By: Vasudha Sharma, Extension irrigation specialist
Late-season irrigation can make or break your yield, and your nitrogen. Learn how much water your crops still need, when to irrigate, and how to avoid costly mistakes like overwatering and nitrate leaching.
Typically, corn at R2 (blister) uses approximately 6 to 7 inches of water to maturity; Soybeans at R4 (full pod) uses approximately 3 to 4 inches of water to maturity; and Potato at tuber bulking uses 4 to 5 inches of water to maturity.
Daily crop evapotranspiration (ETc) rates remain high at this time of the season, especially during hot and windy spells, but start declining after mid-August. Central Minnesota reference evapotranspiration (ETo) is averaging 0.20 inches/day, meaning soils can dry quickly if rainfall is limited.
If soil moisture sensors or hand probes show dry conditions in the top 12–18 inches, strategic irrigation may still be warranted to protect kernel or seed fill.
The tables below provide an overview of water use in the late season for corn and soybeans.
Late-season irrigation can make or break your yield, and your nitrogen. Learn how much water your crops still need, when to irrigate, and how to avoid costly mistakes like overwatering and nitrate leaching.
1. Crop stage and water demand in late season
As of early August, most irrigated corn in central and west-central Minnesota is close to the R2 growth stage. Soybeans are progressing through pod fill (R3–R5), with seed fill just beginning. These stages represent a critical period for grain yield determination.Typically, corn at R2 (blister) uses approximately 6 to 7 inches of water to maturity; Soybeans at R4 (full pod) uses approximately 3 to 4 inches of water to maturity; and Potato at tuber bulking uses 4 to 5 inches of water to maturity.
Daily crop evapotranspiration (ETc) rates remain high at this time of the season, especially during hot and windy spells, but start declining after mid-August. Central Minnesota reference evapotranspiration (ETo) is averaging 0.20 inches/day, meaning soils can dry quickly if rainfall is limited.
If soil moisture sensors or hand probes show dry conditions in the top 12–18 inches, strategic irrigation may still be warranted to protect kernel or seed fill.
The tables below provide an overview of water use in the late season for corn and soybeans.
Table 1. Estimated normal water requirements for corn between various growth stages and maturity in central Minnesota.
Table 2. Estimated normal water requirements for soybeans between various growth stages and maturity in central Minnesota.
The table below shows available soil water capacity and allowable soil moisture deficit of different Minnesota soils at maturity that can be referenced.
| Corn growth stage | Approximate number of days to maturity |
Water use (ET) to maturity (inches) |
|---|---|---|
| Blister (R2) | 50 | 7.00 |
| Milk (R3) | 40 | 4.70 |
| Dough (R4) | 28 | 2.50 |
| Beginning dent (R4.7) | 24 | 2.00 |
| Full dent (R5) | 20 | 1.50 |
| 1/2 milk line (R5.5) | 13 | 0.80 |
| 3/4 milk line (R5.75) | 7 | 0.30 |
Table 2. Estimated normal water requirements for soybeans between various growth stages and maturity in central Minnesota.
| Soybean growth stage | Approximate number of days to maturity |
Water use (ET) to maturity (inches) |
|---|---|---|
| Full flowering (R2) | 51 | 7.25 |
| Full pod (R4) | 37 | 4.40 |
| Beginning seed (R5) | 29 | 2.90 |
| Full seed (R6) | 17 | 1.20 |
| Beginning maturity (R7) | 10 | 0.40 |
| Full flowering (R2) | 51 | 7.25 |
| Full pod (R4) | 37 | 4.40 |
2. Don’t over-apply: Assess your soil profile first
Before applying late-season irrigation, it’s essential to determine how much available water your soil still holds.The table below shows available soil water capacity and allowable soil moisture deficit of different Minnesota soils at maturity that can be referenced.
Table 3. Available soil water capacity and allowable soil moisture deficit at maturity for several irrigated soils Minnesota.
In potatoes, overwatering during tuber bulking can lead to hollow heart, reduced tuber density, and poor skin finish as well as promote pathogens.
Late-season nitrate leaching is a common risk in irrigated fields, especially as crops begin to mature and nitrogen uptake declines. The primary causes include residual soil nitrate left unused by the crop, excess irrigation or rainfall that pushes nitrate below the root zone, and reduced nitrogen demand as the plant shifts its focus from vegetative growth to grain or tuber fill.
