October 17, 2026

Soil water definition for the part your plants can use

Soil water is the water held in the pore spaces between soil particles and in thin films on their surfaces. Soil scientists sort it into three kinds by how tightly the soil holds it, called gravitational, capillary, and hygroscopic. Only the capillary water, the middle band, is water your plants can use.

The three kinds of soil water

Soil is a mix of mineral grains, organic matter, air, and water. The gaps between grains come in many sizes. Water's grip on the soil depends on the size of the gap it sits in. Large pores hold water loosely. Small pores and grain surfaces hold it tightly.

• Gravitational water. After a heavy rain, water fills the large pores and gravity pulls it down through the soil. It drains away before roots can use much of it, and while it sits there it pushes out the air roots need.
• Capillary water. Once the big pores drain, water stays in the small pores, held by surface tension and its attraction to grain surfaces. Roots can pull this water out, so soil scientists call it plant-available water.
• Hygroscopic water. A thin film clings to each grain so tightly that no root can pull it free. Soil that looks and feels dry still carries it.

Two points mark the edges of the useful band. Field capacity is the state of the soil after gravitational water has drained out. The permanent wilting point is where the remaining water is held so tightly that plants wilt and don't recover overnight. The water between those two points is the plant-available water.

How roots pull water from soil

Water leaves a leaf through its stomata as vapor. That loss puts the water column inside the plant's xylem under tension, and the tension pulls water up from the roots. Roots take in water when the pull inside them is stronger than the soil's grip on the water around them.

As soil dries, the loosely held water goes first. What remains sits in smaller pores and thinner films, so the soil holds each remaining drop harder. The plant has to pull harder to get the same amount. Uptake slows well before the soil runs out of water, and leaves can droop in afternoon heat while the bed still holds moisture. Our guide on where water enters a plant follows that path through the root zone.

Texture changes the size of each band. Clay has many tiny pores, so it holds a lot of water in total, and a large share of it is held too tightly to use. Sand has big pores that drain fast, so it sheds gravitational water quickly and holds less overall. Loam and soils rich in organic matter hold a wider useful band than either. Our guide on soil water retention covers how a mix changes that band.

How soil moisture content is calculated

Moisture content has two common definitions. Gravimetric water content is the weight of water divided by the weight of dry soil. You weigh a sample, dry it in an oven, weigh it again, and divide the lost weight by the dry weight. Volumetric water content is the volume of water divided by the total volume of soil. Field sensors report the volumetric figure as a percentage. Our guide to calculating moisture content against a sensor's percent walks through the arithmetic.

Where a probe's percentage sits

A capacitive probe such as the Ecowitt WH51 sends a small electric field into the soil around it. Water stores far more electric charge than mineral grains or air, so the probe's signal climbs as the water around it climbs. The probe converts that signal to a volumetric percentage.

The probe counts every kind of water in its range. It can't tell gravitational water from capillary or hygroscopic water. Right after a soaking rain, the reading includes water that will drain away within hours. At the dry end, a garden bed never reads zero, because the hygroscopic film stays put. The plant-available band sits somewhere in the middle of the scale, and its edges move with your soil's texture.

In our own garden the pattern shows up across a wet summer. July brought 10.89 in of rain. The wettest single day was 2.46 in on August 20, and raised bed 3 read its high of 62% the next day. That peak held gravitational water on its way down. Raised bed 1 read no lower than 39% and no higher than 54% between mid-July and early September. Over those weeks the ground bed ranged wider, with readings as low as 32% and as high as 62%. The beds got the same rain. Soil mix, drainage, and the plants drinking from each bed set the different ranges.

A percentage can't tell you where your soil's wilting point falls on the scale. We have never measured the wilting point of our own beds, so we don't know what reading it sits at in any of them.

What the definition changes for a gardener

The water worth managing is the capillary band. Watering past field capacity adds gravitational water that drains below the roots and can carry nutrients with it, which our guide on leaching explains. Waiting until the soil nears the wilting point stresses the plant before you see it. The aim is to refill a bed while it still sits inside the plant-available band.

With a probe mapped to a bed, Siltwell turns that band into a signal for each bed. The check runs every 10 minutes, after the station's readings come in. If a bed's last two readings both fall under 25%, you get a warning alert and a push notification naming that bed. Watermark tension probes on a Davis station follow the same two-reading rule and fire when both readings are above 60 cb. A single stray reading doesn't fire the alert on its own, and you get at most one alert per bed per day. The alert names the dry bed and leaves valves and hoses alone, so the watering stays with you.

The 25% line is the same for every bed, and it sits at a fixed point on the volumetric scale. In a sandy mix, 25% can sit near the top of the useful band. In heavy clay, plants can run short of usable water while the probe still reads above it. Watch how your plants look the first few times an alert fires, and you'll learn where that line falls for each soil.

To see the bands in your own bed, deep-water it and then watch the probe through a dry week. The fast drop in the first day or so is gravitational water leaving. The slower decline after it is the plant-available water your crops draw down. Our guide on reading a soil water content sensor against a threshold shows how to read that curve bed by bed.