You'd probably need to do some research with farmers. See what type of moisture meters maize and types of phones they have access to. Your app needs to work with whatever is currently in use.

Some use a probe, some are like a jug that contain the grain sample.

As far as I am aware the more modern devices can store to internal memory or SD card and export data via CSV file.

A lot of meters come bundled with software that can help farmers analyse the readings already so you may need to see what software is already in use too.

Ideally you could just use a wifi SD card in the meter and then use the phone to browse straight to the readings on the card which can then be imported into the app. No need for people to connect their phone via USB. This requires a smartphone though.