What Does Soil Moisture Reveal About North Dakota Irrigation Needs

Soil moisture is the most direct indicator of whether a field needs water. In North Dakota, where precipitation, soil texture, crop mix, and evapotranspiration vary widely across the state, properly interpreting soil moisture can mean the difference between profitable irrigation and wasted water, energy, and fertilizer. This article explains what soil moisture measurements reveal about irrigation needs in North Dakota, how to measure and interpret them, concrete numbers to use for scheduling, and practical, field-ready recommendations.

Why soil moisture matters in North Dakota

North Dakota spans climatic and soil gradients: the eastern Red River Valley has deep, fine-textured, high water-holding soils while the western plains and badlands transition to coarser-textured soils with lower stored water. Growing-season rainfall also declines from east to west. These differences make soil moisture the central variable in deciding when and how much to irrigate.
Soil moisture affects plant water availability, nutrient mobility (especially nitrogen), root growth, and soil temperature buffering. Overwatering wastes water, promotes nitrate leaching and disease, and can reduce yield in some crops. Underwatering during critical growth stages (e.g., tasseling to grain-fill in corn) can produce large yield losses. That makes measured soil moisture far more actionable than relying on calendar schedules or estimated precipitation alone.

Regional soil variability and its implications

Soils in the Red River Valley versus the Missouri Plateau

The Red River Valley soils are often silty clay loams with high plant available water capacity (PAWC). That means a shallow moisture decline per day of crop water use and lower irrigation frequency but often larger application depths when irrigating.
Western and southwestern North Dakota have coarser textured soils–sandy loams and gravels–that store much less water per unit depth. These soils require more frequent, smaller irrigations to keep the root zone supplied and to avoid large fluctuations that stress crops.

Typical PAWC ranges (practical estimates)

• Sandy soils: PAWC roughly 50-100 mm per meter of soil depth (0.05-0.10 cm3/cm3).
• Loamy/silt loam soils: PAWC roughly 150-230 mm per meter (0.15-0.23 cm3/cm3).
• Clayey soils: PAWC can be high but much of it may be held at tensions too high for crops; practical PAWC often 150-250 mm per meter (0.15-0.25 cm3/cm3).

Use site-specific soil tests or local extension data to get precise values. Even broad categories above are enough for initial scheduling and sensor calibration.

Measuring soil moisture: methods and best practices

Common measurement methods

1. Gravimetric sampling: accurate, used for calibration. Involves taking soil cores, weighing wet and dry. Not practical for frequent field decisions but critical for validating sensors.
2. Time-domain reflectometry (TDR) and capacitance probes: give continuous volumetric water content (VWC) readings. Low maintenance, widely used on farms.
3. Neutron probe: historically common, accurate for profile measurements, requires licensing and safety procedures.
4. Remote sensing (satellite/sensor-based): provides spatial patterns and trend detection across fields but has lower temporal resolution and needs ground-truthing.
5. Hand-held dielectric sensors: inexpensive and useful for spot checks.

Sensor deployment and interpretation

• Install sensors at multiple depths to cover the effective root zone (common depths: 10-20 cm, 30-40 cm, 60 cm, and deeper if crops root deeply). Soil moisture at deeper layers matters for late-season supply.
• Place sensors in representative locations: avoid wheel tracks, irrigation pivot end-gun areas, depressions, and spots with atypical soil or management.
• Calibrate sensors with local gravimetric samples, especially when soil texture or EC is outside normal ranges.
• Convert VWC to depth-of-water: VWC (%) multiplied by root zone thickness (mm) gives mm of water stored. Example: 20% VWC in a 600 mm root zone = 0.20 * 600 mm = 120 mm available water in that zone.

Interpreting soil moisture for irrigation decisions

Plant available water and depletion thresholds

Plant available water (PAW) = (VWC at field capacity) – (VWC at permanent wilting point), expressed as mm per layer. Farmers should decide a management-allowed depletion (MAD) threshold that triggers irrigation. Typical guidance:

• Long-season, high-value crops (corn, sugarbeet): refill when 40-50% of PAW is depleted (maintain 50-60% of PAWC).
• Medium-risk crops (soybean, sunflower): refill at 50-60% depletion.
• Drought-tolerant or low-value crops: allow up to 60-70% depletion if needed to conserve water.

These thresholds balance yield risk and irrigation cost. For example, in a silt loam with PAWC 200 mm per meter and an effective root zone of 0.6 m (120 mm PAW), a 50% depletion means irrigation is needed when about 60 mm of water has been used since the last refill.

