Question
Hi, I have a lemon tree that I grow form a pip twenty years ago, Its about six foot tall but all the leaves are begin to fall off even some of the new growth from this year! The compost is the correct one for the tree ( acidic ) there seems to be a lot of tiny flys or more like flees in the compost. Please can you help !

Answer
Hi David,  Citrus need full sun, should never be allowed to be dry and require alot of fertilizer. Foliage drop is usually associated with watering inconsistencies or a change in the amount of light it's getting.  If neither of these has changed, then we look to insects, diseases or nutrient problems.  You say the pH is correct, have it checked.  Prefer the pH to be around 5.5.  
The most common insect on citrus is scale.  Check the new growth, stems and leaves, for these insects. Foliage is normally sticky from their secretions.  If you find any, spray the plant with a summer or dormant oil per label instructions.  Most of the soil insects you see are not normally detrimental to plants, just a nuisance.  Spray the soil with permethrin to get rid of them.  If your plant is in a container, you can lay a dog flea collar on the soil surface for up to 3 months control of soil insects.
If you've lost a significant amount of foliage, consider cutting the plant back some and fertilizing it with a good quality fertilizer.  Avoid cheap water solubles that may contain chlorine.  Check with your local garden center for a grower quality fertilizer.  Jim