How to reduce Fe(III) impurity in Iron(II) sulfate?
I recommend you dissolve your iron sulfate in hot water, dissolving as much sulfate as possible in as little hot water as possible. While keeping it hot, dip some coiled iron wire in it as well as some steel wool, and stir gently. The metallic iron should reduce your iron III back to iron II. 

Once the solution is clear green, remove the iron wool, but keep the iron wire in it, and let it cool down as slowly as possible, while protecting it from air the best you can. Finish the cooling by putting it in a fridge

You should be left with beautiful large, green crystals of highly pure iron(II) sulfate heptahydrate, and a clear solution of relatively dilute iron(II) sulfate.

Since you have 50 kg of it, you will likely have to proceed in several batches, so I recommend you keep that solution for the next batch to minimize product losses.

Once you are done recrystallizing your iron sulfate, you can heat up that solution to a boil, until some crystals start forming on its surface. Then, immediately stop the heating and cool down that solution again. You'll have some lower grade iron sulfate, but that should still be perfectly usable for gardening.

And if you want to keep some of the prettiest crystals you made in the process, I recommend you keep them in a sealed container with a small piece of moist cloth. Otherwise, they may tend to slowly dehydrate and turn into powder.