How do I stop my Dr Martens from cracking?

I’ve had my beloved docs for about two years now. I bought them at the Salvation Army. They were fairly new at the time, and the leather was stiff and super shiny, but I’ve since broken them in and they have become very scuffed. I’m fine with the scuffs, but I’ve started to notice right across the middle of the boot, past the toes, the leather is starting to show signs of cracking.

I don’t have any leather conditioner, nor do I have money to purchase any. Are there any DIY remedies for this?