I purchased a jade bonsai (crassula ovata) tree several months ago and since then it has been gradually dropping leaves prematurely. I water it once a fortnight by soaking it in a bucket of room temp rain water for 15 minutes. It lives on my desk which receives bright daylight and sunshine if it's a nice day. Some of the smallest branches are just breaking off now too. The soil is very hard and water just runs off of it so I know it needs re potting in the spring. What else can I do? I want to keep it healthy and stop the leaves from dropping! Thanks

You are watering too much at one time but not often enough. Dunking is not a good way to water. You should be watering when it is dry not to a schedule.

Jade is a plant species that lives under semi-desert circumstances. So they need very little water, and can go for weeks without water. It loves sun.

Note that this is not a species suitable to create bonsai. You can only create a silhouette, not the branch structure. I would recommend keeping this just as an ornamental plant, but not try to create a bonsai. It is not worth the effort.