The wilting point is one of the most critical parameters in irrigation management , yet it is often reduced to a simple threshold value. Understanding exactly what happens to the soil and plants when this limit is approached makes the difference between timely intervention and production damage that is difficult to reverse.
What is the permanent wilting point?
The permanent wilting point (PWP) is the soil water content below which the plant can no longer extract water. This is not a conventional threshold, but corresponds to a soil water potential equal to or lower than -1.5 MPa (about 15 atm, pF 4.2-4.4). At this level, water is still physically present in the soil, but it is retained in the micropores and adsorbed onto the colloids with a force greater than the suction pressure of the roots. The plant is unable to overcome this tension and stops absorbing water.
Soil at the point of permanent wilting isn’t necessarily dry : there’s water, but the soil holds it with a force that roots can’t overcome. This is particularly evident in clay soils, where the volumetric water content can still be high while still being completely inaccessible to crops.
From an operational perspective, this has a direct consequence : relying only on visual observation of the soil or its surface texture often leads to underestimating the severity of the ongoing water stress, especially in medium- and fine-textured soils where the risk is most subtle.
The pF scale and water potential: operational references
To understand the wilting point in daily irrigation management, it’s useful to recall reference pF values. The pF expresses the cologarithm of the tension with which the soil retains water: the higher the value, the more difficult it is for the plant to extract residual water.
| Soil condition | pF | Water potential (kPa) |
| Water saturation | 0 | 0 |
| Field capacity | 2.0 – 2.5 | from -10 to -33 |
| Stress threshold (indicative, varies by crop) | 3.0 – 3.5 | from -100 to -300 |
| Permanent wilting point | 4.2 | approximately -1,500 |
| Air-dried | 7 | approximately -100,000 |
Field capacity corresponds to the water content of the soil two to three days after a rainfall or irrigation event, once the excess water has been removed by gravity. In the most widely used international standards, the soil is considered to be at field capacity when the water potential reaches -33 kPa . It is important not to confuse it with saturation : at saturation, all the pores are occupied by water; at field capacity, air and water coexist in the spaces between the particles.
How the wilting point varies according to soil texture
Texture is the factor that influences both field capacity and permanent wilting point more than any other. Sandy soils have large particles with a small specific surface area: they retain little water in total, and the water retained at the wilting point is a minimal fraction of the volume. A sandy soil reaches field capacity below 10% VWC ; a clayey soil , with very fine particles and a very high specific surface area, can exceed 40% VWC at field capacity and contain a water content at the wilting point that in a sandy soil would already correspond to the field capacity. Silty soils are around 25% VWC .
The following table reports the fraction of available water (cm/cm of soil) for the main USDA textural classes, distinguishing between superficial and deep horizons. These data are essential for calculating the actual water reserve of the layer explored by the roots and correctly setting irrigation rotations and volumes.
| Weaving (USDA) | Available water – surface horizon (cm/cm) | Available water – deep horizon (cm/cm) |
| Sandy | 1.2 | 0.7 |
| Sandy loam | 1.3 | 0.9 |
| Sandy loam | 1.7 | 1.5 |
| Franca | 2.0 | 1.9 |
| Frank limosa / Limosa | 2.3 | 2.2 |
| Clayey silty loam | 1.9 | 1.7 |
| Clay loam | 1.8 | 1.6 |
| Sandy clay loam / Sandy clay loam | 1.7 | 1.5 |
| Clayey | 1.7 | 1.6 |
Approximately half of the water present at field capacity is held too tightly to be extracted by the roots: this percentage defines the permanent wilting point. Of the water actually available between field capacity and PAP, only 40–60% can be extracted without inducing physiological stress . The remainder is technically available but accessible only with increasing root stress, which already compromises photosynthesis and growth.
What happens to the plant: physiology of wilting stress
Approaching the wilting point is not a binary process: water stress sets in progressively, well before the plant reaches the PAP. Understanding the sequence of physiological events helps us intervene appropriately .
The first responses: closure of stomata and reduction of photosynthesis
When the soil water potential falls below the crop’s comfort threshold (which varies by species and phenological stage), the plant initiates a series of defense responses:
- Stomatal closure: the first defense mechanism. It reduces transpiration but simultaneously blocks CO₂ entry, compromising photosynthetic efficiency. This mechanism is closely linked to current atmospheric conditions: barometric pressure and VPD directly influence the intensity of transpiration and can exacerbate ongoing stress.
- Reduction of cell turgor: the cells lose rigidity, the leaves tend to roll up or lose the erect shape typical of the species.
- Reduced cell elongation: Growth slows before any obvious visible symptoms appear.
- Blocked carbohydrate synthesis: With reduced photosynthesis, the energy reserves available for growth and maturation decrease.
The vegetative stop
If stress persists, it leads to vegetative stunting : an almost complete cessation of growth . In agronomic terms, this translates into reduced fruit growth, imbalances in ripening, flower abortion in sensitive crops (such as citrus fruits), and, in the most severe cases, loss of root function . After prolonged stress, there is a point of no return beyond which, even when irrigation is resumed, the plant is no longer able to recover its normal absorption functions .
