Riddle of the King's Seed

Once there lived an ancient king
Without and heir or marriage ring
He was old, and groaned about his throne
And the future of his home

And so one day the king decreed:
"The kingdom's children get one seed.
To the child who's plant is best and faire
I shall declare them my heir!"

Three months later they arrived
From city straights and countryside
And plants of every kind were here
And yet the king was without cheer

Yet last of all a young girl brought
A little empty flower pot
Without a plant...and filled with soil
To her the king gave all the spoil

Why?