Steve,
Fertilizer staining can be a pain. You are going to need an acid either Oxalic or Citric, but the downside is thay can stain/brighten the pavers, using either of these two is ok on regular concrete (like a concrete pad etc) but you would need to do the whole area, you cannot spot clean. I would be very careful on block paving as you could dis-colour them, and it is not easy to reverse the damage.
Don't bleach the area as this can set the fertilizer staining even more.
Tensid do a ferrous stain remover which might be what you need.
Seeing as this is Block paving you could try Phoshoric acid either neat or try an aluminium cleaner as most have a content of phoshoric in them.
But if this is run-off from the farm land it could be an on going problem?