaluated traditional pre-plant urea application against advanced management practices, including split applications and using PCU at pre-plant. We used the same rate of 180 lbs N/ac across the study. Split applications consisted of one-third of the total N rate applied as either pre-plant urea or PCU and two-thirds applied around V6 as urea with a urease inhibitor (Agrotain). We compared their effects on 1) N losses (N2O and NH3 volatilization and NO3 leaching in tile drainage), 2) corn yield, and 3) net economic return. The study was conducted on a continuous corn cropping system in a poorly drained soil in Lamberton, Minnesota.
Urea applied at pre-plant resulted in lower corn yields and net economic returns and was the least efficient practice to reduce N loss. Pre-plant application of PCU reduced N losses, especially in wet years that created ideal conditions for N losses. Corn yield for pre-plant PCU was between pre-plant urea and the split application with PCU. Net economic return for pre-plant PCU was also between pre-plant urea and the split applications. The split applications were not efficient in reducing N loss, but the split application including PCU produced more grain yield and greater net economic returns than pre-plant PCU.
Measurements were done 2015-2020 for nitrate, 2018-2020 for nitrous oxide, and 2019-2020 for ammonia.
For the net economic return calculations, the following was used:
- Corn price = $3.5/bu
- Urea price= $0.34/lb N
- PCU price = $0.54/lb N
- Agrotain price = $0.39/lb N
- Pre-plant application cost = $4.5/ac
- Split application cost = $11/ac
What should you consider before choosing an N management practice?
Both profitability and environmental protection are important. The traditional single application of urea at pre-plant is a lose-lose situation as it has the greatest potential for negative environmental impact and lowest grain yield and net economic return. The advanced N practices were beneficial but not consistent across all three effects being evaluated. Even though the split applications resulted in the greatest net economic return, it did not reduce N loss substantially relative to the traditional pre-plant urea application. Using pre-plant PCU reduced N losses relative to the split applications and the traditional pre-plant urea application. However, pre-plant PCU had a lower net economic return than either of the split applications and only a slightly greater economic return than the traditional pre-plant urea application (the increase in yield with PCU was just enough to pay for the added cost of PCU compared to urea).
Based on what we have learned from this study, we are currently evaluating other practices with an emphasis on various blends of PCU and urea applied at different times to reduce N loss and increase grain yield and net economic return. We are doing this study during two growing seasons at four locations throughout Minnesota. Stay tuned for more as results from the new study start to come in!
Source : umn.edu