Jacob Krumm Nature Preserve has two main trails which loop together for a total of 5.3 miles. There are two parking areas, each with a shelter and restroom.
I found two maps online, the Grinnell College map is more detailed than the Jasper County Conservation map. The west entrance is near the lake. The east entrance is closest to the observation tower. The photo below shows the start of the trail at the east entrance.
As we hiked along the wide grass trails, we really wished we had worn waterproof hiking shoes. Our shoes and socks were totally soaked shortly after we started our hike.
The trails are fairly well marked, but following along on All Trails made it much easier.