How do farmers use pH to choose the right soil to use to grow there crops in?

It is not a matter of choosing the soil, it's not like they can change it out or something.

Amendments can be added either directly to the soil or to the water used on the soil to correct high or low PH conditions. 7 is ideal, lower than that and the soil is acidic and needs to be buffered, higher than 7 and acid can be added to the irrigation to correct the soil.

Different crops thrive in different PH environments and nowadays the farmer can change the soil profile to meet his needs.