This is a respin of one of my projects.
Some things it needs to do:
read seven moisture sensors,
read a flow sensor,
drive seven latching type solenoids,
use solar power,
ditch USB to save some power,
and accept Arduino Uno style headers for communications.
Schematic: http://epccs.org/indexes/Board/Irrigate7/Documents/14320,Schematic.pdf
I have found the pulse output from a capacitive type moisture sensor works while the analog types are not so good. Also, the pulse output from a flow meter works. Both measurements can be made accurately (well beyond reasonable need) if the input capture hardware is used. So after a fair amount of convincing myself I have selected the ATMega1284P which has a second set of capture hardware (ICP3). That will allow reading the moisture sensor at the same time as the flow sensor.
Everyone has their own take on how the 1284 pinout should map to the Arduino header and frankly after looking at a few I gave up and went with the functions shown on the schematic. I have not done the layout yet...