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.