Don't take my word as gospel but ...
If you buy a field with the intention of using it as a strip, the 28 day will not apply as that is its primary use. Unless you have another ligitimate primary use, it would be better to either rent a field from a farmer or rent the farmer your field, but have a strip in it. The 28 day rule when applied to agricultural use, applies to the holding and not on a per field basis, so sorry, it is 28 days only. However, as mentioned most of us operate normally without problems until someone starts counting. That is one of the reasons most strip owners are not always welcoming to other pilots flying in without strict PPR.
Hope this helps ... SS