eh... might have been, Mousa.
Ginger, this is one Unicorn we're talking about here. And consider this: there's a spell that can make you walk on clouds. There's a spell that can give you wings. Why wouldn't there be a spell that could give you a green thumb?
Unicorns are gifted simply in that they have access to a greater range of magic
in theory. Certain kinds of magic are stated to be retardedly hard, even for a powerful unicorn like Twilight. Unicorn power isn't some all-encompassing, "hey look ma I can
do fucking everything" god-power, it's more like a knack. Like how someone can whistle the alphabet backward, or someone can make themselves burp, or someone can wiggle their ears. Magic is largely relegated to a single knack. Yeah, they can theoretically teach themselves how to do it, but it's insanely difficult - I've been trying to teach myself how to wiggle my ears for
years.
By the same virtue it's possible that both earth ponies and pegasi could have similar knacks, and thus teach themselves more tricks as well. You can't tell me that trailing stormclouds is just something that naturally happens, after all, nor that keeping miles of orchard alive on the care of two people is something just anyone can do. All magic is likely more flexible than we've seen, but it happens to exert itself in different ways naturally.