August 28, 2026 · Updated September 13, 2026

What your soil moisture measurement should read before you water

No single soil moisture percentage tells you to water. A reading of 30% is dry in one bed and comfortable in the next, because a capacitive meter reports a relative value that shifts with soil texture, salts, and how the probe seats. The number you act on is your bed's own dry point, and you find it by watching the bed for a few weeks.

Why a meter reading has no universal cutoff

A capacitive probe measures the soil's dielectric response, which rises with water content but also depends on the soil around the probe. Sand drains fast and holds little, so its percentage falls quickly. Clay grips water, so it reads higher at the same real dryness. Two beds can sit at the same number and mean different things for the roots.

In our own garden this showed up across a single season. Bed 1 held a moisture reading between 39% and 54% through late summer. Bed 3 moved between 32% and 62% during that same stretch. Both readings came from the same garden in the same weeks, yet the ranges differ. A cutoff copied from another yard would have fit neither bed.

How to find your bed's number

Read the bed at root depth every day for two or three weeks and note when the plants slow or the leaves soften in the afternoon heat. The reading on the day before that stress is your bed's dry point. Set the threshold a few points above it, so you act before the roots feel it.

• Read the same spot. Four to six inches down, away from the stem and any crust, at the same depth each time.
• Trust the plant over the page. The bed's history plus the plant's behavior gives you the number no chart can hand you.
• Consider tension. A Watermark-style sensor reads centibars instead of percent; above about 60 cb the soil holds water tightly enough that roots struggle.

A capacitive percentage reports relative wetness rather than absolute water content, and two probes in the same soil can read a few points apart. Compare a bed to its own past readings rather than to another bed or a website figure. If you want the reasoning behind picking a starting value, see how to set a per-bed threshold from a baseline, and for whether the cheap meters are worth trusting, how moisture meters perform.

What to do once you have the number

A mapped soil sensor in Siltwell holds a per-bed threshold. The default is 25% capacitive, and you override it per bed with the dry point you measured. To skip a single odd spike, the alert waits for the two most recent readings to both fall below that threshold before it fires. It sends you an alert when the bed has gone dry. It runs no hose.

Start by logging one bed for a couple of weeks before you trust any threshold. Once its low readings line up with the days the plants wanted water, write that number in as the bed's threshold and let the reading call the next watering instead of the calendar. For the difference between a handheld meter and a bed sensor that watches for you, read the meter versus bed sensor comparison.