Field Capacity vs. Permanent Wilting Point: The Soil Moisture Numbers That Tell You When to Water

A clay soil reading 25% moisture can be drier, as far as a plant is concerned, than a sandy soil reading 8%. That is not a sensor error. The difference comes down to two thresholds every soil has: field capacity, the most water a soil can hold against gravity, and the permanent wilting point, the moisture level below which roots can no longer pull water out fast enough to survive. The clay at 25% is three points above its wilting point. The sand at 8% is four points above its own, in a soil that only holds six usable points to begin with. Everything a plant can use lives in the gap between those two numbers, and the gap is different for every soil.

This guide explains the biology of wilting and why plants are in trouble long before the "permanent" threshold, the soil physics behind both numbers with values for common textures, the arithmetic that converts them into a watering trigger and a run time, how the thresholds shift through the season and between lawns, containers, and trees, the mistakes that come from treating one number as universal, and exactly where to set a soil moisture sensor so it watches the right threshold.

Part 1: The biology — what wilting is, and why "permanent" arrives late

A leaf stands up because its cells are inflated with water under pressure — turgor. On a hot afternoon, a plant can lose water through its leaves faster than roots can replace it even in moist soil, and the leaves droop. NC State Extension calls this the protective response of a well-hydrated plant: "If you notice wilting between 12 and 4 p.m., but the plants perk up again by early evening, you're likely seeing a normal, temporary heat response" (NC State Extension, 2025). That is temporary wilting. The soil still has water; the plumbing is just behind demand.

Permanent wilting is a soil condition, not a weather condition. The concept dates to 1912, when USDA scientists Lyman Briggs and Homer Shantz grew indicator plants in drying soil and recorded the moisture content at which they wilted and failed to recover in a humid chamber — the "wilting coefficient" (Briggs & Shantz, USDA Bureau of Plant Industry Bulletin 230). Veihmeyer and Hendrickson standardized the field methods in Soil Science 68:75–94 (1949), and Richards and Weaver showed that the water a soil holds at 15 atmospheres of suction — about 1,500 kilopascals — tracks the permanent wilting percentage closely enough to measure it in a lab instead of with a greenhouse full of wilting indicator plants (Soil Science 56:331–340 (1943)). That 15-bar convention is still the standard.

Two things follow. First, plants are stressed well before the permanent wilting point. Colorado State's irrigation scheduling guide puts it directly: "plants start to experience water stress even before soil water is depleted down to PWP" (CSU Fact Sheet 4.707). Growth slows, stomata close, and in turf the first visible sign is footprinting — "drought-stressed turf does not spring back after foot traffic" — followed by a blue-gray color (University of Nebraska–Lincoln NebGuide G2191). Second, the threshold differs by species. A 2012 meta-analysis of 317 species found that the leaf water potential at which turgor is lost correlates strongly with the water availability of the plant's native habitat — drought-adapted species keep functioning at soil moisture levels that would collapse a hydrangea (Bartlett, Scoffoni & Sack, Ecology Letters 15:393–405).

"Permanent" also does not mean dead. An established Kentucky bluegrass lawn pushed past wilting goes dormant and regenerates from rhizomes when water returns; our summer dormancy guide covers that strategy. A newly planted shrub or a container tomato has no such reserve. The threshold is the same; the consequences are not.

Part 2: The soil physics — two thresholds and the gap between them

Soil holds water in pores and as films on particle surfaces, and it holds that water with a suction called matric potential. Right after a soaking rain, the large pores are full and water drains under gravity for a day or two. What remains when drainage slows is field capacity, held at a suction of roughly 10–33 kPa (Oklahoma State Extension BAE-1537). As roots and evaporation remove water, the remaining films get thinner and cling harder. At about 1,500 kPa of suction — the permanent wilting point — roots can no longer extract water fast enough to keep up with demand. South Dakota State describes the texture effect: "A strong negative force is required for water to be released from a clay soil texture; however, water is easily released from sandy soils" (SDSU Extension).

