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The Cheap Vegetable Gardner created this soil moisture sensor using nothing more than picture wall hanging hooks, a soda straw, hot glue, and some Plaster of Paris.

After a little research there was the classic science class method of creating a homemade sensor by simply putting two galvanized nails in the soil and measuring the resistance between them. A major problem with this solution is the soil composition can significantly vary the resistance and keeping the spacing between the nails could be troublesome.

After a little more research I came across the method that has been working well for over 50 years. This method includes taking two metal probes and inserting them into a gypsum block and measure the resistance between them. The gypsum absorbs the water and provides a decent range of resistance and moisture measurement.

Unfortunately I was fresh out of gypsum, so I looked around the garage and found a good substitute, Plaster of Paris…


How to make a cheap soil moisture sensor

Gareth Branwyn

Gareth Branwyn is a freelancer writer and the former Editorial Director of Maker Media. He is the author or editor of a dozen books on technology, DIY, and geek culture, including the first book about the web (Mosaic Quick Tour) and the Absolute Beginner’s Guide to Building Robots. He is currently working on a best-of collection of his writing, called Borg Like Me.


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Comments

  1. Adrian says:

    I beleive plaster of paris is gypsum but simply heated up first?

  2. s. says:

    Yes, it is. Heated and powdered. To use it, you add water, which makes it harden into “standard” gypsum. Somehow I doubt the original idea involved drilling holes in a gypsum block instead of using plaster and water. Someone didn’t make their homework before submitting the story.

  3. The Cheap Vegetable Gardener says:

    Yes, you both are correct. That was my assumption was that as well but looking at the box and only seeing calcium sulfate hemihydrate on the box I was thoroughly confused. After a quick check on Wikipedia (should have done this before) calcium sulfate hemihydrate + Water + drying = Gypsum. Thanks for bringing this to my attention and I have updated the post to avoid confusion to others.

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