Remove as much of the ammonium nitrate as possible with a shovel.
Then leach (wash) from the root zone. This can be done by overhead irrigation with three to four inches of water. For slight problems, maintain good, uniform soil moisture and avoid drought conditions by irrigation to supplement rainfall; this will gradually eliminate the problem as plants assimilate nutrient portions of the salts.
Water, water , water. The roots have absorbed the ammonium nitrate and transferred it to the stems and leaves. The salt needs to be removed and the way to do it is by overwatering.
take the plant out of the ground wash it off good and then put take out the old soil and add in new soil or you could take the plant out and wash them off and plant them somewhere else...it all depends how high the level is and where at.