Question
What is the best food for leylandii

Answer
Pick up a bag of "Espoma Hollytone". This is an acidifying fertilizer for plants which like acidic conditions, such as hollies, azaleas, and Leyland cypress, etc. Not sure if you have this over there. Use directions on the bag and apply about 1/2 the recommended amount. Try to fertilize at least 3 months before first frost. E.g. if you get frost in November, try not to fertilize after august 1st. Next spring fertilize again.

Make sure you water the fertilizer in well.

If you can not find holly-tone, you can also buy 14-7-7 slow release fertilizer (labeled as azalea food). It is the same, but just not organic like Holly tone. Finally you can use miracid (from makers of miracle grow) although it is not slow release. I really prefer using slow release products when fertilizing Cypress.