Liming growing medium in tubs for Lilies

I have pretty acid soil and grow many Oriental Lilies in huge whisky barrels. I would like to grow some of the Asiatic Lilies especially the Tempo ones which look amazing, but need an alkaline soil. Can anyone advise me as to what (hydrated lime?) I need to add to the tubs to increase the alkalinity and how much to mix in. The tubs are 2ft high by 3ft circumference and I haven't a clue how much I'd need to add to the growing medium which is a mixture of soil, compost and well rotted horse manure which suits the Oriental Lilies very well. I don't know if it's a tablespoonful or shovel full! I'm lucky normally with an acid soil as can grow all the Rhodis, Camellias, Meconopsis, Azaleas, Trilliums etc without any trouble but know with Asiatic Lilies I've grown in the past that the are ok the 1st year and them disappear over the next couple of years. Hoping someone will be able to adviss me. Thanks.