All plants have particular needs when it comes to the potting media or soil blend that they’re grown in. In any given plant’s native habitat, the soil has specific properties - pore space, drainage potential, and fertility, for example - which are best suited to the plant’s needs. So, in blending soils, the goal is to replicate the properties of the plant’s native soils.
Willow trees are water-hogs for example, and grow near water sources. They do best in a soil that can hold LOTS of water. Most ferns grow in shaded and deep woodland soils with high organic (nutrient) content and very little inorganic content. Many carnivorous plants grow in bogs which remain damp nearly always and contain very few nutrients.
Succulents and cacti are no different. The majority of them are native to the rocky, sandy, and fast-draining soils of arid climates - Southern Texas, Mexico, Southern Africa, parts of Australia, etc. The soils (so to speak) that they grow in contain very little organic matter, which means that the soil has low nutritional value and will drain moisture very quickly. These, then, are the properties that should be replicated when blending a soil specifically for cacti and succulents. Fast draining, good pore space, with low organic matter (decaying plants, primarily).
While there are dozens of different ingredients that can go into a succulent blend, the focus should remain on replicating the simple properties of the plant’s native habitat – fast-draining, good pore space, with low organic content. Simple.
One recipe that we’ve used at Flyleaf to grow tens of thousands of plants consists of 2 parts perlite to 1 part peat moss. The perlite (which is inorganic) allows the soil to drain very quickly and provides pore space for roots to grow and breathe, and the peat moss (which is organic but very low in nutrition) holds just enough water to allow the plant a good drink before the rest of it drains away.
This blend is about as simple and basic as a blend can be, but it replicates the properties of the plant’s native habitat, so it works.
Recently (due to limited availability of perlite), we have developed a new blend which consists of 2 parts peat, 2 parts rice hulls (which, though they are organic, still provide excellent drainage and pore space), and 1 part Pro-Mix potting soil with mycorrhizal fungi (these form natural, symbiotic relationships with about 85% of the world’s plant species, and increase the plant’s ability to process water and nutrients, nearly exponentially). This blend is still in the experimental phase, but I believe it’s going to prove to be a championship quality product.
What about the myriad other soil blends and ingredients that people talk about?
While it’s fine and fun to craft your own blend, remember that there are only a few basic properties that need to be replicated. Drainage, pore space and nutrient content. The plant itself doesn’t care if you invest $0.50 or $50.00 as long as the properties of its native soils are kept in mind.