Haworthias need a well-drained yet moisture retentive growing medium (soil) for the best growth. These properties are primarily determined by the structure and the components of the growing medium.
When water in the growing medium moves downwards under gravity, a portion of it is held in the pores of 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. Optimum drainage is achieved through pure non-absorbent particles of ~4 mm size. Any larger particles increase very little to drainage and become mere obstructions to root growth.
When the particle size of the growing medium is under ~2.5 mm, water will form a perched water table (PWT) at the container bottom or above coarse drainage layers, where capillary action overcomes gravity. Perch water can evaporate but will not drain. The smaller the particles size, the greater the PWT height. This saturated water occupies air space and often causes root rot.
Do NOT Add A Coarse Drainage Layer
With the same growing medium, PWT heights are the same in all containers. Adding a coarse drainage layer (for example, gravels) at the container bottom does not improve drainage, but moves PWT higher and closer to roots.
The uniformity of particles is also important. Mixing large particles with small is often ineffective to drainage because the smaller particles fit between the large, increasing surface area and consequently capillary action.
For Haworthias, a growing medium with particles in 2.5~4 mm size range provides the ideal balance between drainage and moisture retention. Sieves can be used for removing the particles that are not in this range.
Haworthias grow the best in physically stable and (bio-)chemically inert mediums, and periodically supplying nutrient solution for plant growth. The structural stability ensures consistent good drainage, and the inert property allows to accurately control the nutrients and the pH in the solution. Haworthia growers widely mineral particles with these properties as the main components of growing mediums.
Pumice is a solidified frothy lava. It contains a large percentage of cavities at its surface and can hold a medium amount of water. Besides, the surface of each particle is soft and roots ramify well. Lava rock is very close to pumice and therefore a good substitution. Decomposed granite holds less water than pumice and can be added to accelerate water evaporation in a humid or cold climate.
On the other hand, perlite is too light in weight and float away while watering. Vermiculite breaks down to fines quickly. And crushed granite has shape edges and when a container is squeezed, these edges could create wounds on the roots and bottom leaves and cause the plant to rot. They should be avoided.
Due to the mining and crushing，mineral components may contain undesired fines. Before using as the growing medium, they should be sifted (and rinsed in water) to remove fines.
In container culture, organic components cannot be relied on as a stable source of nutrients, since their breakdown to the forms that can be used by plants is slow and unstable. On the other hand, some organic components absorb a high amount of water and can be used as an amendment to increase the moisture retention of a growing medium. Fine roots and root tips of Haworthia could die if the growing medium is bone dry. This not only stunt the growth but also increases the chance of rotting after next watering.
Peat is an accumulation of partially decayed vegetation or organic matter. Peats holds a large amount of water and is an excellent amendment. The ideal percentage of peat depends on the growing environment but in general, it should not exceed 25%. Peat contains a lot of colloids and fines. To remove them, soak the peat in water thoroughly and stir until there is no large mass. The colloids will precipitate and fibers will float on the top. After 1 day, take out the fibers, squeeze water out and let them dry. At last, sift the dried fibers to remove fines < 1mm.