Permanent Wilting Point Estimator

About the Permanent Wilting Point Estimator

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 page estimates the lower end of that range so field capacity and depletion targets can be converted into usable available water.

When This Page Helps

Wilting point is rarely measured directly, but it is needed for any real available-water calculation. This page fills that gap from texture and organic matter.

How to Use the Inputs

1. Enter the sand percentage from your particle size analysis.
2. Enter the clay percentage.
3. Enter the organic matter percentage.
4. Review the estimated permanent wilting point.
5. Subtract PWP from field capacity to get available water capacity.

Example Calculation

Tips & Best Practices

• PWP is primarily driven by clay content — more clay means more water held too tightly for plants.
• Sandy soils: PWP ≈ 5–10%. Loams: 10–18%. Clays: 20–30%.
• Organic matter increases PWP slightly but increases FC more, so net AWC still increases with OM.
• Some drought-tolerant crops (sorghum, sunflower) can extract water slightly below conventional PWP.
• PWP is a threshold, not a cliff — crop stress begins well above PWP (at 50–60% depletion of AWC).
• For irrigation scheduling, trigger irrigation at 40–50% depletion (not at PWP).

Available Water Capacity

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).

Managed Allowable Depletion

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.

Drought Resilience

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.

Frequently Asked Questions

• What happens at the permanent wilting point?

  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.

• Is there really no available water below PWP?

  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.

• How does organic matter affect wilting point?

  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.

• Can I measure PWP in the lab?

  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.

• Why do clay soils hold more unavailable water?

  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%.

• How accurate is the Saxton-Rawls estimate for PWP?

  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%).