Over the years I have long experimented with different soils for my growing dragon fruit collection, from trying store bought soil to making my own. With the help of one of my horticulturist friends in Southern California I finally found a mix that I have more confidence in and have used for a few years now. It is well draining, provides good nutrients, and promotes great root growth. The soil mixture is rather simple and easy and isn't too costly. I have found that it works amazingly well with my subtropical and tropical fruiting plants as well.
You take equal parts of Compost, Peat Moss, Coco Coir, and Perlite (I prefer the smaller sized perlite about 0.5-0.75cm). In a wheel barrow or large bucked thoroughly mix these four ingredients together until the mix is soft and fluffy. Then use as needed.
An alternative to the above mixture, following the same instructions is equal parts Compost, Coco Coir, and Perlite.