Last time we had our soil tested - pH, P, K and Mg, for 4 fields, it cost £40. - That was a couple of years back.
Might be worth going that route - we had ours organised by our local agent for Scotgrain Agriculture - and they supplied the Maglime we needed to correct our pH, before using fertiliser.
We're very acid here - right next to a peat grouse moor - the fields had a pH of 5 - much too low for the fertiliser to do any good.
Sue