See this project on my website!

This project will calculate the water content of soil around a plant by measuring the dielectric constant (the soil's ability to transmit electricity) and will alert you with a red LED when the plant needs more water or a blue one when it has too much.

Step 1: Materials

You will need to gather these things in order to build this project.

Arduino UNO or equivalent (x1): https://amzn.to/2DLjxR2
220Ω* resistors (for LED) (x3): https://amzn.to/2S2sV5R
10kΩ resistor (x1) - brown black orange: https://amzn.to/2S2sV5R
RGB LED (x1) or 3 different colored LEDs: https://amzn.to/2Fzbusx
Long Jumper Cables (x2): https://amzn.to/2Q7kiKc
Jumper Cables (x6): https://amzn.to/2Q7kiKc
Breadboard (x1): https://amzn.to/2RYqiSK
Bolts of any size (x2): https://amzn.to/2qWLYTF
Nuts same diameter as bolts above (x2): https://amzn.to/2qWLYTF

Step 2: Create Sensor Prongs

You will need to repeat this step twice (once for each prong).

Begin to tighten the nut around the bolt
Slide the end of the long jumper cable between the nut and the head of the bolt.
Finish tightening the nut until you are unable to pull out the jumper cable

Step 3: Create the Circuit

Follow the schematic or the breadboard image - whichever one works better for you. The wires labeled "out" are the two prongs you just created.

Step 4: Upload This Code

Pretty self explanatory here. Just upload this code to your Arduino!

Step 5: Place Sensor Prongs

Insert the prongs you made about 1" to 1.5" apart in soil close to the plant you want to monitor.
Give the plant a healthy amount of water and open the serial monitor
It should give you readings around 25 - 30% if you gave it the right amount of water
If not, try moving the prongs around to get it right (or you just added too much water)

Step 6: Outdoor Protection

If this is going outside you will want to put your circuit inside of a Tupperware or other waterproof container to protect it from the elements. Then drill some holes for the sensor wires to come through and add a battery box to power it (see here for how to do that). Mine is not going outside though, and will be fine without a container.