Guys, thanks for the answers

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There is 1 thing i don't get. We will place clay over the soil for the pond. I mean the water in the pond won't even be touching to the soil. So, i guess this is not why we need to get the soil tested.
I suppose that we need to get it tested to have an idea about watershed. Is that right?

And another question is, should i get the surface soil tested or some soil in deeper spots too?