When selecting an inert medium for use in Semi-Hydroponic culture, one must consider several factors: particle size, particle size distribution, the ability of the medium to absorb nutrient solutions and the free volume between particles being the primary ones. After that, one must properly prepare the medium by rinsing the dust and presoaking to remove manufacturing residues.
It has been decades since I evaluated such media, and a lot has changed – old brands are no longer manufactured, new one are being produced, and some new materials have been brought into the market.
With the help of a few other orchid growers, I acquired 8 samples for testing:
Cz Expanded Clay Pebbles
Distributed by Cz Garden Products of Lathrop Village MI. Typical spherical clay aggregate.
An irregularly-shaped LECA product distributed by Hydrofarm.
Hydro Crunch Expanded Clay Growing Medium
Spherical LECA pellets distributed by Hydro Crunch of Walnut CA and available from Home Depot and online vendors.
Hydroton Expanded Clay Pebbles
Manufactured in Germany and widely distributed. A spherical clay aggregate that is the defacto standard in hydroponics.
Another spherical aggregate marketed by IKEA.
UltraAgg Claymax #40XL
Calcined clay product marketed online by American Bonsai out of Merritt Island FL.
Calcined montmorillonite clay distributed by Bonsai Jack of Fort Myers FL and sold online and through retail bonsai shops.
Ultra Agg Ultra DE Dark #30 Std
Calcined diatomaceous earth marketed by American Bonsai out of Merritt Island FL.
1. A 470 ml plastic cup was overfilled with the as-received, dry material.
2. A metal straightedge was used to scrap of excess; any particle obstructing the “swipe” was removed.
3. Dry weight was measured.
1. 75 ml reverse osmosis (RO) water was placed in plastic cup.
2. Cup was filled with dry material.
3. Photos were taken immediately and 60 minutes later.
(Testing done at 77°F and 86% RH)
1. Dry samples were placed in plastic bag.
2. 750 ml RO water was poured into bag, which was squeezed to remove all air and then heat sealed.
pH & Electrical Conductivity (EC):
1. Bags were opened and HM-Digital COM-100 EC Meter was inserted into liquid for EC measurement.
2) Hydrion 9200 pH Strips were dipped into the liquid and compared to the manufacturer’s color scale.
1. Soaked material was added to the plastic cup using the same “swipe” technique used for the dry weight.
2. Material was weighed.
1. Perforated lid was placed on cup to prevent flotation.
2. RO water was added to cup to the point of overflowing.
3. Water was poured into another cup and weighed.
1. Bulk Density = Dry Weight (g)/470 ml
2. Absorption % = (Saturated Weight – Dry Weight)/Dry Weight
3. Void Space = Free Volume (ml)/470 ml
Dry Wt g/470 cc
Bulk Density g/cc
Submerged Wt g/cc
Saturated Wt g/cc
Free Volume ml
Void Space %
Visually, the Cz, Hydroton and Odla materials are extremely similar. Add to that the physical properties, and I speculate they are the same material, repackaged under different brand names. The glaring difference in the EC of the Cz material from the others may be nothing more than a manufacturing batch-to-batch variation, and points out the need for thorough preparation of the products prior to use.
The Hydro Crunch material is similar in size and shape to the other brands of LECA, but appears to have been coate in a clay slip prior to firing, apparently reducing the surface porosity, which is supported by the reduce water absorption and possibly the EC, as well.
The Electrical Conductivity and pH numbers, to reiterate, support the requirement of thoroughly rinsing and soaking any inert medium before use, but the fact the that pH levels of the soaking water stayed in such a narrow band is encouraging.
All of the products have similar free void space – essential for health roots, which is attributable to their relative particle size uniformity. Personally, I feel that the calcined clay and diatomaceous earth products are just too fine for the majority of orchids, as the size of the spaces between the particles can be completely blocked by roots and when saturated, might allow for bridging water between the particles.
Looking specifically at the LECA products – Cz, Grow!t, Hydro Crunch, Hydroton and Odla – they all appear to be perfect acceptable for orchid media. The Hydro Crunch material floated far more than the others, in agreement with its lower bulk density and absorption properties. Grow!t also had a lower bulk density, likely due to its irregular shape and surface texture, and absorbed less water when saturated, but the interlocking of the particles might provide some stability compensation that may be lacking in the spherical products.
Based upon my decades of growing in semi-hydroponics using a LECA medium, I would have no hesitation to use the Cz, Hydroton or Odla materials interchangeably, with the Grow!t brand a close second. The Hydro Crunch material is a close third in my book, only because its relatively low absorption capacity.