How to decipher the labels of the substrates?

Choosing a substrate isn’t an easy task and even experimented growers struggle with this sometimes. The unique values of the properties of each substrate are written on the label of this particular substrate. But do you know what each property refers to? In these articles we tell you everything!


The best culture medium for your plant depends on numerous factors such as the type of vegetable material with which you work (seeds, plants, stakes, etc.), plant species, climatic conditions, irrigation and fertilization systems and programs, economic aspects, etc. But, generally, a substrate has to meet the following requirements:

  1. High porosity.
  2. Sufficient air supply.
  3. High water retention capacity.
  4. Low cation exchange capacity (CEC) if the fertigation is applied permanently or appreciable CEC if it’s applied intermittently.
  5. Low salinity (low EC).
  6. Sufficient level of assimilable nutrients.
  7. High buffer capacity and ability to keep the pH constant.

In this first article of this section, we are going to explain all you need to know about the first two physical properties of substrates: porosity and air content.

All the values that you want to know are in the label of all Atami’s substrates.



Porosity or pores fraction is a measure of the void or porous spaces in a substrate. The porous space refers to the percentage of substrate volume not occupied by solids. Within the porous space we can distinguish macropores (larger spaces) and micropores (smaller spaces) where water, nutrients, air and gases can circulate or be retained:

- MACROPORES do not retain water against the force of gravity as they are too large. They are responsible for the drainage and aeration of the soil and constitute the space where the roots are formed.

- MICROPORES function is to retain water, part of which is available for plants. They prevent the soil from drying.

Macropores and micropores.


An ideal substrate should have an adequate proportion of micro and macropores: the macropores will provide good aeration and the micropores will facilitate the retention of water.

It is important to note the fact that a substrate with a very high porosity is not necessarily a good substrate. A substrate with only micropores can be very porous – it would be easily saturated with water and nutrients, but the aeration would be almost nonexistent, because the very high water content would not leave space for the air.

Among the different types of substrates, the ones with nicer porosity are the substrates based on coconut husk. This is because they are composed of particles of different sizes and structure. This allows many pores of various sizes. The coir substrates of Atami present an ideal proportion of micro and macropores and an impressive 94% of pores fraction.

Atami’s soil substrates have a porosity of 91% and substrates for hydroponics, 85%. The porosity of these last one is still really great, but it is lower than the previous ones because substrates for hydroponics are made of clay pebbles and clay is a very compact material, Thus, the porosity of this substrate is mainly due to the spaces between the different pebbles.

Clay pebbles.


Air content

As we have seen above, the air content of a substrate does not depend directly on the porosity value, but of the ratio between macropores and micropores, Normally, micropores are “full” of nutrients and water, so macropores are the voids in which gases, such as oxygen, can circulate and be supplied to the roots. This translates that substrates with a large amount of macropores present higher percentage of air.

Let’s use an example to see how the air content of a substrate does not depend that much on the total amount of pores fraction: Hydro Rokz, a substrate made of clay pebbles, has the lowest pores fraction of all Atami’s substrates – 85%, as mentioned before. But surprisingly, it presents the largest amount of air content – more than 70%! This is because this material presents mainly macropores. Easy to grasp, right?

You can find the value of Air content in the label of all Atami's substrates.


One tip to make your substrate airier is to add perlite to it. Perlite is a volcanic rock used to procure a more heterogenous structure to the substrates. Generally, coir substrates are lighter an airier than soil substrates. An example of this kind of substrate is High Porosity Cocos, a substrate specially developed to provide the maximum possible air supply for the roots. However, soil substrates get high air content when mixed with perlite, such as Janeco Light Mix or Kilomix.