Purple Leaf Basil

We have a purple leaf basil where the lower leaves are going yellow. Does anybody know what is causing it and how to stop it?

 

where are you growing it ? as I bet its not constantly warm , and basil like the heat to grow best

 

It's in a pot on the patio

 

Would a watering of Epsom salts work?

 

Cold , waterlogged roots or lack of food ?

 

I agree, It is either lacking something or doesn't like where it is.

I think I will re-pot in fresh compost and feed, then put it further up the garden where there is more sun. I also took a couple of cuttings and put them in water, just in case.