What Does A Soil Moisture Test Mean For Arkansas Irrigation Plans

Soil moisture testing is a practical, science-based tool that directly informs irrigation decisions, water budgeting, and long-term irrigation infrastructure planning. In Arkansas — where agriculture ranges from flooded rice fields of the Delta to upland row crops and specialty horticulture — knowing what your soil moisture test results mean can increase yields, reduce water use, and help design systems that match crop needs and local water supply realities.
This article explains how to interpret soil moisture tests for Arkansas soils, link results to irrigation scheduling, choose appropriate sensors or lab tests, and incorporate findings into irrigation planning and infrastructure decisions. Concrete examples and step-by-step recommendations are included so producers, consultants, and water managers can act on test results quickly.

Why soil moisture matters in Arkansas

Soil moisture determines whether plants experience water stress, how efficient applied irrigation will be, and how much water must be supplied to replace root-zone depletion. Arkansas has diverse climates and soils:

• Hot, humid summers with high evapotranspiration during peak growing season.
• A precipitation pattern that includes periods of heavy rainfall and multi-week dry spells.
• A wide range of soil textures across the Delta, loess hills, and uplands.

A soil moisture test turns an uncertain condition (“the ground looks dry”) into quantifiable metrics: volumetric water content (VWC), percent of available water remaining, or water potential. Those metrics let you answer operational questions: when to irrigate, how much to apply, whether your pump capacity and storage are adequate, and when to make investments in precision irrigation.

How to interpret a soil moisture test

A soil moisture report typically provides one of the following:

• Volumetric water content (VWC), given as percent by volume (e.g., 22%).
• Soil water potential or tension (e.g., centibars or kilopascals).
• Gravimetric moisture (percent by weight), usually from lab analysis.

To convert those numbers into irrigation actions, you must know three soil hydraulic benchmarks:

• Field Capacity (FC): the water content retained after excess drainage has occurred (upper usable limit).
• Permanent Wilting Point (PWP): the water content where most crops cannot extract further water (lower usable limit).
• Available Water (AW): the difference FC – PWP; this is the water stores plants can use.

Use this formula to determine percent of available water remaining:
Percent available = (Current VWC – PWP) / (FC – PWP) * 100
Irrigate when this percent drops below your chosen threshold. Common depletion thresholds:

• Sensitive crops or turf: irrigate at 30-40% depletion (i.e., keep 60-70% of AW).
• Field crops like corn, soybean, cotton: commonly schedule at 40-60% depletion.
• Drought-tolerant or deep-rooted crops: can tolerate 60-70% depletion.

Always choose thresholds based on crop growth stage, yield sensitivity, and water availability.

Common Arkansas soils and typical values

Arkansas soils vary, but the following approximate volumetric benchmarks are useful starting points. These are generalized; local lab or sensor calibration is essential.

• Sand / sandy loam:
• Field Capacity: 10-15% VWC
• Permanent Wilting Point: 4-7% VWC
• Available Water: ~6-10% VWC
• Loam / silt loam (typical Arkansas Delta topsoils):
• Field Capacity: 20-30% VWC
• Permanent Wilting Point: 10-15% VWC
• Available Water: ~10-20% VWC
• Clay / silty clay:
• Field Capacity: 30-40% VWC
• Permanent Wilting Point: 20-25% VWC
• Available Water: ~10-20% VWC

Implication: a sandy soil holds much less water per inch of depth than a loam or clay. That means more frequent, smaller irrigations on sandy sites, and less frequent but deeper irrigations on heavier soils.

Sensors and testing methods: pros and cons

There are several ways to get soil moisture data. Each has tradeoffs in cost, accuracy, and ease of use.

• Gravimetric lab test:
• Pros: gold-standard accuracy, useful for sensor calibration.
• Cons: destructive, time-consuming, requires oven or lab.
• TDR / Tension Domain Reflectometry and capacitance sensors:
• Pros: provide continuous VWC readings, moderate cost, suitable for on-farm monitoring.
• Cons: require calibration for local soil texture and salinity; point measurement.
• Tensiometers:
• Pros: measure soil water tension directly, intuitive for irrigation start/stop thresholds; good for clay and loam.
• Cons: less useful in sandy soils nearing dry conditions; maintenance-intensive.
• Gypsum blocks:
• Pros: low cost.
• Cons: less accurate over time; require calibration and regular replacement.
• Neutron probe / neutron moisture meter:
• Pros: established methodology, reliable if operated correctly.
• Cons: expensive, requires licensing and safety protocols.

Best practice: run an initial gravimetric test to characterize local FC and PWP by depth, then install daily sensors (TDR/capacitance or tensiometers) and calibrate them against lab results.

Scheduling irrigation based on tests

Follow these steps to convert a soil moisture test into an irrigation event:

1. Determine the effective root zone depth for the crop and growth stage.
2. Establish FC and PWP for that depth (measure or use calibrated values).
3. Get current VWC (from sensor or soil sample) for the same depth increments.
4. Calculate current percent of available water remaining.
5. Compare to your depletion threshold for that crop and stage; if below threshold, plan irrigation.
6. Calculate irrigation amount required to return to FC, adjusting for system efficiency and expected rainfall.

