Josh: Do you know any other method to check the soil dampness? (I can't use the "finger test" method because the soil is so hard and full with roots so it's impossible to stick my finger down there). Johnmerr: Thank you for the suggestion! I will give it a try. There is another thing. There is a instruction tag came with the tree that says I should only repot it after 2 years. However the soil doesn't seem to have good drainage to me (when i water it, the water doesn't go right down but stay on the surface of the soil for a few minutes, I then have to wait until it's absorbed by the soil then repeat again until I see some water running down the drainage hole at the bottom). Is it normal so? Or should I repot it and replace it with better soil?

Agree. leaf miners. They don't pose any real threat to the plants. As mentioned just remove the leaves and/or squish the worm. As for the yellow bottom leaves, that is usually from over-watering, a common problem when using containers as you are. Since nutrients leach out of the pot every time you water you need to replace them regularly and make sure your water pattern is one of consistency - not too much and not too little. Plus there is clearly lots more room in your pot for more soil. The more soil in the container the easier it is to stabilize the watering. Lots of info about all this over on the Container Gardening forum here. Dave

You may want to repost this to the vegetable forum. There's lots of helpful people there.

It looks like underwatering to me. Typically brown leaves means not enough water, yellow leaves (without brown tips) means too much water. This is under-watering. When new leaves grow in, look at their condition. If they don't look fresh and healthy, then it's probably a different issue. * I'm not a plant specialist. Just speaking from experience.