Is this a stupid notion?

do the temp markings go down low enough  maybe to zero deg? This would be my only concern. Other than that i don't see why not just poking probe into soil instead of chicken flesh.....................but what do i know.............there's only one way to find out

i don't think the meat thermometer will have an accurate enough scale.
I'm guessing here but I think it will go roughly between 0 and 100C - you need to measure between 0 and 10C if you are thinking of getting stuff in as early as poss.
Common sense is a good soil thermometer - if it feels cold it is probably too cold.
What are you thinking of putting in?

Dont know.  Would have been a blessed relief on the allotments I knew in UK!