Field capacity, wilting point and available water
How soil holds water, what saturation, field capacity and wilting point mean, how texture changes available water, and how to convert between water units.
On this page
A soil is a reservoir. After rain or irrigation it fills; plants draw it down; when too little is left, they wilt. The amount of water a soil can store and release to plants, its available water, decides how long a crop can go between rains, how often to irrigate and how deep roots must grow.
The soil water constants (saturation, field capacity, permanent wilting point) and the arithmetic that links them are among the most used ideas in agronomy and irrigation engineering, and they appear in almost every soil physics question.
How soil holds water
Water is held in the pores between soil particles by two forces: adhesion to particle surfaces and cohesion between water molecules, which together produce capillary forces. The smaller the pore, the more tightly the water is held. Plants must pull against these forces, so what matters is not only how much water is present but how strongly it is held.
That strength is the matric potential (or, as a positive number, the suction or tension), measured in kilopascals (kPa). Wet soil holds water at low suction; as it dries, the remaining water sits in ever smaller pores and films and is held at higher suction.
The constants
| State | Approximate suction | What it means |
|---|---|---|
| Saturation | about 0 kPa | all pores full; no air |
| Field capacity (FC) | about 10–33 kPa | water left after free drainage stops, typically 1–3 days after soaking; 33 kPa is usual for medium and fine soils, 10 kPa for sands |
| Permanent wilting point (PWP) | about 1,500 kPa | plants can no longer extract water and wilt without recovering overnight |
| Hygroscopic coefficient | about 3,100 kPa | water held as thin films on air-dry soil |
| Oven-dry | — | soil dried at 105 °C, the reference for water content |
- Water between saturation and field capacity is gravitational water: it drains away under gravity within a few days, so plants use only a little of it, and it carries leached nutrients with it.
- Water between field capacity and wilting point is available water, the reservoir plants can actually use.
- Water held beyond the wilting point is unavailable.
These are useful conventions, not sharp physical boundaries. "Field capacity" depends on the layering of the profile and how long drainage is allowed; the wilting point varies somewhat among plants. Plants also become stressed well before the wilting point, which is why irrigation is usually scheduled when only part of the available water has been used.
Texture, structure and organic matter
| Texture | Field capacity (m3/m3) | Wilting point (m3/m3) | Available water (m3/m3) |
|---|---|---|---|
| Sand | about 0.10 | about 0.04 | about 0.05–0.08 |
| Sandy loam | about 0.18 | about 0.08 | about 0.10 |
| Loam and silt loam | about 0.28–0.32 | about 0.12–0.14 | about 0.15–0.20 |
| Clay | about 0.40 | about 0.25 | about 0.15 |
Typical values; real soils vary widely.
The pattern is examined often:
- Sands drain quickly and hold little water at any suction; available water is low.
- Clays hold the most water in total, but so much of it is held tightly in tiny pores that their available water is often no greater than a loam's.
- Medium-textured soils, loams and silt loams, usually have the highest available water, which is part of why they are prized.
- Organic matter and good structure increase available water, especially in sandy soils; compaction reduces it by destroying larger pores.
Units and conversions
Water content can be expressed three ways, and confusing them is the commonest error in exams and in practice.
Because the density of water is 1 g/cm3, θv = θm × bulk density (in g/cm3 or t/m3).
Available water in a root zone
A loam has gravimetric water contents of 24 per cent at field capacity and 11 per cent at the wilting point, with a bulk density of 1.4 g/cm3. The crop roots to 60 cm.
- Volumetric water at FC = 0.24 × 1.4 = 0.336 m3/m3; at PWP = 0.11 × 1.4 = 0.154 m3/m3.
- Available water = 0.336 − 0.154 = 0.182 m3/m3.
- Depth of available water in 600 mm of soil = 0.182 × 600 ≈ 109 mm.
If the crop uses 5 mm a day and irrigation is applied when half the available water is used, the interval is 109 × 0.5 ÷ 5 ≈ 11 days, and each irrigation must replace about 55 mm (550 m3 per hectare).
Measuring water content in the laboratory
A moist soil sample weighs 150.0 g; after drying at 105 °C to constant weight it weighs 125.0 g.
- Water = 150.0 − 125.0 = 25.0 g. Gravimetric water content = 25.0 ⁄ 125.0 = 0.20 (20 per cent), always on the dry weight.
- On a wet-weight basis it would be 25.0 ⁄ 150.0 = 16.7 per cent. Soil science uses the dry basis; grain and food science often use the wet basis. Always say which.
Measuring soil water
- Gravimetric (oven-drying): sample, weigh, dry at 105 °C for about 24 hours, weigh again. Direct and accurate, but destructive and slow.
- Tensiometers: a porous cup filled with water, connected to a gauge, measures suction directly in the wet range (up to about 80 kPa), useful for irrigation scheduling.
- Electrical sensors: time-domain reflectometry (TDR), capacitance and frequency-domain probes measure volumetric water through the soil's dielectric properties; widely used for continuous monitoring.
- Neutron probes: measure hydrogen, hence water, in access tubes; accurate but need licensing for the radioactive source.
- Pressure plate apparatus in the laboratory: holds saturated samples at set suctions (such as 33 and 1,500 kPa) to measure field capacity and wilting point.
- Satellite and remote sensing: estimates of surface soil moisture over large areas.
Key points
- Soil water is held by adhesion, cohesion and capillarity; the strength is the matric potential or suction, in kPa.
- Field capacity is about 33 kPa (10 kPa for sands); permanent wilting point about 1,500 kPa.
- Available water = water at FC − water at PWP; gravitational water drains away.
- Loams usually have the most available water; sands the least; clays hold most water but much is unavailable.
- θv = θm × bulk density; depth of water = θv × soil depth.
- Gravimetric water content in soil science is on the oven-dry basis.
What the exams ask
- School-leaving and entrance papers: definitions of field capacity, wilting point, available and gravitational water, and how texture affects them.
- Degree and research-fellowship entrances: matric potential and suction values, retention curves, gravimetric and volumetric conversions, and measurement methods.
- Irrigation and agricultural engineering examinations: available water in a root zone, irrigation depth and interval, and sensor-based scheduling.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does a clay soil feel wet but the crop wilts?
Clay holds a lot of water in very small pores at high suction. A clay can contain more water than a loam at the wilting point, but that water is held too tightly for roots to take it up.
Is field capacity a fixed number for a soil?
Not exactly. It depends on how drainage is defined and on layers in the profile; a sandy layer over clay, for example, can hold more water above the boundary. The 33 kPa value is a laboratory convention that approximates field behaviour for many soils.
How can I increase my soil's available water?
Building organic matter and good structure helps most in sandy soils, as do mulches that cut evaporation and practices that deepen rooting. Texture itself cannot be changed at field scale.
Sources
- Hillel, D. Introduction to Environmental Soil Physics. Academic Press, 2004.
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 5, Soil water: characteristics and behavior.
- Allen, R. G. et al. Crop Evapotranspiration. FAO Irrigation and Drainage Paper 56, 1998. Chapter 8 (soil water availability).
- Saxton, K. E. and Rawls, W. J. "Soil water characteristic estimates by texture and organic matter for hydrologic solutions." Soil Science Society of America Journal 70, 1569–1578 (2006).
- Dane, J. H. and Topp, G. C. (eds) Methods of Soil Analysis, Part 4: Physical Methods. Soil Science Society of America, 2002.
More in soil science
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.