[bfu]

Instead of measuring soil moisture try:

    - measuring air moisture of small upside down cup on top of the soil 
    - measuring weight of the whole pot

Or don't measure at all and instead use precise amount of water on precise times (RTC module, medical grade piezo-electric pump). My similar project is on hold at 20 or so sketches of various types of water pumps.

[rwmj]

Wouldn't the weight of the pot increase as the plant grows (because it's absorbing carbon from the air in fact).

[jbeales]

This could be accounted for - for example, if the weight of everything drops by 100 grams, then at least 100 grams of water has been used or evaporated. Eventually any increase in weight from the plant could probably be calculated, but for many plants, (like bush beans), it's a relatively small amount, probably less than a pound.

OTOH indeterminate tomato plant, for example, weighs a lot, and the weight would also be affected by picking the fruit.

[criddell]

I wonder if you could use the camera to detect when the plant needs water? I have a plant on my desk that I dump water into whenever it starts looking sad and it's been alive for years now.

[Diederich]

> I have a plant on my desk that I dump water into whenever it starts looking sad and it's been alive for years now.

I've grown some glorious plants on my desk at work with exactly that method. When the ends get a little droopy, I water them heavily.

[cbanek]

How would measuring the weight account for the plant growing in size and weight?

You can also use a peristaltic pump for accurate watering and adding in nutrients, or adjusting pH. Sparkfun even has a kit (https://www.sparkfun.com/products/12915) for its automated bartender.

[kpil]

Wouldn't two temperature sensors tell a bigger difference for wet soil, as the wick temperature is lower in damp soil. If it's little difference it's probably dry.

That's at least partially how my finger works as a sensor. A finger test probably picks up sponginess too.

[hvidgaard]

> Or don't measure at all and instead use precise amount of water on precise times

This may or may not work depending on location of the plant. If it's in changing temperaturs and direct sunlight, there will be periods where it needs more water.