Haworthias need a very well drained yet moisture retentive soil for the best growth. Soil choices are all about soil structure, where particle size plays a primary role.
There are two forces that cause water to move through soil – gravity pulls water down through larger soil pores; and capillary action retains water in smaller soil pores as a result of the natural forces of adhesion and cohesion.
A given volume of large soil particles has less overall surface area when compared to the same volume of small particles and therefore less overall adhesive attraction to water. So, in soils with large particles, gravitational flow potential (GFP) more readily overcomes capillary attraction. …..[insert bonsai statement here]….
When soil particle size is under about .1 inch, there will be a naturally occurring “perched water table” (PWT) in containers. It is there because the capillary attraction of the soil at some point will surpass the GFP. The water can evaporate or be used by the plant, but will not drain. The smaller the size of the particles in a soil, the greater the height of the PWT.
Besides the size, the uniformity of soil particles is important. Mixing large particles with small is often very ineffective because the smaller particles fit between the large, increasing surface area which increases the capillary attraction and thus the water holding potential.
Is Coarse Drain Layer Needed?2>
Pumice is a volcanic rock that consists of highly vesicular volcanic glass strands permeated with countless tiny air bubbles. As the growing medium, it has several key advantages. Its hpores vary in size and shape. The large pores drain quickly and takes in air. The microscopic pores holds moisture with nutrients. It is lightweight but substantial enough to support root systems and not float away. Finally, it is enduring and hardly compacts or breaks down in a couple of years.
Pumice should be sifted to 1/8 – 1/4 inch size to achieve optimum drainage. Besides, pumice is crushed from larger pieces and often come with undesired fine dusts. It should be rinsed in water until the runoff becomes clear.