Rock wool and other inert substrates

CANNA SUBSTRA nutrient line is specially formulated to provide the best results possible when used in combination with inert substrate. Rock wool, of all the inert substrate available, is the most common one. It is widely used in general horticulture because it ensures rapid development of good root systems. Additionally, it is easily managed and has good supportive properties. Plants grown in rock wool remain upright without extra support as long as the medium itself is supported.

There are many other inert substrate available to work with besides rock wool. In this section we will discuss some of them, but considering the popularity and suitability of rock wool we will discuss cultivating on this specific substrate in particular.

The origins of growing on substrates

The first farmers of mankind quickly discovered that plants grew better on the remains of other plants and manures; still it took thousands of years before people understood exactly why. Research into plant food began many years ago, long before our time, but only recently, about 150 years ago, researchers found out exactly which substances in the compost actually feed the plants. As a result of these discoveries, the artificial fertilizer industry was born.

Shortly before the dawn of this era, in the Netherlands, Napoleon introduced monoculture, where one crop per field is cultivated. The combination of these new systems increased farming production to new levels. Initially the new developments produced tremendous results, but this success was short lived. No one was familiar with these cultivation methods and all were certainly unaware of the drawbacks. The damage in the cultivation of vegetables was particularly noticeable. Year after year an excess of artificial fertilizers was applied creating problems in the soil structure and in the fertility of the ground. The same crops were grown year in - year out. In turn the monocultures brought on a multitude of plagues. Soil-bound pathogens were particularly difficult to counteract. A solution was urgently required. Growers began placing the crops in separate compartments and cultivating them on growing medium instead of in the open ground. This was the beginning of growing on non-mineral soil substrate.

Growing on artificial substrate was put into practice for the first time in the first half of the 20th century. As plastic containers became available growing on substrate made considerable progress. Production could be scaled up and automated. As it turned out, growing on these substrate generated up to 25 percent greater yields compared to cultivating in the open ground. This is because nutrients can be adjusted directly to the needs of the plant or crop at any particular time.

When CANNA SUBSTRA was introduced in the 1980s, serious small scale cultivation became possible on inert media and rock wool in particular. This CANNA SUBSTRA formula has been used successfully worldwide for many years and while many have attempted to copy the formula, an equal has yet to be developed.

Clay pebbles

infopaper-hydro_text_3Clay Pebbles are made by forming clay into pellets and then firing these in a hot kiln. This causes the clay to expand and become porous. Clay Pebbles are available in various shapes and sizes and with two types of surface; smooth and coarse. Clay Pebbles have been used in horticulture since 1936. They have the advantage that they can be reused for up to five years as long as they are well cleaned. The greatest disadvantage of Clay Pebbles is that they absorb almost no moisture, making them unsuitable as a run-to-waste substrate.

However, they are widely used in recirculation systems where the nutrients continually pass through the roots. This is because Clay Pebbles have good supportive properties and are heavier than water and, therefore, do not float. Additionally, Clay Pebbles are used extensively as a drainage layer at the bottom of pots when growing in coco coir, soilless mixes or soil.


infopaper-hydro_text_4Perlite is a glassy, volcanic rock that is ground and then baked at high temperature. Perlite is also inert, but due to its poor supportive properties it is relatively vulnerable as a growing medium itself.

It does make, however, an excellent soil improver and is used particularly to increase the air ratio in the soil; though, nowadays there are environmentally friendlier methods to do this, such as adding white peat.


infopaper-hydro_text_5Mapito is a very light medium with limited water retention capacity. This means that the substrate will dry out faster and needs to be watered more frequently. Mapito is a mixture of Polyurethane (PU), rock wool and sometimes coco or even perlite.

Most types of Mapito are not “clean”, in that they often have a higher EC and a lower pH than the ideal growing medium; this is the great disadvantage of Mapito. So it is always essential to determine the exact pH and EC values of the Mapito and to rinse it thoroughly before use!

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