My wife has had success starting an avocado from seed. It grew fast. But now the leaves are starting to turn brown. (See photos.) Any ideas why this is happening? I'm thinking it may be time to put into a larger pot, but that may be a separate matter. (Looking for advice here too: what size pot should I put it in so I am not transplanting every year?) I don't think avocado roots go deep, so am planning to put it into a pot that is wide, but shallow. And does it need plant food?