Just a few thoughts further on the topic. 

As I read the definitions of fallow, I would summarize as "Not in use."

Note that this is not equivalent to "Useless."

Some ground is "fallow" because it has never been prepared for growth.  Some, perhaps, never will.  Other, given the right circumstances, might one day be placed in use. 

Some ground is fallow because it has been used, and is being given the opportunity to "rest" as nutrients and minerals are replaced.  Rotating of crops allows different contents of the soil to be used by different types of plants with differing needs, and part of the process might include leaving a plot bare for a season.  This is to prepare it to be ready for future use.    A skilled farmer or gardener, would understand the process well, and assign crops appropriately with purpose and insight.  A novice, like myself, would do so a little more randomly, I imagine...call it guess and check.

There are those hearts out there which have never heard the gospel...God, by design and purpose, will deal with each one, and in His time, they will have that opportunity to receive Him.  There are those hearts out there which are devoted to Him, but at times, by His plan and design, are permitted to step back and rest, not in a negative way, but in preparation for the fruit He intends to cultivate and produce in their lives.  

I guess I haven't fully assimilated the thoughts and ideas, but I do enjoy thinking about such things when I'm about gardening business.