Could adding irrigation to Minnesota corn fields help reduce nitrate losses to groundwater?

By: Vasudha Sharma, Extension irrigation specialist

The number of irrigated acres in Minnesota is increasing as more and more growers look for ways to ensure high crop yields during dry years. This blog post provides an update on an ongoing field research study being conducted at two irrigated corn sites in Minnesota’s central sands region. I will focus on how different reduced irrigation strategies impact nitrate leaching and residual soil nitrate. After four years of data (one wet year and three dry years) at one of the sites, we’ve seen some interesting results on how different irrigation strategies could alter water usage and nitrate leaching losses, saving farmers money and benefiting the environment.

Where is this study conducted?

A field research study is being conducted at two locations in central Minnesota: The Sand Plain Research Farm in Becker, Minnesota, and the Rosholt Research Farm in Westport, Minnesota. Both sites are situated in the heart of Minnesota’s irrigated acres. The funding is being provided by the Agricultural Fertilizer Research and Education Council (AFREC) and the Minnesota Department of Agriculture (MDA).

Figure 1. Study layout at Becker (left) and Westport (right)

The main goal of this research is to investigate the interaction of different irrigation and nitrogen (N) fertilizer rates on nitrate leaching, corn yield, and water and N use efficiency in coarse-textured soils. This will help us develop best management practices aimed at creating resilient agricultural systems that optimize corn production and groundwater usage and minimize nitrate leaching. This project is evaluating six nitrogen rates (0, 70, 140, 210, 280 and 350 pounds per acre) and four irrigation levels:

100% Full Irrigation (FI), i.e., filling the soil profile to 100% field capacity)
75% of FI
50% of FI
Rainfed conditions

We are looking at how different nitrogen rates and irrigation levels impact corn production and nitrate leaching, as well as how they interact with each other. Below are the findings from the Becker site.

What are we measuring?

The evaluated variables consist of corn yield, nitrate leaching, soil moisture, plant nutrient status, and soil nitrate. Nitrate leaching is determined using permanent suction cup lysimeters (at a depth of four feet) which are sampled every week during the growing season. Soil moisture is monitored weekly using a Neutron Moisture Gauge from 0 to 4 ft depth, to estimate irrigation requirements, crop evapotranspiration, and drainage. Plant samples are collected at V8, R1, and R6 growth stages to determine total N uptake, and soil samples were collected post-harvest for soil nitrate. Additionally, proximal and remote sensing data is being collected using a wide range of in-season non-destructive technologies such as unmanned aerial vehicles (UAVs) and leaf sensors.

What did we observe?

The results from the study offer important insights into how different irrigation and nitrogen application strategies impact residual nitrate in the soil and nitrate leaching in sandy soils, which are prevalent in central Minnesota.

The direct correlation between increased nitrogen application and residual soil nitrate suggests that applying excess nitrogen can lead to inefficient use by crops. When nitrogen is applied in amounts beyond what crops can take up, it remains in the soil, increasing the risk of leaching into groundwater (Figure 2). This highlights the need for precise nitrogen management to match crop requirements, reducing excess nitrate that could contribute to environmental contamination, especially in regions where nitrate leaching into groundwater is a concern.

Figure 2. Effect of nitrogen application rate on residual nitrate in the soil after corn harvest. Soil nitrate after corn harvest increased as the N application rate increased.

The results show that no irrigation (rainfed) and full irrigation (100%) lead to higher residual nitrate levels than limited irrigation treatments. This implies that crops either experience water stress (rainfed) or excessive water supply (100% irrigation), both of which limit their nitrogen uptake efficiency. We suspect that in 100% irrigation, when we refill the soil profile to 100% of field capacity, we do not leave any room for potential precipitation, resulting in excess water or saturated conditions if it rains after irrigation. Limited irrigation (50-75% of full irrigation) seems to create an optimal condition where nitrogen uptake is maximized, reducing residual soil nitrate (Figure 3). These findings suggest that moderate irrigation can improve crop efficiency in nitrogen uptake while mitigating the environmental risk of excess nitrate. Figure 3 also suggests that unirrigated fields with sandy soils may experience less N uptake and more nitrate leaching than irrigated fields deploying limited irrigation strategies (Figures 3, 4, and 5).

