Estimate soil field capacity from texture and organic matter using Saxton-Rawls pedotransfer functions. Plan irrigation scheduling.
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. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Field capacity is the key number for irrigation scheduling — it defines how much water your soil can hold. Estimating FC from easily obtained soil texture and OM data eliminates the need for expensive laboratory retention curves for routine planning. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Saxton-Rawls simplified (2006): θ33 = θ33t + (1.283 × θ33t² − 0.374 × θ33t − 0.015) Where θ33t = −0.251 × S + 0.195 × C + 0.011 × OM + 0.006 × S×OM − 0.027 × C×OM + 0.452 × S×C + 0.299 S = sand fraction (0–1), C = clay fraction (0–1), OM = organic matter fraction (0–1)
Result: FC ≈ 27.5% (volumetric)
Using Saxton-Rawls with 40% sand, 20% clay, 3% OM: the intermediate value θ33t is computed, then adjusted to give a field capacity of approximately 27.5% volumetric. This means every 12 inches of soil depth can hold about 3.3 inches of water at FC.
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.
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.
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.
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.
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.
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).
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.
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.
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.