problems with an avocado in a pot

Hi, I need some advice concerning my 2-year-old avocado growing in a pot in a flat. The whole plant looks nice, is quite big, seems strong and new leaves are healthy and pretty but older ones get brown on the edges and dry. I cut them off because it is not a nice seight. What can I do? I don't water them too much, I think, the soil is always humid but not wet, it has enough of light... Any ideas?
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