[gpos]

My best guess is that SpaceX had to buy a lot of land in the buffer region surrounding their testing facilities in McGregor, and they're just looking to put it to good use.

In addition to the direct income from farming, farming the land would maintain its eligibility for an "open-space appraisal" under Texas law, which would reduce its property taxes by changing the valuation methodology for the land. Although there are two types of agricultural property tax exemptions in Texas, only the open-space appraisal eligibility requirements allow the owner's principal source of income to be something other than farming.

Sources: http://recenter.tamu.edu/pdf/1361.pdf http://bigskyranchtexas.com/everything-you-want-to-know-abou...

[wsh]

I think your guess is correct. As the announcement says:

   This position will be required to work around test schedules as
   necessary to ensure the successful crop production does not
   interfere with testing progress.

[salimmadjd]

Probably to get additional tax exemptions on property taxes of the land they own [0]

[0] http://exiledonline.com/property-taxes-are-for-parasites-bil...