(1) Ammonium nitrate contains nitrogen and is a nitrogen fertilizer;

The mass of this element in each bag of fertilizer is at least: 50kg×34.3%=17.15kg;

(2) According to the information in the question, we know that ammonium nitrate is easy to store in a cool, dry place, so the fertilizer is easily soluble in water and easy to decompose when heated;

(3) From NH4NO3+NaOH= NaNO3+NH3↑+X and the law of conservation of mass show that each X contains 2 hydrogen atoms and 1 oxygen atom, which is water; ammonium nitrate can react with sodium hydroxide to generate ammonia, so when applying this type of fertilizer, Avoid mixing with alkaline substances, otherwise the fertilizer efficiency will be reduced.

So the answer is: (1) B; 17.15; (2) AC; (3) H2O; alkali.