I have to agree with the others about being more specific.
Soil chemistry and physical properties are a well documented subject - literally tons of published books/ papers etc., etc.
That should be your first port of call. A simple Google search on "how to measure soil pH" will produce reams of answers which might not be a blessing.
You want temperature, moisture (content?), pH. Of these, temperature is the easiest, spoilt for choice.
Moisture trickier. The "classic" method was to weigh and dry. Conductivity might be used as an indicator. Simply putting the probes of a DMM on resistance range will tell you roughly how well the soil conducts - generally conducts due to moisture, but certain dry soils with say a lot of gold nuggets in it will also conduct.
pH classically, dry soil sample, add a specific quantity of water and measure with a glass pH (combined) electrode. There are reinforced glass electrodes for direct insertion into moist soil, sometimes called "spear" electrodes.
There are many items on the web on horticulture outside the Arduino Forum that will give you clues.
For example, I am looking to run an MSF clock on Arduino - first port of call? Amateur radio websites - the wireless bit is the trickiest, the Arduino bit relatively easy - with yours, the tricky bit is the sensor side and with pH in particular, buying a probe is the easy bit - getting a very high impedence output at femptoamp levels into a microcontroller is the first challenge, then follows stuff like calibration, compensation etc. etc.