It's really quite simple.
Before I was born, or my Mom even got married, her dad died. He had built my grandmother a house on some acreage in a rural area. They had a garden area, seasonal pond, pie cherry trees, mile long driveway, old barn and a handful of cows.
Because my Grandmother had no man around the house to do upkeep on the "farm" my parents would spend a lot of time on the weekends out there helping her out. I absolutely ADORED my grandmother, being outdoors and mucking around the farm. I loved helping her feed the cows.
Now, everytime I pass by a farm I have to roll down the windows and take a big whiff. Naturally, the smell that I detect is the strongest one: cow poop.
It reminds me of my grandmother.....sort of like apple pie would remind you of home....