Field Capacity Estimator

About the Field Capacity Estimator

The Field Capacity Estimator uses Saxton-Rawls pedotransfer functions to approximate the volumetric water content at field capacity (approximately −33 kPa or −1/3 bar tension) based on soil texture percentages and organic matter content. Field capacity (FC) represents the amount of water a soil holds after drainage by gravity has ceased, typically 24–48 hours after saturation.

Field capacity is the upper limit of readily available water for plant growth. Irrigation scheduling aims to refill the soil to field capacity without exceeding it (which wastes water to deep percolation). Knowing FC for your specific soil allows precise calculation of irrigation amounts.

The Saxton-Rawls equations are empirical relationships developed from thousands of soil samples. They provide reasonable estimates when laboratory measurement is not available, with typical accuracy of ±2–5% volumetric water content. This page estimates FC from texture and OM when you do not have a full water-retention curve, which is often enough for routine irrigation planning.

When This Page Helps

Field capacity matters because it sets the top of the refill range. This page gives a practical estimate from ordinary soil-test inputs.

How to Use the Inputs

1. Enter the sand percentage from a particle size analysis.
2. Enter the clay percentage.
3. Enter the organic matter percentage from a soil test.
4. Review the estimated field capacity (volumetric %).
5. Use this value along with wilting point to calculate available water capacity.

Example Calculation

Tips & Best Practices

• Saxton-Rawls estimates are approximate — lab-measured retention curves are more accurate for high-value crops.
• Each 1% increase in organic matter can increase FC by 1–2% volumetric.
• Coarse-textured soils (sands): FC ≈ 10–18%. Medium (loams): 22–32%. Fine (clays): 30–45%.
• FC decreases with compaction because large pores are compressed, reducing drainage equilibrium.
• Use FC together with wilting point to determine available water capacity (AWC = FC − PWP).
• In the field, FC can be measured by saturating a leveled area, covering to prevent evaporation, and sampling after 48 hours.

The Saxton-Rawls Model

Developed by K.E. Saxton and W.J. Rawls (2006), this pedotransfer function estimates soil water retention from easily measured soil properties: sand%, clay%, and organic matter%. The model was calibrated on the USDA NRCS soil survey database encompassing over 2,500 soil samples. It provides FC (−33 kPa) and PWP (−1500 kPa) estimates.

Applications Beyond Irrigation

Field capacity data is used in hydrological modeling, erosion prediction (USLE and WEPP models), contaminant transport studies, and engineering design. Wetland delineation, septic system design, and stormwater management all require knowledge of soil water retention properties.

Improving Field Capacity

While texture is fixed, management practices that increase organic matter and improve structure will increase FC. Cover crops, compost application, reduced tillage, and diverse rotations build OM over years, gradually increasing the soil’s water storage capacity and drought resilience.

Frequently Asked Questions

• What is field capacity exactly?

  Field capacity is the soil water content after free drainage has essentially stopped, typically 24–48 hours after saturation in well-drained soils. It corresponds to approximately −33 kPa (−1/3 bar) matric potential. At FC, macro-pores have drained but micro-pores remain water-filled.

• How does organic matter affect field capacity?

  Organic matter increases FC primarily by creating micro-pores and increasing water-holding surface area. Each 1% OM increase typically raises FC by 1–2% volumetric. This is one of the strongest arguments for building soil organic matter.

• Is the Saxton-Rawls method accurate?

  The Saxton-Rawls pedotransfer functions have been validated on thousands of U.S. soils and typically estimate FC within ±3–5% volumetric water content. Accuracy is lower for extreme soils (very sandy, very high OM, or high-shrink-swell clays).

• Can I measure FC in the field?

  Yes. Saturate a representative area (at least 1m × 1m) to full depth, cover with plastic to prevent evaporation, wait 48 hours, then sample and measure volumetric water content gravimetrically. This is the field-based definition of FC.

• Why does field capacity matter for irrigation?

  FC defines the maximum soil water storage your irrigation can refill. Applying more water than (FC − current moisture) causes deep percolation — wasted water and leached nutrients. Applying less leaves the soil below full storage.

• What is the difference between field capacity and saturation?

  At saturation, all pores (macro and micro) are filled — typically 45–55% volumetric. At FC, macro-pores have drained to air, leaving only micro-pores water-filled — typically 20–40%. The difference drains freely by gravity.