Together, these conditions create an environment where valuable nitrogen is easily lost to leaching, potentially polluting groundwater and reducing fertilizer efficiency. To minimize these risks, avoid overwatering and aim to maintain soil moisture at 50–60% of available water capacity.
It’s also important not to "top off" irrigation ahead of forecasted rainfall, as this often results in unnecessary leaching. If nitrogen was applied late in the season or in higher amounts, the risk of loss is even greater, impacting both yield potential and environmental sustainability. Smart scheduling and nutrient management are key to protecting your crop and water quality during the final stages of the season.
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| Soil type | Total available water (inches) | Allowable soil moisture deficit (60% depletion) (inches) |
|---|---|---|
| Becker (fine sandy loam) | 4.00 | 2.40 |
| Dakota (loam) | 5.75 | 3.45 |
| Estherville (sandy loam) | 2.50 | 1.50 |
| Hubbard (loamy sand) | 2.60 | 1.56 |
| Renshaw (loam) | 3.75 | 2.25 |
| Sioux (loamy sand) | 1.20 | 0.72 |
*Water capacity in the top 3 feet or less for soils that have a root restrictive layer, like course gravel.
Use a soil probe, sensor, or the U of M Irrigation Management Assistant Tool to estimate current soil water status. If recent rainfall filled the profile, you may not need to irrigate again. However, if your sensors show that the top 1–2 feet are drying out and ET demand remains high, a small irrigation (0.5–0.75 inch) may be justified.Keep in mind that overwatering may delay physiological maturity and increase dry down time, impacting harvest scheduling.
In potatoes, overwatering during tuber bulking can lead to hollow heart, reduced tuber density, and poor skin finish as well as promote pathogens.
3. Be cautious: Late-season irrigation can increase leaching risk
Late July and August rains can be surprisingly intense. When combined with unnecessary irrigation, nitrate leaching risk increases, especially in sandy soils.
Late-season nitrate leaching is a common risk in irrigated fields, especially as crops begin to mature and nitrogen uptake declines. The primary causes include residual soil nitrate left unused by the crop, excess irrigation or rainfall that pushes nitrate below the root zone, and reduced nitrogen demand as the plant shifts its focus from vegetative growth to grain or tuber fill.
Together, these conditions create an environment where valuable nitrogen is easily lost to leaching, potentially polluting groundwater and reducing fertilizer efficiency. To minimize these risks, avoid overwatering and aim to maintain soil moisture at 50–60% of available water capacity.
It’s also important not to "top off" irrigation ahead of forecasted rainfall, as this often results in unnecessary leaching. If nitrogen was applied late in the season or in higher amounts, the risk of loss is even greater, impacting both yield potential and environmental sustainability. Smart scheduling and nutrient management are key to protecting your crop and water quality during the final stages of the season.
4. Irrigation scheduling tools: Keep it simple and smart
Minnesota growers have access to several tools that help schedule irrigation with more precision. Options to consider:- Soil moisture sensors – track trends and visualize soil drying.
- U of M Irrigation Management Assistant Tool
- NDAWN crop water use and weather station data
- Checkbook method
- Open ET
5. Takeaways for this season and beyond
- Check soil moisture weekly. Don’t assume your crop still needs water.
- Target irrigation to the root zone, not beyond.
- Avoid irrigation before large rain events. Check weather apps frequently.
- Use tools (apps, sensors, or hand probes) to support your gut feeling.
Want to learn more about irrigation and nutrient management?
- Attend our upcoming Field Day at the Rosholt Research Farm in Westport, Minn. August 21st, 2025. Register at popeswcd.org
- Register for the 2025 Minnesota Irrigator Program: z.umn.edu/MIP25REG
- Fill out the Minnesota Irrigator Program feedback survey: z.umn.edu/MIP_2026
For the latest nutrient management information, subscribe to the Nutrient Management Podcast. And don't forget to subscribe to the Minnesota Crop News daily or weekly email newsletter, subscribe to our YouTube channel, like UMN Extension Nutrient Management on Facebook, follow us on X (formerly twitter), and visit our website.
If you have questions or comments, please email us at nutmgmt@umn.edu.
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