The rock wool substrate

After making our entry on hydroponic cultivation  in which we wanted to make a slightly more technical guide on growing without soil, now we are going to make some articles on the main substrates that can be used. In this case we have to talk about one of the best known and used in this system, rock wool. 


The origin of the use of stone wool as a substrate for hydroponic cultivation was born in Denmark and soon moved to the Netherlands, where they have a great culture of this type of crops and have impressive greenhouses with the best available technology. Today it is one of the most widely used substrates, due to its cost, its quality and its characteristics, which we will see throughout this article.


Rock wool is composed of the following elements, mixed and melted at 1,600 degrees

  • Diabase 60%
  • 20% coking coal
  • 20% limestone

All this mixture of components and at that temperature results in very fine fibers 0.005 mm thick. In addition, some more materials are added to balance the mixture and give it more properties. When ready to sell it is considered safe and pathogen free. Imagine, at 1,600 degrees …


Let’s go to what interests us. In the article on hydroponic cultivation, the characteristics that a good substrate should have to fulfill its function appeared. Let’s see what are the properties of rock wool.

  • Reserve water: 0.9%
  • Aeration capacity: 35-45%
  • Total porosity: 96%
  • Water holding capacity (readily available): 30%
  • Apparent density: 0.08 g / cm2
  • Has no buffer power
  • Easily assimilated water:> 95%

Under these characteristics, we can say that it is a good substrate, has very good porosity , good aeration capacity, water retention capacity and a low percentage of reserve water . Perfect!

It has some cons, yes (like everything). It has a limited duration , so when the time comes, its properties are lost and must be replaced to ensure the timely growth of the plant. Furthermore, it is also weak in terms of its mechanical stability. 

However, the one that strikes us the most is that it is not biodegradable,  so you have to be careful with it. In Europe there are some recycling plants dedicated to taking advantage of already used stone wool, reusing it for construction materials, although it is not found in Spain . Given the growth that stone wool is having in Spain (especially in greenhouses in southern Spain), it is necessary to take measures in this regard and guarantee a reuse of this substrate.


At first, stone wool has alkaline reactions that can increase the pH of the nutrient solution, but it is gradually corrected by providing irrigation with more acidic nutrients than normal, around 5.5-5.8. It does not have a buffer capacity, so variations in pH can be achieved with little nutrient solution.


In Spain, rock wool became known in hydroponic cultivation in 1982. In these current dates it is the main substrate and has had an exponential growth in use since its year of introduction.

It is usually presented in tables that usually last 2 years and are 1 meter long and between 15 and 25 cm wide. In this table, 2 to 6 plants are usually grown, but for high-density plantations, 9 or 10 plants can be reached.

The fact that it is the most widely used substrate material in Spain and in Europe is due to its great free water retention power (where the plant can assimilate it without any problem), which is over 95%. This means that there may be long periods when watering is not needed again. However, one of its biggest cons is that, given its low buffer capacity, greater control of the irrigation water and the nutrients used is required.

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