Particularly sensitive crops
Not all crops react in the same way to pre-wilting stress . The following table summarizes the sensitivity of the main Mediterranean crops and the water potentials of concern threshold:
| Culture | Stress threshold (kPa / MPa) | Main consequence of pre-PAP stress |
| Tomato | -30 / -60 kPa | Reduced flowering, blossom-end rot, decreased yield |
| Screw | -600 / -1200 kPa | Reduction in bunch growth, sugar concentration (manageable as controlled stress) |
| Olive tree | variable by variety | If moderate: manageable; if prolonged, compromises productivity and longevity |
| Citrus fruits | -50 / -100 kPa | Flower abortion, fruit drop, reduction in commercial quality |
| Corn | -30 / -60 kPa | Rapid vegetative stop, significant yield losses |
| Onion (surface roots) | -40 / -60 (layer 0-60 cm) | Quickly exhausts useful reserve due to limited root depth |
How to determine the wilting point in the field
There are two main approaches to estimating PAP under operating conditions :
1. Estimation from soil texture
For most production contexts, a texture-based PAP estimate is sufficiently accurate . Useful data can be obtained from the USDA classification and the available water values reported in the previous table. This approach requires soil particle size analysis , which should be available on every farm with structured agronomic management.
2. Direct measurement with dedicated instrumentation
For those working in research or precision irrigation management , accurate determination of PAP requires specific instrumentation. The reference method involves the use of a pressure chamber (Richards plate) or instruments such as the WP4C (dew point potentiameter), which directly measures the soil sample’s water potential. Field estimation is possible with volumetric moisture sensors and water potential sensors installed at multiple depths in the root zone.
The most widespread operational methodology for estimating field capacity in the field includes:
- Install soil moisture sensors at multiple depths (early, mid, and underroot zones) before the growing season.
- Monitoring water content immediately after an irrigation event or rainfall.
- Read field capacity after 3-10 days (depending on texture: 3 days for sandy soils, up to 10 for fine-textured soils), when the water content stops decreasing due to gravity.
- Estimation or direct measurement of PAP via WP4C or soil moisture release curve.
Detect stress before wilting point becomes a limit
The distance between field capacity and permanent wilting point represents the margin within which all irrigation management takes place. The problem is that the readily available water is only a fraction of this interval: irrigating when the crop has already reached the stress threshold means intervening late compared to actual physiological needs. Plantvoice works to monitor the plant’s internal water status through biosensors that detect early biochemical signals of stress , anticipating irrigation decisions.
Factors that modify the useful water capacity of the soil
Knowing your soil’s wilting point is the starting point. But the actual water capacity of the plot depends on other factors that the agronomist can partially modify:
- Organic matter: Every 1% of organic matter increases available soil water by approximately 10%. Increasing SOM through organic amendments or covercrops structurally improves the water reserve available to crops.
- Compaction: Compacted soil reduces effective porosity and therefore available water capacity. Subsoiling in compacted layers increases effective root penetration and accessible water reserves.
- Root depth: A soil with a root depth of 150 cm and low available water can hold more useful water reserves than a soil with high availability but roots limited to 50 cm. Compaction management directly impacts this parameter.
- Rock fragments: reduce the available water capacity in direct proportion to their volume, unless the rocks are porous.
- Salinity: an electrical conductivity above 2-4 dS/m begins to reduce effective water availability due to osmotic pressure, with significant impacts on sensitive crops even at relatively low values.
FAQ – Frequently Asked Questions about the wilting point
1. Is the wilting point the same for all crops?
No. The conventional value of -1.5 MPa is a general reference valid for most cultivated herbaceous and tree crops. Some xerophytic or drought-adapted species can extract water at much lower potentials (as low as -3 or -4 MPa). For some sensitive horticultural crops , production stress begins well before the PAP, at -0.03/-0.06 MPa. The operational irrigation threshold must therefore be calibrated to the specific crop and phenological stage .
2. Does clay soil have a higher or lower wilting point than sandy soil?
Clay soil reaches wilting point at a higher volumetric water content than sandy soil. This means that, in absolute terms, clay soil contains more water at the PAP, but that portion is completely inaccessible to the plant. In sandy soil , the PAP threshold corresponds to a very low volumetric water content (often below 5%), meaning that the available water is quickly used up, but the remaining water is still inaccessible.
3. How is the wilting point estimated without specific instruments?
The most accessible estimate is based on soil textural classification (particle size analysis) and tabulated values of available water by USDA class . For a more accurate estimate in the field, volumetric moisture sensors can be installed and soil behavior observed after rainfall or irrigation events. For research applications or to set up precision irrigation systems, direct measurement via WP4C remains the gold standard.
Do you water when you see symptoms or when the plant needs it? Plantvoice monitors the plant’s internal water status using biosensors, so you can intervene at the right time. Book a demo!
