Pioneering Hydroponics

Since 1976

Rockwool Slabs and Nutrient Application

Did you know that Rockwool is the most used substrate when it comes to growing tomatoes hydroponically?

Rockwool slabs are also a great growing medium for a wide variety of other plants due to being easy to set up and use.

It does, however, need to be monitored and its irrigation adjusted accordingly to make sure you make the most out of Rockwool’s ability to hold moisture and aeration simultaneously.

Rockwool and Moisture Gradient

Rockwool slabs usually have a lot of moisture at the bottom while the top has drier conditions. This allows for better oxygen and aeration for root respiration and uptake. This moisture gradient, from top to bottom, makes Rockwool a great substrate for a lot of growers.

The moisture gradient is why many growers often believe Rockwool is dry, leading to over-irrigation of plants. The nutrient solution is often held deep down in the root system.

When irrigated correctly, Rockwool shouldn’t sit in a pool of nutrient solution by being soaked from top to bottom. This growing medium needs to drain completely for excess nutrients to leave the slab. This allows for fresh air to be drawn to the material’s top layers for fresh oxygenation to the roots.

Letting Rockwool drain completely also prevents overwatering, however, you need to be careful to not allow large quantities of nutrient drainage from the bottom of the slabs.

Nutrient Application

Factors such as temperature, plant size, light, humidity, and growing conditions affect the amount of nutrients required and drive plant transpiration and water uptake. 

The irrigation for Rockwool slabs needs to be adjusted for the specific plants and growing conditions in your grow room. You may find you’re underwatering your plants if you simply follow guidelines for nutrient application.

You can help Rockwool reach ‘field capacity’, which means that the substrate has completely drained but still holds a good moisture level for the roots until the next irrigation. It’s advised that Rockwool should be irrigated with short and frequent nutrient applications so that it can reach this capacity.

Optimal conditions are considered to be 10-15% of nutrient solution being fed to plants with the slab being drained at each irrigation. This allows for a fresh nutrient solution to be flushed through the Rockwool slab without a lot of wastage, which keeps the EC level fairly stable.

Irrigation and natural drainage of Rockwool allow for it to retain 80% of the nutrient solution, 15% of air pore space, and 5% of Rockwool fibres. 

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