Accounting for nitrogen in irrigation water offers an opportunity to optimize nitrogen use to crop needs and minimize nitrogen leaching to groundwater, particularly in well-drained soils.Have you ever wondered if there is nitrate in your irrigation water? Irrigation is used to grow specialty crops as well as commodity crops such as field corn, soybeans, and alfalfa. Yet, nitrogen in irrigation is often overlooked as a nitrogen input. In this article, we will discuss the presence of nitrate in irrigation water and opportunities to budget for nitrogen in irrigation to improve nitrogen use efficiency.
Consider what’s available
Nitrogen is present in groundwater mostly as nitrate (NO3). Nitrate-N in groundwater re-enters the nitrogen cycle in irrigated fields through irrigation. In Figure 1, irrigation nitrogen, like other nitrogen inputs (nitrogen from fertilizer and manure), can add nitrogen to the soil nitrogen pool.
In 2020, a survey of six farms in the Wisconsin Central Sands region conducted by Chris Kucharik and Tracy Campbell from the Department of Agronomy at the University of Wisconsin-Madison found NO3-N levels ranging from 4 to 45 mg/L in irrigation wells. The level of nitrogen from the same wells was relatively stable year to year, even though it can vary from well to well. To determine nitrogen levels in your irrigation water, test different wells annually.
Crunch the numbers
You can calculate the amount of nitrogen from irrigation through the following formula:
Irrigation N (lb. N/acre) = Irrigation amount (inches) * Irrigation water NO3-N concentration * 0.23 (mg/L)
For every inch of irrigation water containing 10 mg per liter of NO3-N, 2.3 pounds of nitrogen per acre is added through irrigation. The amount of irrigation nitrogen varies across wells with different nitrate-N levels and crops with different water needs. The table presents nitrogen applied through irrigation from various rates on irrigated corn.
Consider that nitrogen from irrigation can enhance crop uptake efficiency. In a dry year, close to 80 pounds of nitrogen may be applied through high-nitrate irrigation water on cornfields in this scenario. Ignoring this nitrogen source when estimating total nutrient supply can lead to over-fertilization, increasing the risk of groundwater contamination and reducing profitability as excess nitrogen is likely to leach into the groundwater.
Look for opportunities to credit
The following example highlights corn production on coarse-textured soils in Wisconsin, where irrigation water contains elevated levels of NO3-nitrogen. It demonstrates how to account for nitrogen from irrigation and use that information to adjust nutrient applications effectively.
Figure 2 presents the irrigation amount and the corresponding nitrogen applied through irrigation on a sandy cornfield. In this case, the field received irrigation every three to four days from mid-June through harvest, averaging 1.7 pounds of nitrogen per acre per application. Over the season, 78 pounds of nitrogen were applied through 17 inches of irrigation water, with a nitrate concentration of 20 mg per liter.
Keep in mind that irrigation nitrogen can vary significantly with weather. In the previous, wetter year, the same field received less than half that amount of irrigation, resulting in only 32 pounds of irrigation nitrogen.
In drier years, when irrigation ramps up, it’s critical to account for the nitrogen it contributes. Adjusting in-season fertilizer rates based on nitrogen already applied through irrigation helps crops reach optimal growth with fewer inputs. This not only saves time and money but also protects water quality by reducing the risk of nitrate leaching.
Incorporating irrigation-derived nitrogen into nutrient management plans requires further research. We need to better understand how much credit irrigation nitrogen provides across different crops and soil types, such as potatoes and corn in the Wisconsin Central Sands region. Still, the approach outlined here offers two practical applications:
1. In-season: Adjust fertilizer rates based on nitrogen already supplied through irrigation.
2. Off-season: Use irrigation nitrogen data as a hindcasting tool to evaluate total nitrogen use.
For many growers, considering nitrogen addition from irrigation is a new concept. It’s important to be aware that fertility doesn’t just come from fertilizer bags. It may also come from the water we apply.
TAKEAWAYS:
- Test nitrate concentration in water from each irrigation well: To accurately quantify the nitrogen addition from irrigation, conduct annual testing of nitrate levels in water from each well used to supply irrigation — research shows these nitrate-N levels fluctuate from year to year. Take water samples from irrigation wells at or after the second irrigation event to avoid biases. This will help you understand the variability of nitrogen levels and make informed decisions regarding nitrogen management.
- Budget and reflect on nitrogen content in irrigation water: This hindcast approach offers an opportunity to optimize nitrogen use to crop needs and as well as minimize nitrogen leaching to groundwater, especially in well-drained soils.
This article appeared in the August 2025 issue of Journal of Nutrient Management on page 6-8.
Not a subscriber? Click to get the print magazine.
