I am going to start with a simple moisture meter one can carry about and test all their plant soils with, rather have one committed to one plant and needing one for each plant, as I had originally planned.
Insert the probe into the soil, push the button and it gives a go / no go signal whether the soil is damp enough or not.
Step 1: Parts
IC1: 4001 CMOS 2 input quad NOR gate (14 pins)
D1: Red LED
D2: Green LED
R1: 22 K resistor
R2, R3: 200 ohm resistor
R4: 100 ohm resistor
R5: 200 K trimmer resistor
One 14 pin IC socket
Misc: copper or copper clad probe, 9v battery connector, 6 lengths of insulated wire, and a pc board, box (I used an empty SwingLine staple box).
NOTE: IF you use a section of copper for the probe rather then a copper CLAD board, you will need 2 of them, and a bit of insulation attached in between the two pieces.
Step 2: Assembly
I normally try to eyeball just where components will be on the pc board, and place the Socket in what I feel is the best position for it. Everything else can be soldered into place in the order most convenient for each person.
One caution: When inserting the IC into the socket, care must be taken not to bend any of the pins or misalign them. It is easy to put a 555 into a socket, since there are only 4 pins to a side, but with a 14 pin IC, there are 7 on each side and it can be a little more tricky.
Step 3: Completion
Once soldered into place, on can cut the holes needed for the LEDs and the push button switch into the top of the case, and install.
In this case, I am able to house the 9 v battery on the inside.
Step 4: Adjusting and Testing
Once everything is ready; put the battery clip on a battery, insert the probe into soil that is of the proper moisture content; and press the button. If the red light comes on, adjust the trimmer pot. until you get a green. Then do the same for one that is not quite the proper moisture.
I few times around with get it just where you want it. Red LED for too dry, and Green for enough.
EDIT: Once I housed everything into the box, I found it was hard to get to the adjustment trimmer, so I bored a hole in the end of the device, as a small access hole as shown in the last picture. The trimmer has a slot for using a small screwdriver or similar tool on. Extra Note: on projects that have inductive sensitivity, one normally would not use a metal screwdriver for adjustment, but rather a plastic or nylon tool.
Since I made this a simple project, I did not build in any hysteresis, so adjustment to the proper moisture level is imperative.
The tester should be used before watering, since the plants roots, most likely will be in the bottom 2/3's of the soil. Press the probe in as far as you can without burying the contacts (unless you have waterproofed them with hot glue or something) before pressing the button. It is imperative that you get to that level if possible, and to always measure from the same level each time.
Watering tips can be found on line at places like this link to watering different types of plants.
Eventually, the probe will tarnish some, and the conductivity will not be as good, thus lowering sensitivity. This will make the monitor inaccurate.
You have two choices then: using a product like tarn-x to convert the tarnish back to the original metal (copper cleaner is OUT of the question as you only have a very thin layer to start with), but then you mush make sure the probe is SUPER cleaned afterward. Tarn-x and similar products are highly toxic to humans, pet, and plants.
OR you can replace the probe.