August 30, 2026 · Updated September 13, 2026
What a soil moisture probe tells you about watering
Push a soil moisture probe into a bed and it reports, as a percentage, how much water sits in the soil touching its tip right then. That single number tells you whether the root zone at that spot is wet or dry. To turn it into a watering decision you need a threshold and a run of readings.
What the number measures
A capacitive probe measures how easily the soil around it holds an electric charge. Water changes that charge, so the probe converts the signal to a moisture percentage. The reading covers the soil touching the probe, at one depth, at one moment. Move the probe a foot over or a few inches deeper and the number shifts.
What percentage should trigger watering
No single percentage is right across every bed. Sandy soil holds less water than a compost-rich raised mix, so the same 30% reading means different things in each. Siltwell uses a default of 25% capacitive moisture as the dry line, and you can set a per-bed threshold to match your soil. Below that line the root zone is running short.
In our own garden the probe beds settled into steady ranges through late summer. Between mid-July and early September, raised bed 3 moved across a 32% to 62% band. The ground bed covered that identical 32% to 62% band, and raised bed 1 stayed narrower, from 39% to 54%. Those ranges tell us what normal looks like, so a drop toward the low end is our signal to water.
From one reading to water this bed
One reading can mislead, because you might catch the bed right after rain or right before it dries out. A dry decision needs a pattern. When a bed's two latest readings both sit under its threshold, Siltwell sends a dry alert for that bed. On a Watermark tension probe the same logic uses 60 cb. The alert names the bed, so you water that bed and skip the ones still wet.
Siltwell points you to the dry bed, and you open the valve. See how a handheld meter differs from a mapped bed sensor, and how depth and timing change the reading.
A probe reads its own few inches of soil. It cannot tell you that the corner of the bed drained faster, or that the surface crusted while the root zone stayed wet. Place it in the root zone of the plants you care about, and add a second probe for a large bed.
Pick the threshold from your own soil rather than a chart. Log a week of readings per bed, watch where each one sits after rain and after a hot day, and set the dry line a little above the point where plants start to wilt. Then let the two-reading rule turn the number into a decision. Connecting the probe through a station is how Siltwell watches it for you between checks.
Common questions.
- Do soil moisture meters work?
- Yes, a capacitive probe measures the water held in the soil at its tip and shows it as a percentage. Cheap two-prong meters that read conductivity drift with fertilizer salts and corrode over a season, so they are less trustworthy. A capacitive probe left in the bed gives you a steady, comparable number over time.
- How do I check soil moisture?
- Push a probe into the root zone, a few inches down where the roots feed, and read the percentage. Check the same spot each time so your readings compare. For a large bed, take two or three readings, since one corner can drain faster than another.
- Is there a device that can tell me when to water my plants?
- A soil probe mapped to a bed can. Siltwell watches each bed and raises a dry alert once its two latest readings both drop under the threshold you set, naming the bed. It tells you which bed to water, and you turn on the water yourself.
- What should your soil moisture percentage be?
- It depends on your soil, so there is no universal number. Siltwell uses 25% capacitive moisture as a default dry line and lets you set a per-bed threshold. In our own garden the probe beds ran between 32% and 62% through late summer, and the low end of that range is our cue to water.
Related guides
What a soil moisture sensor for irrigation system tells you
Learn what a soil moisture sensor reads, the threshold that triggers a dry-bed alert, and why it tells you which bed to water rather than opening a valve.
How to calculate soil moisture content at home
Weigh a soil sample wet, dry it, and divide to get its moisture content, then see why your bed sensor's percent is a different number.
Soil moisture sensors for agriculture in a home garden bed
How field-scale soil moisture sensors translate to a single bed, and what a per-bed probe alert can and can't tell you at home.
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