August 30, 2026 · Updated September 13, 2026

How soil moisture monitoring works in a home garden

A single probe reading tells you what one spot of one bed holds at one moment. Continuous soil moisture monitoring reads that same spot every few minutes, so you watch the trend as the bed dries and see the drop before a plant wilts. That trend is what lets software warn you which bed needs water tonight.

What a soil moisture reading measures

A capacitive probe senses the water held in the soil around it and reports it as a percentage. Water changes how the soil stores an electric charge, and the probe reads that change. The number rises after rain or watering and falls as roots and evaporation pull water out.

One reading is a snapshot of one depth. The top inch dries within hours of a hot afternoon while the root zone below still holds water, so where you push the probe changes the answer. A probe left in place reads the same depth on a fixed schedule, which removes that variation and shows you the shape of the drying curve.

Why continuous reading beats a spot-check

A spot-check catches the bed at whatever moment you happened to walk out. Continuous monitoring records the low points you would miss, like the reading at dawn after a dry, windy day. It also shows how fast each bed drains, which differs by soil and by sun. Our raised beds and our ground bed dried at different rates over the same weeks.

The practical payoff is timing. You water when a bed crosses into dry. A weekly habit overwaters one bed and starves another. For the per-bed side of this, see our guide on reading soil moisture per bed.

What triggers a dry-bed alert

A cron checks mapped sensors every ten minutes. Siltwell fires a "soil is low" warning and a push when the two most recent readings for a bed are both below that bed's threshold. The default is 25% moisture, and you can override it per bed. A single dip below the line does not trigger the alert, which keeps a brief reading from a stirred-up probe from waking you at night.

If you run a Watermark tension sensor instead, the same rule applies against soil suction, with both recent readings above 60 centibars standing in for dry. Either way the alert names the bed that needs water. You still turn on the hose yourself. Connecting a station and mapping soil probes happens on the connect a station page.

What percentage your soil should read

Chasing a universal target number wastes effort, because a capacitive percentage shifts with the sensor and the soil packed against it. Watch your own bed's range instead. In our own garden the probes ran between 32% and 62% across mid-July to mid-September, and each bed had its own band.

• Ground bed. Swung widest, from 32% on a dry August day to 62% after rain in late July.
• Raised bed 1. Held a tighter band, 39% to 54%, and hit its low in early September.
• Raised bed 3. Ran 32% to 62%, the full spread of the ground bed in a raised frame.

Those bands tell you where to set each threshold. A bed that rarely drops below 39% wants a higher dry line than one that regularly touches 32%.

One limit to keep in mind: the probe reads only the depth it sits at. A single-depth reading leaves out whether water soaked down to deeper roots or ran off the top, so check how the bed takes water alongside the number. To turn a reading into a watering decision, read our guide on when a soil measurement means it is time to water, then set each bed's threshold from its own summer range.