I don't support the idea of writing on multiple niches. Just pick one niche, establish yourself as an authority and see traffic flock to your blog. I am a poultry farmer. If I want to pour out all my knowledge of poultry farming for years into a blog, you can imagine how rich that blog would be. But if I write on poultry farming and tomorrow, I write on gold, that blog wouldn't be rich.You can excel in any niche, however, make sure you are too good with the niche you are working on. If you want to become successful you need to work on multiple niches.