Hi,
I did few of measuring station using Arduino board with different sensors.
For temperature i like to use 1-Wire DS18B20 sensors, no meter where you measure i got relay precise results, plus you will need only one digital pin for few of them.
For pH, i use one self created sensor, without precise results.
For air humidity I use AM2301 sensor and I am very satisfied with working parameters.
For soil humidity I also use few different sensors but all of them measure resistance, what dissolve one of electrode after some time, so right now i work on capacitance sensor.
Hope that help.
Regards,
Dubravko