That is because if you plant when your farm is fertilized, those seeds remain fertilized until they are harvested. Hence you can fertilize on day one (lasts 3 days) and plant a 4 day crop and it stays fertilized (with the F next to it). The actual "farm" itself stops being fertilized after 3 days. You don't have to re-fertilize until you plant again, even though the farm itself is no longer glowing.
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