Dear Benjmain. you need 200 Kg of Ammonium sulfate for your 3500 m2 area at the rate of 120 Kg N/ha. The calculation is like this:

1 ha = 10,000 m2

So, for 3500 m2 area @ 120kg N, you require 120/10,000 x3500 = 42 kg N

Your ammonium sulphate is having 21% N

So, in terms of ammonium sulfate you need : 42x100/21 = 200 Kg of ammonium sulfate.

check this link may be help you

http://pubs.ext.vt.edu/424/424-035/424-035.html

Dr. Tarafdar correctly calculated.

Please Mr. J. C. Tarafdar thank u very much for your answer but how did u get to know you need 200kg of Ammonium Sulphate for your 3500m2.

Please look at the calculations, I have given the details of calculation. 1ha is equivalent to 10000 m2.

I am also agreed with Dr Tarafdar sir for giving a simplified procedure for calculating doses of fertilizer.