what should the soil mix be for a chinese elm in a place that gets rain about 6 times out of seven days a week?
Have you read the article that I suggested? This should answer many of your questions, make sure to follow the links provided to the other resources.
In short, loose inorganic components will retain less moisture than dense organic ones. Both size and composition effect water retention. You have several months before this becomes an issue.
Look here, about 2/3 of the way down there is a link to another article regarding kitty litter as bonsai soil, read them both.