Adam, If you do not want to overdo it, shoot for 100 lbs/a of Nitrogen. Urea is 46% N, so assume 50% to make the math easy. 30 x 20= 600 ft2. 600 ft2/43560 ft2 = 0.014 acre. 100 lbs N = 200 lbs urea. 200 lbs urea x 0.014 acre = 2.75 lbs urea.

For 2.75 lbs of urea, it can be put on anytime. Work it into the soil or apply before a rain so it doesn't volitalize. On a large area where there are frequent spring rains, side dressing when corn is 6-12 inches tall is the best way to apply urea.