Effect of variable rate irrigation and nitrogen fertilizer rates on crop productivity and water quality

This research project set out to answer a key question: Can we adjust irrigation and nitrogen rates to keep yields high and nitrate loss low?

Why Was This Study Done?

As more Minnesota farmers start using irrigation—especially in sandy soils—it’s become more important to understand how irrigation and nitrogen fertilizer work together. Farmers know that:

• Corn needs both water and nitrogen to grow.
• But applying too much of either wastes money and can harm the environment.
• And with rising input costs and increasing regulation, it’s more important than ever to apply both resources wisely.

In sandy soils of central Minnesota, water drains quickly and nitrogen can easily leach below the root zone. That nitrate can end up in drinking water, which is a health risk and a concern for both rural families and local communities.

Where and How the Study Was Done

The research was conducted at two irrigated sites in Minnesota:

• Becker – Sand Plain Research Farm
• Westport – Rosholt Research Farm

Both sites have sandy soils and are in areas with a lot of irrigated corn production.

Each year, researchers tested:

• Four irrigation levels:
  • 100% (full irrigation, no water stress)
  • 75% of full irrigation
  • 50% of full irrigation
  • Rainfed (no irrigation)
• Six nitrogen rates: 0, 70, 140, 210, 280, and 350 lbs of N per acre

They tracked:

• Corn grain yield
• Soil moisture
• Nitrogen uptake by the plants
• Nitrate leaching into the soil
• Weather and rainfall patterns

They also used drones and sensors to gather more information without damaging the crop.

What the Weather Was Like

Over three growing seasons (2020, 2021, and 2022), researchers got to study both wetter and drier conditions:

• 2020 was considered a wet year, with decent summer rainfall at key crop stages.
• 2021 and 2022 were dry years, with rainfall 4–6 inches below average during June, July, and August.

This range in conditions helped them see how irrigation and nitrogen interact in both wet and dry seasons.

What They Learned

1. Irrigation Timing and Amount Matter More Than Just Total Rainfall

You might think that rainfall totals are the biggest factor, but timing is just as important. For example:

• In 2020, more rain came in June, July, and August—even though the total rainfall wasn’t much higher than in 2022.
• In 2022, August was wetter, but June and July were dry—meaning the crop still needed irrigation early.

Bottom line: Small, well-timed rains reduce irrigation needs more than big rains that come too late.

2. You Can Save Water Without Sacrificing Yield

This might be the most important takeaway for farmers: 75% irrigation often gave the same yield as 100% irrigation.

In fact, across all three years:

• The highest yields came from 75–100% irrigation with nitrogen rates of 280 lbs/ac.
• In Becker, the top yields were 222–226 bushels per acre using those rates.
• At Westport, even in dry years, 75% irrigation held up just fine.

This suggests a smart irrigation strategy can reduce water use without hurting your bottom line.

3. Nitrogen Response Was Strong—But More Isn’t Always Better

Corn yield increased with nitrogen, but only to a point. After about 280 lbs N/ac, yield gains flattened out or even dropped slightly.

This classic “plateau” effect means:

• You’re not getting a return on investment beyond that point.
• Extra nitrogen can increase the risk of leaching, especially in dry years.

Best practice: Don’t overapply nitrogen—especially if you’re not irrigating at 100% rate.

4. Too Little Water Can Lead to More Nitrate Loss in Dry Years

This may surprise some people. We often assume that nitrate leaching is worse when there’s too much water. That’s true in wet years. But in dry years, researchers found that:

• Plots with less irrigation had more nitrate leaching at first.
• As water application increased, nitrate loss went down—but only to a point
• Then, at very high irrigation levels, nitrate leaching started going up again.

Why? In dry years, roots may not grow as deep or take up as much N. So unused nitrate builds up and moves down suddenly when water is applied.

Bottom line: Don’t go too low or too high on irrigation. Mid-levels are often best for minimizing N loss.

Practical Takeaways for Minnesota Farmers

Here’s how you can use this research in your operation:

✅ Consider 75% irrigation as your starting point

• You’ll save water.
• You’ll likely maintain high yields.
• You’ll reduce pressure on local water supplies.

✅ Match N rates to your irrigation level

• Don’t use full N rates if you’re cutting back on water.
• Aim for around 210–280 lbs N/ac unless you’re in an unusually wet year.

✅ Watch rainfall timing closely

• Well-timed rains can reduce irrigation needs more than you might expect.
• Track soil moisture and evapotranspiration to avoid overwatering.

✅ Don’t assume dry years mean less nitrate loss

• Use a soil nitrate test when possible.
• Manage N to match crop uptake, especially under low water conditions.

What’s Next for the Study

This research is still ongoing. The goal is to build more years of data and develop:

• Decision tools for N and water use
• Irrigation recommendations for sandy soils
• Region-specific strategies that protect water quality and farm profitability

Funders include AFREC, the Minnesota Department of Agriculture, and the Irrigators Association of Minnesota.

Final Thoughts

Farming in Minnesota’s central sands means walking a tightrope with water and nitrogen. This research offers reassurance: You can use less—smartly—and still win.

By adjusting your irrigation and fertilizer practices based on local conditions and crop needs, you’ll save money, protect water, and maintain strong yields year after year.