The difference between the two thresholds is total available water (TAW), sometimes called available water capacity or plant-available water. It is the only number that tells you how much a soil can actually offer a plant. The table below gives U.S. average volumetric values from Oklahoma State's compilation (BAE-1537, Table 1), alongside SDSU's figures for inches of available water stored in the top foot of soil.

Volumetric values: OSU BAE-1537, Table 1 (U.S. averages, after Ratliff et al. 1983 and Hanson et al. 2000). Inches per foot: SDSU Extension, Table 1; SDSU's own laboratory values differ slightly from the U.S. averages, which is itself the point — your soil will too.

Read the table and the opening paradox resolves. Clay holds 40% water at field capacity but keeps 22% locked away below the wilting point, so a clay reading of 25% means the plant has used 15 of its 18 available points — 83% depleted and wilting. Sand at 8% has used 2 of 6 points — a third depleted, comfortably fine. The loams and silt loams in the middle of the table are the soils gardeners love because they hold the most usable water, not the most water. Our guide to clay versus sandy soil works through what that means for schedule design, and the organic matter guide covers the one lever that widens the gap over time.

Part 3: The practical math — from two thresholds to a watering trigger

Irrigation science does not wait for the wilting point. It waters when a set fraction of the available water is gone — the management allowable depletion, or MAD. For turf, the number is remarkably consistent across sources: FAO Irrigation and Drainage Paper 56 lists a depletion fraction of 0.40 for cool-season turfgrass and 0.50 for warm-season (Allen et al., 1998, Table 22); Oklahoma State uses the same 0.4 and 0.5; and Utah State states the rule in plain language: "irrigation is recommended when 50 percent of the water has been used from the root zone" (Hill & Kopp, Utah State University Extension).

Step 1: Calculate your trigger reading

Oklahoma State gives the formula that turns MAD into a sensor setting: trigger VWC = field capacity − (MAD × total available water). For a loam at 50% MAD: 27 − (0.5 × 15) = 19.5%, call it 20%. For sand: 10 − (0.5 × 6) = 7%. For clay: 40 − (0.5 × 18) = 31%. Three soils, three triggers, and the clay's "water now" reading is higher than the sand's "completely saturated" reading.

Step 2: Calculate how much to apply

The refill is the depleted fraction times the available water in the root zone. Utah State's rule of thumb: soils hold "about 1 inch per foot of depth in a sandy soil to about 2 inches per foot of depth in a loam soil," and most turf roots are in the top 1 to 1.5 feet. For a loam lawn with roots 12 inches deep, 1.7 inches of available water is stored; at 50% depletion, apply about 0.85 inch net. For the same lawn in sand, 0.6 inch is stored and the refill is 0.3 inch — which is why sandy lawns need small, frequent cycles and loam lawns can take a deep soak once or twice a week. One inch over 1,000 sq ft is about 623 gallons (from the USGS figure of 27,154 gallons per acre-inch, USGS Water Science School), so the loam refill is roughly 530 gallons per 1,000 sq ft and the sand refill about 190.

Step 3: Convert inches to minutes

Run the catch-cup test to measure your zone's precipitation rate, then runtime = inches ÷ (inches per hour) × 60. A zone delivering 0.5 inch per hour needs about 100 minutes for the loam refill above — split into cycles if the soil can't absorb that fast — and 36 minutes for the sand.

Step 4: Check the plant before you trust the number

Published thresholds are averages; your soil is not. The two-minute check is the one NC State recommends: "Stick your finger into the soil up to your second knuckle. If it feels damp, the wilting is probably temporary." If the soil is damp and the plant is still wilted the next morning, the problem is not the water supply — skip to Part 5.

Part 4: How the thresholds shift through the season and across the yard

Spring. Roots are shallow after winter and the soil column holds plenty of water, so the effective root zone — not the soil — is the limit. Use the lower MAD (40%) and shorter cycles until roots deepen. Newly seeded or sodded areas are a special case: keep the surface near field capacity, not at a depletion trigger, until establishment; our new seed and sod guide has the schedule.

