Get them off of your property. It will be very noisy, with flashing lights and much excitement, just the situation to get any horse freaky. Either load them into your trailer and drive them a mile down the road, or figure out where you can safely place them. Do not let them lose in the field since they are liabvle to return to the barn. Best way to keep your horses safe in a barn fire is to keep them outdoors rather than in a barn.
Barn fires can spread extremely fast.
It is not worth dieing for to save a horse. Fire departments also respond to other emergencies in barns. Even if there is no fire, the fire fighters may be in turnout gear. Be aware of what problems your fertilizers can cause. Some will explode on contact with water.