Private airfields are not at all uncommon in the U.S., especially the rural parts (which is most of the U.S.) These are probably for crop dusting or because the owner also happens to be a private pilot and wants to keep their plane at home rather than paying the local FBO for hangar or tie-down rental. Of the ones I've seen, though, the markings vary from very simple (old tires painted white placed along the edges) to non-existent. Sometimes smaller ones can be hard to see on Google Maps, but, at least in my experience, you can usually find them if you look hard enough. They're probably easier to spot in places with a lot of trees rather than in places where the land is predominately wide-open farmland.
Here's an example of what it looks like on a Sectional Chart and on Google Maps:
One Grand Field on a Sectional Chart
One Grand Field (yes, it's literally a field) on Google Maps. The long, narrow clearing in the trees is the grass runway.
To give an idea of just how common these things can be, take a look at this sectional chart clip:
There are no less than 12 private airports here within a roughly 25 nmi x 25 nmi area, despite having two nearby decent-sized public uncontrolled airfields (one of which is home to a large flight training school,) and a class D airport (with a busy-ish Class C visible in the Northwest corner of the clip.)