September 27, 2026

Measuring soil moisture in a garden bed

You measure soil moisture where the roots are, at 4 to 6 inches down, and you read it more than once. A single scratch at the surface tells you almost nothing, because the top inch dries first while the root zone stays wet. Every method below gets you to that root-zone number a different way, and each one misleads you somewhere.

What you are measuring

Two different things wear the same name. One is volumetric water content, the percentage of the soil that is water, which a capacitive probe reports. The other is tension, the suction in centibars that a root must overcome to pull that water out, which a tensiometer reports. A high percentage means wet soil. A high tension means dry soil, so the numbers run in opposite directions.

The percentage is easy to log and easy to compare within one bed. Tension maps more closely to plant stress. For most home gardeners a percentage from a fixed probe is enough, as long as you read the same bed the same way each time.

Steps to measure soil moisture

• Step 1. Reach the root zone. Dig or push down 4 to 6 inches to reach where the plant drinks.
• Step 2. Feel it. Squeeze a handful. Soil that forms a ball and stains your palm is moist; soil that crumbles dry is ready for water.
• Step 3. Read a handheld meter at several spots. Take three or four readings across the bed and average them, because one prong hits one pocket of soil.
• Step 4. Check tension if you want plant stress. A tensiometer reads centibars; roots start working hard as the number climbs, and past about 60 cb the bed is dry.
• Step 5. Bury a capacitive probe for the trend. A fixed probe mapped to a bed reads the same soil every few minutes, so you see the fall between waterings instead of one snapshot.

A handheld meter and a buried bed sensor answer different questions. The handheld tells you the moisture at one spot right now. The buried probe tells you which direction the bed is heading and how fast.

When a capacitive probe is mapped to a bed in Siltwell, it flags the bed as dry when the two most recent readings both sit below that bed's threshold, which defaults to 25% and you can set per bed. It tells you which bed to water and which to skip. It alerts you and leaves any irrigation to you. The continuous history that shows the trend is a Pro feature.

What goes wrong

Cheap two-prong meters measure conductivity, and fertilizer salts raise that reading whether or not water is present. So a freshly fed bed can read wet while the soil is parched. Corrosion on the prongs drifts the reading further over a season.

A capacitive percentage measures each bed on its own scale. It shifts with soil type, compaction, and how tightly the probe sits against the soil, so 40% in one bed does not match 40% in another. Treat each number as a trend for that one bed and set the floor where that bed's plants start to struggle.

Placement fools every method. A probe against a dry root ball reads low while the bed around it is fine, and a probe in a settled low spot reads high. In our own garden the ground bed swung from 32% to 62% between mid-July and September, while raised bed 1 held a tighter 39% to 54%, because the deep raised soil drains and dries differently from the shallow ground bed.

Pick your driest bed, push a probe to root depth, and note the number the next time the plants wilt at midday. That reading becomes the floor you water against. From there, set a per-bed threshold and let the trend, not the clock, tell you when to water.