Haworthias grow best a growing medium that is well-drained yet moisture-retentive, and these properties are mainly determined by its particle size and components. Choosing the right growing medium should be largely based on the local climate and also taken the development phase of individual plants into consideration.
When water moves downwards under gravity, a portion of it is held in the pores of the growing medium by capillary action. A given volume of growing medium with smaller particles has a bigger surface area than those with larger particles, and therefore holds more water.
The optimum drainage is achieved through ~4 mm particles. Any larger particles increase very little to drainage and become mere obstructions to root growth. And when the particle size is under ~2.5 mm, water will form a perched water table (PWT) at the container bottom, or above any layer of more coarse-textured, where capillary action overcomes gravity. Perch water can only evaporate but will not drain. It occupies air spaces and can cause root rot. The smaller the particle size, the greater the PWT height, regardless of the sizes and shapes of containers.
In general, seedlings and plants being rooted need smaller particles than mature plants with healthy roots, because such particles hold more moisture and allows new roots to better elongate. It is recommended to first use sieves to removing the particles out of the desired size range and then wash the growing medium in running water to remove fine dust.
Since Haworthias are perennial, a good growing medium should be physically stable within a few years. Therefore some common materials that are easy to decompose, like peat and vermiculite, should not be used as the main component of a growing medium.
A growing medium can be composed of just a single component or a combination of components. The baseline is that it should approach dryness 1~3 weeks after watering under the local average weather condition. The table below compares the water retention properties of some common materials with the same particle size.
In Northern California, I use a mix of 80% 2~6 mm pumice and 20% 1~3 mm coir for mature plants.