Example calculation (practical):

• Crop: soybean mid-season.
• Effective root zone: 18 inches (about 450 mm).
• Soil type: silt loam.
• FC = 30% VWC, PWP = 12% VWC => AW = 18% VWC.
• Current VWC = 20% VWC.

Percent available = (20 – 12) / 18 = 44% available remaining. If your depletion threshold is 50% (i.e., irrigate when <=50% remaining), you should irrigate soon.
Irrigation depth needed to refill to FC:

• Deficit = FC – current VWC = 30% – 20% = 10% VWC.
• Convert to water depth = 0.10 * 450 mm = 45 mm (~1.8 inches).
• Adjust for application efficiency: if pivot efficiency = 85%, applied depth = 1.8 / 0.85 = 2.12 inches.
• Account for expected rainfall: subtract expected inches from applied depth.

Where soil moisture tests change irrigation plans in Arkansas

• Rice production:
• Traditional flooded rice uses standing water; soil moisture tests are most relevant when using water-saving strategies (alternate wetting and drying) or dry-seeding. Tensiometers and soil probes help time re-flooding or irrigation to prevent yield loss while saving water.
• Corn and soybean:
• Soil moisture tests enable variable-rate irrigation scheduling, reduce unnecessary irrigations during frequent summer rains, and prevent mid-season stress during tasseling and pod fill.
• Cotton:
• Deep root systems mean deeper monitoring (24-36 inches) is important. Maintain moderate depletion during vegetative growth; avoid severe stress during bloom and boll formation.
• Horticulture and specialty crops:
• High-value crops benefit strongly from precise soil moisture control; sensors and automated controllers often pay back quickly.
• Turf and landscaping (urban Arkansas):
• Frequent shallow depletions indicate need for irrigation; maintain >60% available water for appearance-sensitive turf.

Practical steps and checklist

Follow this practical checklist to make your soil moisture test actionable:

• Perform an initial gravimetric soil moisture profile to determine FC and PWP by depth.
• Install at least 3 sensor locations across field variability (slope, soil type, irrigation coverage).
• Place sensors in the effective root zone and at deeper layers for crops with deep roots.
• Calibrate sensors to local lab values and check calibration each season.
• Decide crop- and stage-specific depletion thresholds (e.g., 50% for corn/soybean).
• Integrate sensor data with local ET estimates and rainfall records for scheduling.
• Factor irrigation application efficiency when converting target water to run-time or pivot settings.
• Keep records of soil moisture, irrigation events, rainfall, and yields to refine thresholds and demonstrate ROI.

Infrastructure and planning implications

Soil moisture testing should influence not only immediate irrigation decisions but longer-term investment and water management planning:

• Pump and storage sizing: tests help determine peak seasonal demand and whether on-farm storage is needed to smooth pumping loads during dry spells.
• System selection: sandy soils and frequent irrigation favor drip or frequent pivot sets; heavier soils may be suited to occasional, deeper irrigations.
• Water allocation and compliance: quantifiable data supports water use planning, helps justify efficiency measures, and informs permit applications or water-saving programs.
• Capital investments: sensor networks, automated controllers, variable-rate pivot packages, and telemetry can be prioritized where soil moisture tests show large potential water or yield gains.
• Scheduling to flatten peaks: coordinated scheduling based on soil moisture across multiple fields reduces simultaneous peak pump demand and can reduce fuel or electricity costs.

Common pitfalls and how to avoid them

• Relying on a single sensor: soils are spatially variable; use multiple sensors and map differences.
• Ignoring rooting depth variation: sensors too shallow or too deep give misleading signals for crop uptake.
• Not calibrating sensors: factory curves may not fit Arkansas soil textures or salinity levels.
• Applying full refill every irrigation when not needed: aim to refill the usable root zone rather than saturating beyond FC, unless crop management requires saturation.
• Neglecting application efficiency: failing to adjust applied depth to system efficiency leads to under- or over-irrigation.

Final takeaways

Soil moisture testing moves irrigation from guesswork to precision. In Arkansas, where soils and crops vary widely, a measured approach provides:

• Better timing: irrigate when the crop needs water, not on a fixed calendar.
• Smarter volumes: apply only the water required to refill the usable root zone after accounting for efficiency.
• Resource planning: use soil moisture data to size pumps, storage, and automation, and to justify investments in precision irrigation.

Action plan summary:

1. Get a gravimetric profile to define FC and PWP for your fields.
2. Install and calibrate sensors across representative spots.
3. Adopt crop-specific depletion thresholds and calculate irrigation amounts in depth units.
4. Incorporate ET, rainfall, and application efficiency into run-time calculations.
5. Use data to guide long-term system design and water-use agreements.

Implementing soil moisture testing and acting on the results can reduce irrigation volumes, protect yields during critical growth stages, and inform better investments in irrigation infrastructure — essential outcomes for sustainable and profitable Arkansas production.