How to interpret the water retention capacity of substrates?

In this article, we are going to discuss one of the most important physical property of substrates: the water retention capacity. Many times, it gives rise to confusion due to the way it is measured, but then we will show you how to know which substrates are the best and those that are worse at retaining water.


A nice soil- or coco-based substrate should have the capacity to retain water and stay moist. This property depends on several characteristics of the substrates, such as their structure and composition:

  • Porosity: As we already commented in the previous article about properties, the small spaces between the particles of a substrate, also called micropores, have the function to retain water, part of which is available for plants.

Water (blue) is filling the micropores between the substrate particles.


  • Organic matter content: Generally, organic material retains much more water than inorganic material. Thus, substrates with high proportion of organic matter (coco, humus…) will retain more water and will stay moist longer than inorganic substrates (clay pebbles).

Of course, these are general rules and there are always exceptions. For instance, rockwool is an inorganic substrate, but as it has a certain structure and porosity, it retains a lot of water.

We can find the value of water retention on the bags of all substrates.


Normally, a soil or coco substrate with a heavy texture and rich in organic compounds retains more moisture in comparison to a light substrate which is poor in organic matter. This is the reason why organic substrates as Kilomix or Cocos Substrate retain water so nicely and stay moist longer, which means a lower watering frequency is needed!

An easy way to increase the water retention capacity of your substrate is to add Worm Delight to it – a substrate improver which is almost completely made of organic matter: worm manure.


A tricky value  

Water retention is measured as the grams of water retained per gram of dry substance (g/g DS). The consequence of this method is that water retention capacity on the bags is actually the water rate only of the organic matter present in the substrate.

So, substrates with low proportion of organic matter will present an apparently high water retention value even if they do not retain water well. This is the case of the totally inorganic substrate Hydro Rokz: although the label indicates a water retention of 860 g/g DS, the reality is that clay pebbles do not retain water well. Hydro Rokz is used in hydroponics or to facilitate drainage and aeration.

On the contrary, substrates with high proportion of organic matter will present an apparently low water retention value even if they retain water great. This is the case of Worm Delight, whose label marks only 4.5 g/g DS, but the reality is that it retains water nicely.

Tip: To avoid confusions, you just have to look for the value of moisture content in the label of each substrate, which is expressed in percentages (%) in product’s Declaration. Moisture content is only 5% for Hydro Rokz, while it is 80% for Worm Delight – looking at this is much easier to decide which substrate to use for drainage and which for moisture!

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


Understanding the meaning of the substrates properties is essential for every grower. This knowledge will help you choose the right grow medium for your plants and know how to treat it properly, obtaining maximum yields when harvesting. Now that you know what is the water retention capacity, you can adjust the irrigation amount and frequency for your crop. This way, you will save a lot of money and time!