Josh Lofton and Beatrix Haggard
Corn grown in Louisiana has the potential to be very productive. Additionally, the Midsouth states, especially Louisiana, have the ability for corn to be planted and harvested much earlier compared to other corn-growing regions. This is due to warm conditions typically experienced in the latter part of February and early March. In Louisiana, however, this warmer weather is often associated with wet conditions. While moist conditions are needed for optimum early season growth and productivity, this excessive moisture cannot only be detrimental to the corn crop but can negatively influence corn nitrogen management and result in high nitrogen loss. This excessive rainfall can frequently occur during the early spring months, with events often exceeding 5 inches within a 24-hour period (Figures 1a and 1b). This results in growers having to consistently decide on how to manage following these potential nitrogen-loss events. However, little information has been available on how much nitrogen is actually lost following these events as well as the effect on the corn growth.
To address the need for more information, trials were established to determine nitrogen loss and movement in the soil following saturated conditions as well as yield loss associated with high soil moisture. Simulated rainfall events were applied through irrigation to ensure nonhigh precipitation plots were not saturated during the treatment application. Simulated high precipitation events occurred at two distinct times within the growing season, early and mid. The early season saturation event was intended to simulate saturation conditions experienced due to natural precipitation events that occur in early spring, while the later saturation event should simulate saturation that occurs near the irrigation pipe during initial irrigation events. Additionally, two nitrogen rates, 240 and 300 pounds per acre, were applied two to three days prior to the first saturation event. The typical rate is 240 pounds per acre, while the 300-pound rate was applied to represent a guaranteed nonlimiting nitrogen application rate. Soil samples and grain yields were used to evaluate the influence of saturated events on soil nitrogen and corn productivity.
Saturated conditions greatly affected both corn yields and nitrogen in the soil profile. These implemented saturated conditions negatively influenced corn grain yield, regardless of when the saturation occurred (Figure 2). When nitrogen applications were made at the current recommended rate, saturation events at either early season or midseason decreased corn yield by nearly 90 bushels per acre compared to the unsaturated treatments. However, when application rates were increased to the nonlimiting rates, saturated conditions decreased corn yields by 77 bushels per acre when saturated conditions were experienced early but only decreased yields by 39 bushels per acre when saturated conditions were only experienced at midseason. These results demonstrated that corn that experiences saturated conditions early in the season is greatly affected, even with excessive nitrogen application rates. However, saturated conditions experienced during midseason still had a significantly negative influence on corn grain yields, but may be more associated with diminished available nitrogen compared to inhibited plant growth.
The concept of nitrogen loss during saturation events can be demonstrated when looking at soil profile nitrogen prior to and following saturated events (Figure 3). Immediately following nitrogen fertilization, high amounts of soil nitrogen were present in the top 6 inches with a drastic decrease in soil nitrogen at 12, 18 and 24 inches. As expected, following the saturation event, plots that were nonsaturated changed little in overall soil nitrogen profile, with higher concentrations of soil nitrogen in the top 6 inches and a sharp decline for the 12-, 18- and 24-inch levels. However, it should be noted that there was a slight decrease in total nitrogen concentration in the surface soil compared to pre-saturation. This indicates that nitrogen movement and loss within the profile are natural occurrences and can be expected with even modest precipitation events.
The largest differences were associated with plots that experienced saturated conditions. These saturated conditions resulted in a drastic decrease in surface nitrogen concentrations compared to pre-saturated conditions. Additionally, a notable increase in soil nitrogen concentration was evident at both the 18- and 24-inch levels. This indicated that deep translocation of the soil nitrogen from fertilization occurred in as little as one saturation event. However, the overall soil profile nitrogen concentration decreased as a result of these saturated conditions (1,192 ppm compared to 407.6 ppm for pre-saturation and post-saturation, respectively), indicating that nitrogen was lost through either deep leaching or denitrification.
With this somewhat somber information, what can we do to minimize this nitrogen loss due to these heavy precipitation events? Currently, research at the LSU AgCenter is investigating two management practices. First, is a split or subsequent applications. As the crop productivity has drastically declined following these events, it is still unknown whether nitrogen applications following early season saturation will allow the crop to recover. Additionally, enhanced-efficiency nitrogen fertilizers have been shown to be quite successful in Louisiana production systems and could be an option to manage around these saturated conditions because they maintain nitrogen in a safe but plant-available form and, therefore, minimize loss. While both of these options have promise, when the crop has experienced these saturated conditions, yield loss should be expected. However, the goal prior to and following these events is to minimize productivity and nitrogen losses to ensure continued sustainable productivity of the system.
Josh Lofton and Beatrix Haggard are former researchers and extension specialists with the LSU AgCenter based at the Macon Ridge Research Station in Winnsboro. They have since both moved to Stillwater, Oklahoma, to take faculty positions at Oklahoma State University.
The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages