Indoor plants are notoriously tricky to keep healthy and alive, especially in winter. You're attempting to replicate a complex and naturally accommodating biome inside your home. Plants that thrive in nature wither on your countertop, and sometimes, it's really difficult to pinpoint what it is you're doing wrong.
One of the most common culprits is moisture. Not only do individual plants' needs vary, but if you see a problem with a plant, the first (over)reaction is often to water, which can lead to overwatering, or pulling back on watering, which can lead to dehydration.
I myself am certainly guilty of this habit cycle, so I figured why not buy a moisture sensor and see if I can build a watering schedule based on actual data: Would my plants benefit? Could a cheap soil sensor help me rescue the orchid on my kitchen table that's been struggling for weeks?
How soil moisture sensors actually work
I never heard of this brand, but the soil sensor worked perfectly to measure dryness and wetness. Alan Bradley/CNET
Common capacitive soil moisture sensors rely on the electrical properties of the materials they're built from. Typically, they consist of two conductive surfaces separated by a nonconductive material, called a capacitor.
Dry soil acts as a dielectric, a substance that is a poor conductor but can hold an electric charge. That ability to store a charge grows as moisture levels increase, which affects the capacitor's electrical properties. The sensor measures the dielectric constant. It then translates it into a percentage, number or, in the case of the $13 sensor I got on Amazon, a simple reading from a gauge with a range between 1 (dry) and 10 (wet).
The length of the probe on the sensor I bought is a nice advantage, as it allows me to read the soil moisture level all the way to the bottom of even very deep pots. This is crucial because roots can pull differing amounts of water from the soil throughout a pot, and soil may be saturated during watering differently at different depths.
Let data tell you when to water
Prior to this test, I was relying on the typical indicators of when to water. I tried to follow a schedule with most of my houseplants, even as I accumulated plants with different needs that needed to be watered at different frequencies. I also relied on anecdotal evidence, like the finger test, where I tried to feel how moist my plant's soil was, or by testing the weight of the pot or watching for signs of withering or dryness in the plant itself.
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