If you live in the state of Missouri, there is a free magazine published by the Missouri Department of Conservation called XPLOR. This nature magazine is aimed at children, and I would say it's suitable for PK-5th or 6th grade. All you have to do is sign up at the website for your free copy.
We just received our first copy for Feb./Mar. and it's full of all kinds of neat stuff. To celebrate valentine's day, it talks about the different courting rituals of animals. It also has stories about how owls digest food, the edibility of dandelions, black bears, and a lot more little tid-bits of fun nature information. This is a great subscription to start if you're currently studying Missouri or plan on studying Missouri in the near future.
And if you're in a surrounding state that doesn't publish its own magazine, you can get a subscription to XPLOR for only $5, so that's fairly cheap. Happy Homeschooling!