Scheduling basics: timing and amount

• Timing: schedule when measured depletion reaches the crop-specific MAD. If mid-season daily ET is high (e.g., 5-7 mm/day for corn during peak), depletions can occur quickly; check sensors daily to every few days during peak demand.
• Amount: apply water to refill the root zone to near field capacity but avoid over-irrigating. Target recharge to 80-100% of PAWC depending on risk of leaching and capacity of the irrigation system. For example, if the root zone has 60 mm depletion and the goal is to refill to 95% PAWC, apply about 60-70 mm minus expected effective rainfall.
• Frequency: coarser soils -> more frequent, smaller applications. Fine-textured soils -> less frequent, deeper irrigations.

Practical rules and steps for field use

• Regularly monitor VWC at multiple depths and convert to mm water in the root zone.
• Determine crop-specific MAD thresholds before the season (corn 40-50%, soybean 50-60%, small grains 50-60%, sugarbeet 40-50%).
• During rapid growth or heat stress, lower the threshold (irrigate sooner).
• When storms are forecast, adjust schedule: do not irrigate just before a predicted effective rain >10 mm.
• Account for system application efficiency: if a pivot has 85% efficiency, increase planned application to compensate, or run longer to achieve effective root-zone refill.
• Track cumulative seasonal irrigation relative to crop water use and local historic rainfall; this helps with budgeting and yield projections.

Examples for major North Dakota crops

Corn (maize)

• Typical peak ET: 4-7 mm/day in North Dakota during critical stages.
• Rooting depth: 0.6-1.2 m depending on year and soil; design sensors at 0.3, 0.6, and 0.9 m.
• Management: aim to refill at 40-50% depletion. Avoid water stress from tassel to grain fill; a single prolonged deficit then can reduce yield significantly.

Soybean

• Moderate drought tolerance but yield sensitive during pod fill.
• Refill at 50-60% depletion. More flexible than corn but still benefits from consistent supply in mid-season.

Wheat and barley

• Shallower rooting than corn; benefit from stored soil moisture in fall and spring. Apply irrigation to maintain moisture into booting and heading; allow slightly deeper depletion early to conserve water.

Sugarbeet and potato

• High value and water-sensitive during bulking/tuber enlargement. Use tighter control (refill at 35-45% depletion), frequent checks, and avoid wide swings in VWC.

Management considerations and common pitfalls

• Over-reliance on a single sensor: one sensor well placed is useful, but soil variability means several sensors across soil types and field positions are recommended.
• Ignoring rooting depth changes: roots deepen as the crop grows; the effective root zone increases. Sensors should represent the full active root zone.
• Not calibrating sensors: VWC readings are sensor- and soil-specific. Periodic gravimetric checks prevent systematic errors.
• Failing to account for nitrate leaching risk: in sandy soils, excessive irrigation can move nitrate below the root zone. Time applications to avoid leaching after fertilizer applications.
• Neglecting system capacity: plan irrigation amounts to match pivot application rate and pump capacity. It is better to irrigate more often with the system’s maximum allowable depth than to try to apply large depths that the system cannot deliver evenly.

Practical takeaways and recommended actions

• Install soil moisture monitoring at representative points and at multiple depths (e.g., 15, 30, 60 cm). Calibrate with at least one gravimetric sample per texture type.
• Use crop-specific depletion thresholds: corn 40-50%, soybean 50-60%, high-value root/tuber crops 35-45%.
• Convert VWC to mm of water in the root zone for simple arithmetic: VWC (%) * root zone depth (mm) = mm water.
• Schedule irrigation to refill to around 80-95% PAWC rather than to saturate, accounting for upcoming rainfall forecasts.
• For sandy soils, favor shorter intervals and smaller depths to reduce leaching and increase irrigation uniformity. For fine soils favor deeper, less frequent applications.
• Keep records of sensor readings, irrigations applied, and yields. Over several seasons this dataset will refine thresholds and demonstrate economics.
• When in doubt, prioritize critical growth stages: for most row crops in North Dakota these are flowering and grain/tuber fill stages.

Final words

Soil moisture is the most actionable measurement for irrigation management in North Dakota because it integrates soil, weather, and crop demand into a single, interpretable variable. Proper measurement, sensor placement, calibration, and interpretation against crop-specific depletion thresholds let producers apply the right water at the right time. That approach increases yield stability, reduces unnecessary water and energy use, and lowers the risk of nutrient leaching. Implementing a disciplined soil moisture-based program is a practical and cost-effective step toward resilient, efficient irrigation on North Dakota farms.