Mate, all my girls are planted in organic manured soil.
Legumes green manure enrich soil with large amount of N and less P.
Buckwheat and comfrey's green manure enrich with N and less K.
You have two ways to do it:
In Autumn sow your seeds, let the plants grow not more 50 cm then chop and bury them. After 3 months did the soil twice.
If u want to add only potassium you can make a comfrey macerated and spray the plant.
Hope this could solve your question. Au revoir