After getting off my flight, I waited in a line for an initial passport scan at a self-service booth. I had filled out a customs form on the plane, and I reentered important customs information on the touch screen there. I got a paper from that machine with a large X on it.
I was then directed to wait in the line for people with an X on their papers. I assume that the X means they have something to declare, like my stroll through farmland. I got to the front of the line and told the immigration officer that I had been on a farm when he asked that I clarify the nature of my agricultural declaration. He wrote an A on the paper that had an X on it.
I picked up my checked baggage and then waited in line to go through customs. I'm assuming the A on my X paper stood for 'agriculture', because upon presenting it, I was given a green-edged transparent folder to put my X paper in and told to go wait in the green folder line.
I didn't actually have anything agricultural in my luggage, but the forms I had filled out hadn't made any differentiation between having walked through a field and physically carrying a dozen cobs of fresh-cut Mongolian corn. In any case, the agricultural examination area was interesting to look at.
When it was my turn to present my documents, I said that I wasn't carrying any food or plants, but had walked through a farm in Nigeria. The man on duty there called back to a colleague, "Hey, we have a guy who was on a farm in Nigeria." There was a brief summit and an answer was returned: "We can disinfect your shoes if you want."
Having spent all that time waiting in lines, I thought it would be a shame to leave without having anything done, so I held my shoes over a floor-level sink and they were sprayed with something in a Windex-style bottle. I walked out of the airport feeling that something great and bureaucratic had been accomplished. I also had squeaky-clean shoes.