Figure 3. Effect of irrigation rate on residual nitrate in the soil after corn harvest. In all years, soil nitrate levels under the rainfed treatment and the 100% irrigation treatment are higher than the limited irrigation treatments. This implies that N uptake was lower in the rainfed and high irrigation treatments than in limited irrigation treatments.

Similar to residual nitrate post-harvest, higher pre-plant soil nitrate was found in rainfed treatments, suggesting that without irrigation, more nitrate is left over in the soil from the previous growing season (Figure 4). This could be due to the reduced water movement through the soil profile in the winter, however, this residual nitrate poses a potential risk for leaching during periods of snow melt and high rainfall in the spring. These results underscore the importance of irrigation for crop yield and managing nitrate levels in the soil post-harvest.

Figure 4. Pre-plant soil nitrate under irrigation and nitrogen treatments in spring 2024. The rainfed treatment had higher residual nitrate than irrigated treatments.

The results also highlight that full irrigation (100%) leads to the highest levels of nitrate leaching, except in a wet year (2020) when irrigation did not significantly impact leaching. In rainfed treatments, leaching was higher than in the limited irrigation treatment (50%) (Figure 5). These findings are significant because they show that moderate irrigation can help reduce nitrate leaching by optimizing the balance between water supply and crop uptake. In sandy soils, where water movement through the soil is rapid, over-irrigation or no irrigation can lead to greater nitrate losses. Moderate irrigation (50-75% of full) appears to be the most effective strategy in reducing nitrate leaching, thereby improving environmental outcomes.

Figure 5. Effect of irrigation rate on nitrate leaching. In all years (except the wet year 2020, when irrigation did not have a significant effect on leaching), the 100% irrigation treatment had the highest leaching losses compared to the limited irrigation treatments. Also, nitrate leaching in the rainfed treatment was higher than the 50% irrigation treatment in all years, indicating that irrigation has the potential to reduce nitrate leaching in sandy soils.

The study’s findings on nitrogen application rates and nitrate leaching are critical. As nitrogen application increases, nitrate leaching below the root zone also increases, except for the 2022 season, which saw heavy precipitation (more than the long-term average) in May and August at the Becker site (Figure 6). This relationship indicates that applying more nitrogen than what the crop can use results in a greater risk of nitrogen being leached from the soil, particularly in wet years. In dry years, less nitrate is leached due to reduced water movement through the soil. These results suggest that nitrogen application needs to be carefully managed in years with high precipitation to avoid exacerbating leaching risks (see the U of M’s corn fertilizer guidelines). They also emphasize the importance of tailoring nitrogen application rates to match crop needs and environmental conditions, such as expected rainfall.

Figure 6. Effect of nitrogen application rate on nitrate leaching below the root zone. Nitrate leaching increased as the N application rate increased with an exception in 2022 due to heavy late and early season precipitation. We observed higher leaching losses in wet years (2020) and lower leaching in dry years (2021-2023)

Best Practices

These findings suggest that moderate irrigation not only supports crop growth but also reduces the risk of nitrate contamination in groundwater, an important environmental concern in regions with sandy soils. Managing nitrogen application carefully, especially in wet years, can further mitigate nitrate leaching, protecting water resources from contamination.

Farmers should consider adopting limited irrigation (50-75% of full irrigation) strategies to optimize water and nitrogen use efficiency. This can be particularly beneficial in sandy soils, where excessive water can lead to nutrient loss.

Utilizing tools such as soil moisture sensors and remote sensing technologies can help in fine-tuning both irrigation and nitrogen application, ensuring that crops receive the right amount of water and nutrients at the right time, improving overall sustainability. These results point to a clear need for integrated water and nutrient management strategies that balance crop productivity with environmental stewardship, especially in areas with vulnerable sandy soils.

Additional resources:

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