Summer. Deeper roots let you use the full 50% depletion, and the refill gets bigger. Kentucky bluegrass concentrates most of its roots "in the top 6 to 10 inches of soil when mowed regularly," while tall fescue can root "3 feet or more in depth if soil conditions allow" (UNL G2191) — so a tall fescue lawn in loam has roughly three times the available water reservoir of a bluegrass lawn in the same soil, and can go days longer between irrigations. FAO-56 also notes that at high evapotranspiration rates the depletion fraction should drop (its adjustment is p + 0.04 × (5 − ETc), with ETc in mm/day), which is the formal way of saying: in a heat wave, water a little sooner.

Fall. Evapotranspiration falls, intervals stretch, and the trigger stays the same — the soil just takes longer to reach it. Resist the urge to keep summer run times; see our fall watering guide.

Trees and shrubs. Deeper roots, bigger reservoir, slower cycles. The same thresholds apply, but the sensor belongs 6–8 inches down at the edge of the root ball or under the canopy, and the refill is measured in gallons per inch of trunk diameter rather than inches of depth.

Containers. Potting mixes are engineered for drainage, not storage. NC State's substrate laboratory recommends a container capacity of 45–65% by volume with available water of only 25–35% and an equal share — 25–35% — unavailable below 1,500 kPa (Bilderback et al., HortTechnology 15:747–751). A small pot in July can travel from container capacity to wilting in a day; our container watering guide explains the pot-weight method that beats any chart.

Part 5: Common mistakes with field capacity and wilting point

1. Treating "15" as a moisture percentage. The 15 in the classic definition is 15 bars (atmospheres) of suction, not 15% water content. At 15 bars a sand holds about 4% water and a clay about 22%. A sensor threshold copied from a forum post about a different soil is a threshold for a different soil.
2. Waiting for wilt to water. By the time an established plant wilts and stays wilted, it has been losing growth for days. The MAD approach exists because stress begins at 40–50% depletion, not at 100%.
3. Watering a plant that wilts in wet soil. Root rot and salt both mimic drought. UC IPM on Phytophthora: "The leaves of plants affected by Phytophthora root and crown rot appear drought stressed," and "unlike plants suffering from water stress, plants don't recover when irrigated" (UC IPM Pest Notes 74133). Utah State calls high soil salinity "chemical drought," because "affected plants show visual symptoms similar to those plants suffering from a lack of water" (USU Extension). More water makes the first worse and does nothing for the second. Our overwatered lawn recovery guide covers the first case.
4. Using a national average for your backyard. Oklahoma State publishes a second table of measured Oklahoma soils next to the U.S. averages: its silt loam has a field capacity of 23%, not 30%. Compaction, organic matter, and gravel content all move the numbers. Averages set the starting point; a 24-hour soak test sets your actual field capacity.
5. Confusing total water with available water. A soil probe that reads "moist" in clay may be reporting the 22% a plant can never use. Judge depletion against the gap, not against zero.
6. Letting a new transplant or a pot approach the wilting point. A nursery root ball is often a soilless mix with a tiny reservoir sitting inside native soil with a bigger one; the sensor in the bed says fine while the root ball is at 15 bars. Check both until roots have grown out.
7. Placing the sensor where the soil is not representative. Next to a sprinkler head, in a low spot, or in the one patch of imported topsoil. The threshold is only as good as the soil the sensor is sitting in.

Part 6: Sensor placement and thresholds

Place a soil moisture sensor in a representative part of the root zone, away from sprinkler heads, low spots and unusual patches of soil. UF/IFAS recommends about 3 inches for the turf sensor controllers covered in EDIS AE437; follow the installation depth for your own device. Calibrate after the soil has drained following a thorough watering, then compare the reading with plant condition and a root-zone check. A Soildrops sensor has a published accuracy specification of ±3% volumetric water content under normal conditions. That uncertainty can be important in a narrow available-water range, especially in sand, so do not assume a one-point change is precise enough to control watering on its own.

