1. Because solenoid valves that can work with a very low pressure differential (a few feet worth of gravity often isn't enough to "force open" the valve) are expensive and require higher power draw than an Arduino / RPi. If you already need an external power source, might as well go with a pump. A pump is going to be a more consistent flow rate, and thus easier to control.
2. Generally not; a plant's water consumption is not steady over its lifetime.
I eventually moved on to hydroponics. Soil is just too "analog" for most of this stuff to work. Hydroponic kits are available on Amazon for cheap, though the main measurement you end up doing with hydroponics is pH measurement -- and I am not aware of any "hands off" sensors capable of accurate pH measurement that are remotely affordable.
Aren't the kinds of plants you can grow in hydroponics limited, though? I like having flowers on my balcony, and I'm not sure I could get the same stuff with a hydroponic setup.
Not sure, but pretty much any plant with a root system will grow in hydroponics (which includes most flowers). The fertilizer mix may be different for different plants, but the concept is the same.
The biggest pain with hydroponics is that you do have to totally change out the water periodically. That gets difficult, because you have to add nutrients, and those nutrients are pretty caustic, cannot be pre-mixed and actually need to be mixed into the solution in a certain order...
After a while, these "automated garden projects" end up becoming basically industrial control/automation projects with complex rulesets, programming schedules, etc. It was fun and I learned a lot, but it ends up being a much bigger project than you assume when you start.
I had imagined a ball valve mechanically hooked up to a servo - afaik, ball valves take the same amount of force to operate, independent of pressure (within sane limits).
Thanks for your reply, though. I'm starting to think about doing something like this, despite all the warnings in the original post and all the comments that it's basically doomed to failure.
It is basically doomed to failure; but you'll learn a lot and be happy for the experience. The fun part is that it seems so simple on the surface if you're someone who doesn't know much about gardening. But without the benefit of experience in gardening, you make mistakes like the OP that no amount of technology can solve. Automatic watering won't help you if your soil isn't draining.
And the ironic part is that it's actually not hard to cobble together an effective automation setup using some analog electronics like a cheap light timer, an AC fan controller, and a $20 automatic watering gizmo from the hardware store. I had a hard time building a more effective solution using an RPi or Arduino.
2. Generally not; a plant's water consumption is not steady over its lifetime.
I eventually moved on to hydroponics. Soil is just too "analog" for most of this stuff to work. Hydroponic kits are available on Amazon for cheap, though the main measurement you end up doing with hydroponics is pH measurement -- and I am not aware of any "hands off" sensors capable of accurate pH measurement that are remotely affordable.