Effect of Amaze Mulch: "Has the effects of the other three Mulches in one. It increases your Berry harvest, dries out the soil, and makes mutations more likely."
Unless you can't read quotes, you would have read that the Amaze Mulch increases your Berry Harvest, dries out the soil and makes mutations more likely. :3
And for your second question, this li'l quote makes your answer:
Berry Fields composter (using one Maranga or Kee Berry and two other Berries)
So... yeah. You just need two more random berries to create Amaze Mulch. :D
Hope I helped. :)
Source & Source