Triggers are field capacity minus MAD × total available water, using OSU BAE-1537 Table 1 values and the FAO-56 depletion fractions of 0.40 and 0.50. Replace the field capacity column with your own 24-hour soak reading and recompute. In Autopilot mode, the Soildrops controller uses paired sensor readings, weather and zone settings to plan irrigation. These calculated depletion thresholds are educational starting points, not a claim that every controller implements a real-time two-threshold start/stop loop; follow the documentation for your model and check the resulting root-zone moisture. The research on this approach is unusually strong: in University of Florida trials on bermudagrass, soil moisture sensor control cut irrigation 69–92% during wet weather (Cardenas-Lailhacar, Dukes & Miller, J. Irrig. Drain. Eng. 134:120–128) and 16–83% during dry weather depending on brand and period (J. Irrig. Drain. Eng. 136:184–193), and Dukes' review of controlled studies found savings "anywhere from 40% to more than 70%" (Transactions of the ASABE 55:563–569). Savings vary with the previous schedule, weather and site conditions; the trial results above are not a measured outcome for Soildrops customers. Our sensor placement guide covers the siting details that make the difference.

Frequently asked questions

What is the difference between field capacity and permanent wilting point?

Field capacity is the water a soil holds after gravity drainage stops, at roughly 10–33 kPa of suction — the top of the usable range. Permanent wilting point is the water left at about 1,500 kPa, when roots can no longer extract it fast enough — the bottom. The difference is total available water, and it ranges from about 6% by volume in sand to 15–20% in loams and clays.

What soil moisture percentage should I water at?

There is no universal number. Water when 40–50% of available water is gone: about 7–8% VWC in sand, 15–16% in sandy loam, 20–21% in loam, 22–24% in silt loam, and 31–33% in clay. Confirm your soil's field capacity with a 24-hour soak reading and compute from there.

Is the permanent wilting point always 15%?

No. The 15 refers to 15 bars of suction, the laboratory standard established by Richards and Weaver in 1943. The water content at 15 bars is about 4% in sand, 12% in loam, and 22% in clay.

Can a plant recover from the permanent wilting point?

Often, if water arrives soon. Established lawns go dormant and regrow; perennials and shrubs lose leaves and may recover from buds; seedlings, new transplants, and container plants are the most likely to die. Prolonged time at the wilting point kills fine roots even in survivors, which is why irrigation scheduling triggers at 40–50% depletion.

Why does my plant wilt when the soil is wet?

Because the roots cannot use the water. Root rot from saturated soil, high soil salinity, and transplant root damage all produce drought symptoms in moist soil. Check the soil first: wet soil plus persistent wilt means stop watering and investigate drainage, salts, and roots.

How do I find my own soil's field capacity?

Soak the sensor location with at least 5 gallons, wait 24 hours for drainage, and read the sensor. That reading is your field capacity. Set your trigger 40–50% of the way down toward the wilting point for your texture class, then adjust by a point or two based on how the lawn looks the day before it would water.

The bottom line

Field capacity and permanent wilting point are the two numbers that make a soil moisture reading mean something. Between them lies the only water a plant can use, and the size of that gap — six points in sand, fifteen in loam, eighteen in clay — decides how often and how much to water far more than any calendar. Irrigation science waters at 40–50% depletion of that gap, not at the wilting point, because stress starts early and recovery is cheapest before it begins.

The audit you can do today: identify your soil texture with a jar test or your county soil survey, soak one spot in each zone, and read it with a sensor or a soil probe after 24 hours — that is your field capacity. Find your texture row in the Part 6 table, compute your trigger, and compare it to whatever your controller is doing now.

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About this article. Researched and written by the Soildrops content team, drawing on Oklahoma State University Extension BAE-1537, FAO Irrigation and Drainage Paper 56, South Dakota State University Extension, Utah State University Extension, Colorado State University Extension, University of Nebraska–Lincoln NebGuide G2191, NC State Extension, UF/IFAS EDIS AE437, UC IPM, USGS, and peer-reviewed research in Soil Science, Ecology Letters, HortTechnology, the Journal of Irrigation and Drainage Engineering, and Transactions of the ASABE, including the original 1912 USDA bulletin that defined the wilting coefficient. All scientific claims are linked to original sources. Last updated October 2026.