Choosing a right potting soil is all about root health. When planting Haworthias in containers, the most important considerations are soil drainage and aeration. They are both greatly influenced by soil particle size and the nature of ingredients.
When water moves down in soil via gravity, a part of it is retained in the soil pores by capillary action. Excess soil water could cause roots to decay since it occupies air space, and prevents roots from breathing.
The key to good drainage is size and uniformity of soil particles. Clearly, small particles drain more slowly than large ones. Optimum drainage is achieved through pure non-absorbent mineral with particles of ~1/8 inch. Besides, Mixing large particles with small is often ineffective because the smaller particles fit between the large, increasing the capillary action.
When the particle size is under ~1/10 inch, water will occupy a layer of soil at the bottom of containers or above coarse drainage layers and remain saturated. This “perch water table” does not drain and only evaporates over time. This also implies that adding a coarse layer (like gravels) at the bottom of a container doesn’t really improve the drainage but mistakenly moves possible waterlogged layer closer to root.
For Haworthias, a good potting soil should drain fast and hold just adequate moisture. The recommended particle sizes are 1/8 – 1/4 inch.
Mineral vs. Organic
There is no physical advantage in using any organic matter (e.g. peat) in the soil, but there are some disadvantages. After a time any organic matter will break down into fine particles and increase the water retention. Besides, once bone dry, most organic matter actually repels water and is difficult to re-wet.
Haworthias grow greatly in pure minerals. Since there aren’t much nutrients at all in minerals, regular fertilization is needed.
Pumice is a volcanic rock that consists of highly vesicular volcanic glass strands permeated with countless tiny air bubbles. As the growing media for Haworthia, Pumice has several key advantages. Its natural variety of pores in size and shape not only provides good drainage and aeration, but also captures moisture with nutrients. It is lightweight for easy access when planting, but substantial enough to support roots. Finally, it does not compact or break down over time so does not need to be replenished.
Horticulture pumice is made by crushing larger pieces. This process often produces undesired fine dusts. It is recommended to rinse pumice in water before using.
In contrast, other common mineral grow mediums have some problems when used alone. Granite is heavy and its flat surface doesn’t hold enough water and nutrients for plant growth. Sand and vermiculite hold too much water. Perlite drifts around when watering and doesn’t secure the plant.