Hi Peter -
When you say soft sand, how soft do you mean? Mudflats or deep sand? To a point, the sand is a great way to see how your technique is doing. If you leave nice clean prints, and no dishing at the heel or toe, you’re doing it right. That is known as the Sandpit Exercise and can be found on page 89 of the Chi Running book.
Really soft sand will make your run harder, just as it did before Chi Running. But you might find it easier compared to your previous way of running, especially with a quicker turnover in cadence, the lean (less need to push off), and the midfoot landing/ankle lift (again, no push off).
Not sure if that answered your question, but go give it a try and report back how it felt.
Certified Chi Running and Chi Walking Instructor