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FAQ about stonewool

Does stone wool remain disease-free during cultivation?

Stone wool arrives at the greenhouse disease-free. Whether soil diseases are able to penetrate is dependent on the level of hygiene within the company. Naturally, the plant material must also be absolutely disease-free. The careful setting up of the system, the use of ground sheets and properly packaged slabs will limit the risks still further. Should any problems nevertheless arise, they are then relatively simple to resolve. That is the advantage of the small root volume.

What is stone wool?

Stone wool is a mineral wool made of volcanic rock. Stone wool consists of 5% solid material in the form of stone fibres. The remaining 95% is made up of pores.

How is stone wool made?

The basic materials for Grodan stone wool are Basalt and Limestone. These are melted to form lava in an oven with a temperature of 1,500°C. The lava is then poured over a number of fast-rotating discs. Thanks to the centrifugal force, the drops fly off the discs and lengthen to form fibres. The process can be compared with candy floss being made at a fair. The fibres are then compressed to form a sturdy mass, from which the slabs and blocks are then cut.

People often mention capillary action capacity. What does that mean?

Stone wool slabs not only need to absorb sufficient moisture but also need to distribute it over the entire volume of the slab. Grodan stone wool has been specially developed to satisfy these requirements. If the moisture were to sink to the bottom straight away, there would be dry or salty spots in the slab.

The roots would not  thrive in such an environment, so the “rootable” volume would be reduced. A good “capillary action capacity” (water distribution) ensures that the entire slab volume can be controlled and thus also fully rooted.

What do growers gain from the “controllability of the substrate”?

With a controllable substrate, the crop has access to whatever it needs at that moment. The plant will therefore respond optimally and thus also be more productive. With Grodan stone wool substrate, the slab volume can be controlled right down to every little corner of the slab volume. Comparable results may be possible with other substrates, but would require around 10 to 15% more water and nutrients.

Why is a water buffer important in the cultivation system?

In the first instance, the water buffer must be able to cushion a temporary disturbance in the nutrient unit. A substrate with a lower “capillary action capacity” is more susceptible to drying out. In addition, the shocks in EC, pH and temperature in a substrate with good capillary action and with a water buffer are less extreme.

Savings in terms of water and nutrients thanks to the controllability of stone wool substrate are of course also important if recirculation is to be applied. With a water buffer, there are far fewer cubic meters of water to be disinfected, so the disinfection costs per square metre are lower.

Why is full rooting important?

Compared with soil, substrate slabs have a smaller volume. However, it is important that that volume can be used fully by the roots. After all, if the weather suddenly changes, a plant needs all its roots in order to be able to cope with the extra transpiration. If the substrate is not fully accessible for root development, there will be risks in the event of changes in the weather leading to a significant difference in evaporation.

Are slabs with a high capillary action capacity too wet for Winter cultivation?

No, since the moisture content of a slab is determined by the evaporation and the watering. Not using trickle irrigation and allowing extreme transpiration in the plant (hot pipes) will automatically lead to the slab drying out.

In other words: the moisture content in the slab is controllable. In principle, we strive for a lower moisture content in the winter. We can achieve this by allowing the crop to transpire sufficiently (minimum pipe temperature, fast ventilation) and through holding long pauses in between the watering sessions.

How do I determine the moisture content?

The water content in the stone wool slab can be assessed with the aid of the Grodan water content meter (WCM). This indicates the current water content of the slabs precisely so that the water content variations can be closely controlled. The WCM also measures the EC and the temperature of the stone wool slab.

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