Estimate soil permanent wilting point from texture and organic matter using Saxton-Rawls pedotransfer functions for irrigation planning.
The Permanent Wilting Point (PWP) Estimator uses Saxton-Rawls pedotransfer functions to approximate the volumetric water content at which plants can no longer extract water from the soil, approximately −1500 kPa (−15 bar) matric potential. Below this water content, most crop plants wilt irreversibly.
PWP depends primarily on clay content and organic matter because these components have the strongest water-holding forces at low matric potentials. Sandy soils have low PWP (5–10% volumetric) while clay soils have high PWP (20–30%), meaning clay soils hold more water but much of it is unavailable to plants.
Together with field capacity, PWP defines the range of plant-available water: AWC = FC − PWP. This is the fundamental number for irrigation scheduling, drought resilience assessment, and crop water budgeting. 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.
Knowing the wilting point along with field capacity gives you the available water capacity — the usable water reserve for your crops. This determines how much water each irrigation should apply and how long crops can go between irrigations. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Saxton-Rawls simplified (2006): θ1500 = θ1500t + (0.14 × θ1500t − 0.02) Where θ1500t = −0.024 × S + 0.487 × C + 0.006 × OM + 0.005 × S×OM − 0.013 × C×OM + 0.068 × S×C + 0.031 S = sand fraction, C = clay fraction, OM = organic matter fraction
Result: PWP ≈ 13.2% (volumetric)
Using Saxton-Rawls with 40% sand, 20% clay, 3% OM: PWP is approximately 13.2% volumetric. Combined with FC of 27.5%, the available water capacity is 27.5 − 13.2 = 14.3%, or about 1.7 inches per foot of soil depth.
AWC = FC − PWP is the most important derived soil water property for agriculture. It determines how much water the soil can supply between irrigations or rain events. Typical AWC values: sands 0.5–1.0 inches per foot, loams 1.5–2.5 inches per foot, clays 1.5–2.0 inches per foot (clays have high FC but also high PWP).
Irrigation is typically triggered when 40–60% of AWC has been depleted. For example, if AWC is 2.0 inches per foot in a 3-foot root zone (6.0 inches total), and MAD is 50%, irrigation triggers when 3.0 inches have been used. This prevents stress while avoiding over-irrigation.
Soils with high AWC provide greater drought buffer. Building organic matter increases AWC by 0.5–1.0 inches per foot per 1% OM increase. Deep-rooted crops access a larger soil volume, effectively multiplying AWC. Combining high-AWC soils with deep roots and mulching creates maximum drought resilience.
At PWP, the matric potential is so low (−1500 kPa) that roots cannot generate enough suction to extract water. Most crop plants wilt irreversibly. Some native plants and succulents can function at even lower potentials.
Technically, some water remains as thin films around particles, but it is held too tightly for plant roots to extract. The water between FC and PWP is considered "plant-available water." The water below PWP is "unavailable" under normal conditions.
OM slightly increases PWP because organic surfaces hold water at high tensions. However, OM increases FC more than it increases PWP, so the net effect of more OM is more available water. Building OM is always beneficial for water storage.
Yes. A pressure plate apparatus applies 15 bars (1500 kPa) of pressure to a saturated soil sample and measures the remaining water content. This is the standard lab method but requires specialized equipment.
Clay particles have enormous surface area and strong electrical charge, creating tight water films. While clay soils hold more total water than sandy soils, a larger fraction is held too tightly for plants. A clay soil at 25% moisture may have less available water than a loam at 20%.
Typical accuracy is ±2–4% volumetric water content. The model performs best for moderate-textured soils and may be less accurate for extreme textures, high-shrink-swell clays, or soils with very high OM (>8%).