Why are the leaves of my jade turning yellow?
Yellowing or wilted leaves typically result from overwatering but, confusingly, may also indicate underwatering. A jade plant (Crassula argentea) needs thorough watering when the compost begins to dry out, every 3 to 5 days, from spring to autumn but very infrequent watering, only once every six weeks, during the winter.
If the leaves look rotted, "bleached" to yellow and are dropping, you are overwatering. The solution may be to simply water less but if the symptoms don't improve or get worse you will need to remove the plant from its pot to see if the roots have rotted. If they are still white and healthy, repot with proper, well-draining soil. If they are rotted, the plant can not be saved though partially rotted roots may be trimmed away and the plant repotted.
If the leaves are yellow and shrivelling and the plant's stem looks weak, the issue is underwatering. Water thoroughly until water runs out of the pot's drainage holes and then resume a proper watering schedule. Do not allow the plant to sit in the water runoff.
Courtesy of NYBG Plant Information Service