First, Obama mis-answered the question, referring the farmer to the Federal Dep't of Agriculture. Politico followed up for two days to a dead-end. It is EPA which is doing the deeds, not D of Ag. (And it was the Fed Dep't of Transport which 'asked for comments' on forcing farmers to obtain CDL's to drive a friggin' farm-tractor...)
And, of course, Lisa Jackson lied about EPA's involvement.
...EPA is dissatisfied with Maricopa County’s Best Management Practices (BMPs) to control dust emissions from tillage, harvest, and transportation from non-cropland and cropland. EPA claims Maricopa County has insufficient specificity to control agricultural practices creating dust.
According to a February Federal Register notice, EPA is taking final action to find that Arizona failed to make a State Implementation Plan (SIP) submittal…for the Maricopa County non-attainment area for particulate matter (farm dust). This dust standard is called PM-10. EPA claims the Maricopa area, which includes Phoenix, has a serious air quality standard violation.
This is the very same EPA of "DOOM!!!" air-pollution
(Not-really-an-aside: How much money has been spent by EPA and Maricopa County on this twiddle-twaddle? Do you think that that money could have been spent more productively--like, e.g., on food-banks?)