It is probably to late for nitrogen fertilizer to help your corn. Being short plants ears will be small too. You still have time to plant again corn is a 65 day crop. If you plant at the correct time to make corn mature in 85 degree weather you get the largest harvest according for farmers. Corn likes nitrogen and water which reminds me my corn is 6" tall and it rained I should sprinkle urea in the garden soon maybe when it is about 10" tall.