Rear Tire Repair

[coufray]

loader has a slice in one of his rear ag tires. it is not too deep, can't see any of the cords, but does go through one of the cross lugs.
It is not leaking and the tires are not loaded. Is there anything i can use to seal the slice to keep sharp rocks/dirt out. I probably looks worse than it is but just want to be safe. I've heard of vulcanizing compounds but not sure were i can get them. Can't afford to take off or have a tire repairman come in at this time.. tire is on a 2003 Kubota L4610.

Thanks for any advice!