In order to implement a low cost heating element that could be made widely available inspiration was taken from heated printer beds. A PCB heater can be made for very little at a Chinese fab house. To start with some calculations were done to determine the length of trace needed to get roughly 1 Ohm.

Resistance = Resistivity × Length/Area

Resistance= 16.8n × 0.5/(35μ × 0.25m)= 960m ≈ 1 Ohm

Where Resistivity is in ohm meters, Length in meters, Area (Copper Thickness × PCB Trace Width) in m^2.

The trace width was based on a 0.25mm trace width which is safely above most fab houses 8 mil trace width constraints.

The photosensitive board was exposed and developed.

Then etched in a warm bath of FeCl, as a sanity check I always pull the board from the acid to make sure all the areas that are unmasked have gone a nice red color.

Etching went well and I ended up with the board below ready for some 'testing'

After measuring the resistance of each section of 10 x 5cm = 0.5m they were all around 1 Ohm, not bad. All the sections were joined together (I should have done it from the beginning, fail!) 5 Volts was applied to the terminals and the heater warmed up such that it was hot to the touch, success! 

Next I plan to get a sample of soil in my propagator with a temperature sensor and determine the length of time it will take to heat given it cant kill the plant.