soil temperature and moisture probes

I have asked this before, but generally, this year i can refien my question a get a better answer.

Basically i have 2 probes of each (temp and moisture).
I want to put them in the garden and actually use the station this year as it should be (to help gardeners).
In the garden we grow potatoes, leaks, onions, carrots, lettuce, cauli, spring onions, (list goes on).

Can someone recommend what soil depths i should put my probes into?

The soil we have here is “sandy”.

At the moment, i have the probes at 50mm and 300mm (maybe 250mm) will check this out.

If you mention a depth, any chance you could explain the reason behind this, to help me understand my new hobby of “gardening”, please!

Many thanks,
Stuart