I'm a keen gardener and was given a large sack of farm fertilizer by someone
who was clearing out his shed.

So you don't know how old it is or what it might be contaminated with.

unfortunately everything I put it on dies.

Do the obvious. Throw it out.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)