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.
Hydroponics is a method of growing plants in physically stable and (bio-)chemically inert mediums, and periodically supplying nutrient solution for plant growth. The structure stability leads to consistent good drainage, and the inert property allows to accurately control the nutrients and the pH in the solution. In fact, container culture, regardless of growing mediums, is close to hydroponics because the breakdown of organic components is slow and unstable. Haworthia growers widely use growing mediums of which the main components are mineral particles.
Pumice is a solidified frothy lava. It contains a large percentage of cavities at its surface and inside, and can hold substantial quantities of moisture. The surface of each particle is soft and roots ramify well in pumice. Lava rock is very close to pumice and therefore a good substitution. Decomposed granite holds less water than pumice and can be added to speed up water evaporation in a humid or cold climate.
On the other hand, these are less suitable: perlite is too light in weight and float away while watering; vermiculite breaks down to fines over time; crushed granite has shape edges and when containers are squeezed, the edges could make wounds on the roots and bottom leaves and lead to rot.
Due to the mining and crushing，mineral components could carry undesired fines. Before using as the growing medium, they should be sifted (and rinsed in water) to remove fines.
The purpose of adding organic components is to increase the moisture retention of a growing medium. Fine roots and root tips could die if left dry for days, and this increases the chance of rotting after watering. Hence, organic components with larger particles like bark which have similar water holding capacity as mineral components are not useful.
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.