plato_03:
i have an arduino mega... and the reason i'm doing it this way is i want to be able to go beyond the 16 analog and 54 digital outputs of the mega, and i also want to make this project feasible for someone who also just has a uno or something else...
OK, fair enough, and very altruistic of you!
plato_03:
secondly, i'm using the analog multiplexers because i've got more than enough of them, and i'd like to get more familiar with them... that and i have way more multiplexers than i do diodes at the moment...
Well, I think that kind of goes against your first point. Not everyone will have lots of analog multiplexers and diodes are much cheaper.
plato_03:
i like the idea of having a common diode output for all the sensors... really simplifies the coding for the project and takes out the necessity of the multiplexers altogether...
I don't know for sure the idea will work at all, that's why I suggested experimenting with it before abandoning your multiplexers.
If it does work, you could extend the idea to a matrix. 8 rows could be driven by 8 shift register outputs, and 6 columns connected (via diodes on each sensor) to 6 analog inputs, giving 48 sensors. Again, experiment needed to see if this works well, especially if the sensors are close together in the soil.