Phosphorus Deficiency

Hi - I seem to have a young sapling experiencing Phosphorus defiency. Its dark green and taking ages to get taller compared to the other sapling i have.
Is there any "NATURAL" way for me to balance this defiency? i want to avoid using chemical fertilisers if poss. One idea i had was to transplant it carefully from the small pot its in, straight in the earth where i prepared some compost underneath - good idea???