What is the difference between an inert and an active substrate?

You may have heard people saying that inert substrates are better for hydroponics, but do you really know what is the difference between an inert or an active substrate? You're two minutes away from finding it out!


They are two types of substrates depending on whether they are reactive or not. The chemical reactivity of a substrate is defined as the transfer of matter between the substrate and the nutrient solution. 

  • Reactive or active substrates: These substrates retain and store mineral nutrients, transferring them according to the demands of the plant. The chemical activity contributes adding additional elements to the nutritious solution by chemical processes of hydrolysis or solubility. Some examples of active substrates are white and black peats, pine bark, coir, vermiculite, lignocellulose materials, etc.
  • No reactive or inert substrates: The chemically inert substrates act as support for the plant, not intervening in the process of adsorption and fixation of nutrients, so they all have to be supplied by the fertilizer solution. Some examples of inert substrates are granite or siliceous sand, gravel, volcanic rock, perlite, expanded clay, rock wool, etc.

Generally, inert substrates are preferred for hydroponic cultivation as they are more stable and do not alter the pH and the salinity of the nutritious solution. In hydroponics it is very important to keep steady the properties of nutritious water, since a small change in pH or temperature could mean the loss of your crop. As for other cultivation techniques, the most suitable substrate depends on your preferences and the requirements of your plants.

What makes a substrate active or inactive is a chemical property of all substrates called cation-exchange capacity (CEC). If the CEC value is high, this means the substrate is able to store nutrients (it is active) and if it is low or non-existent, the substrate is inert.

Chemically, cation-exchange capacity (CEC) is the ability of a substrate to retain and release positive ions, thanks to its content in clays and organic matter (mainly humus).


Clay-humus complex

Clay bonds to hummus forming the clay-humus complex. Substrates with higher concentrations of clays and organic matter exhibit higher cation exchange capacities and are more active chemically.  

The clay-humus complex presents negative electric charges on its surface, so it is able to attract and fix positively charged particles, a phenomenon known as adsorption. The negative charged ions are not retained so they can be dragged dissolved in the water to deep layers. Remember that "opposites attract", so the particles with the same charge repel each other while those with different charges are attracted.

The purpose of the fertilizers is to provide the soil with the following particles or ions:

  • Positive ions: Ammonium (NH4+) and potassium (K+).
  • Negative ions: Phosphate (PO43) and nitrate (NO3).

From these ions, the plants obtain their very essential elements: nitrogen (N), phosphorus (P) and potassium (K).

Ammonium, potassium and phosphate are fixed by the clay-humus complex. Ammonium and potassium are fixed because of having the opposite charge. Phosphate, in spite of being negatively charged, is fixed to the complex by chemical “bridges” formed by calcium, which is positively charged. In contrast, nitrate is not retained and washes away quite quickly. So, substrates that are chemically active, have more clay-hummus complexes and are able to retain fertilizers better and therefore, the plant has greater facility to absorb the nutrients it needs.


Which one is best for me?

If you are applying fertilizers manually intermittently, you would like these fertilizers to stay fixed on the root environment so the plant can use it over time. In this case, an active substrate will keep your plant nourished until the next fertigation.

The substrates with higher chemical activities are the ones containing black peat, such as Bi-Grow Mix, as these peats have an elevated content of organic matter. On the other hand, if you would like to increase the CEC of your substrate, you can mix it with Worm Delight , which is organic matter. Adding 10-20% of this worm manure to the substrate provides good and rapid growth and increases the yield considerably.

Regarding to coir substrates, they also present high value of CEC, although not as much as soil substrates do. Coco substrates are composed by a great mix of microscopic particles that are negatively charged, and the same way as clay-humus complex does, these particles attract positive charged particles. Coconut trees grow naturally close to the coast, where there are higher levels of salts, and the ions sodium (Na+) and potassium (K+) get attached to the coconut. If the coir is not properly treated before being used, these ions are released to the root environment, leaving “gaps” in the in the coir. Finally, these gaps are filled with calcium (Ca2+) and magnesium (Mg2+), that get attached to the coconut fibre and stop being available for the plant.

As a consequence, the plant has less calcium and magnesium for itself and deficiencies can appear. If this happens, calcium and magnesium fertilizers such as Calmag can be applied, but it is certainly best to prevent and use a high quality buffered coir substrate from the beginning, which ensures that it will not absorb the plant nutrients.

If you are growing using hydroponic systems, you may be applying fertilizers permanently. In this case, substrates with low CEC are recommended in order to avoid over-fertilization of your crop. This is the reason why chemically inert substrates are used in the hydroponic culture. Hydro Rokz  is an ideal substrate for use in both passive hydroculture applications and active hydroponic systems including aeroponic, NFT, drip, and ebb and flow.