Are you referring to the starter solenoid when you say "ignition relay"?
The most troublesome part of the starting system is the transmission safety switch, and your tractor probably has a PTO safety switch. Is the trans in neutral and the PTO disengaged? Temporarily bypass these safety switches, and see if that solves the problem. Take care have the transmission in neutral as it can start in gear when bypassed.
Oxidized/Corroded connections can also give problems. Clean battery, ground, & starter connections thoroughly. Also check fuse connections.
If all else fails, trace power back from the relay till you find 12V.