Ask Mr. Smarty Plants

Western soapberry, Sapindus saponaria var. drummondii is well known and much appreciated in some areas for its yellow-orange fall foliage. Your tree may actually be exhibiting a sort of early "fall foliage" response to the shock of being transplanted. It is likely to begin dropping many of those yellow leaves soon.

Actually, the loss of foliage now is probably a good thing as it is the tree's way of reducing stress caused by dessication. When a plant of any kind (even a container-grown one) is newly transplanted, the roots are inevitably damaged to some extent. The remaining roots are no longer able to meet the water demands of the top growth and the plant suffers what is commonly referred to as transplant shock. Removing some of the top growth by pruning at the time of transplanting (it's not too late to do it now) is the best way to reduce the stress to your new tree and help it become established more quickly. Giving the new transplant more water to compensate for the loss of roots is usually a sure way to kill the plant by "drowning" the roots.

It is possible that your tree is exhibiting a response to a soil nutrient issue, but that is not as likely. If the soapberry produces yellowed foliage next year, then you should suspect a nutrient problem. In that case, you might want to have your soil tested to determine what kind of soil chemistry issues you're dealing with.