Substrates

In Hydroponics, cultivation is done without soil. Still, plants need some sort of mechanical support depending on the system at issue. What is used then are inert materials (culture or growth media) that do not interfere with the chemical composition of the nutrient solution.

These substrates can be either natural or mineral, and must ensure good aeration of the roots, allow a good retention of water and a good availability of nutrients and yet be pathogen-free. The several alternatives of substrates can be mixed in different proportions so as to ensure the desired environment for the crop. For instance, if we use only coir, there will probably be a lot of water retention in the substrate, little ventilation and may not be compatible with watering times, so we can add clay or perlite in equal or lower proportion to ensure adequate drainage and aeration.

The use of substrates has its advantages, particularly in average / long life cycle such as tomatoes, aubergines / eggplants, onions or kale, as they offer more stability to the plant roots, which get more protected from heat, diseases and possible system failures. Many of them can be reused, giving more sustainability to systems.

There are hundreds of different culture media; anything in which a plant can grow can be considered as such medium. Even the air can be an appropriate culture medium. Some of the most commonly used natural substrates are peat, pine bark, sawdust, coir, rice straw and other cereals. As for minerals, some common ones are rock wool, perlite, vermiculite or expanded clay. The following is a description of some of the most used substrates.

 

COCO PEAT / COIR

 

  It is made of the material that is left of coconut shell and is 100% natural. It is composed of millions of micro-sponges that absorb and store up to eight times its weight in water. It is usually colonized with the Trichoderma fungi, for protection and stimulation of the roots. Its ability to retain water vs ventilation is very good, and so it is very difficult to have the plants drowned. It is also a substrate with a high exchange level of cations and can store some nutrients when the plant needs. It is a durable material and can last up to 3 times more than peat.

 It is available in several shapes, but the most common ones are small compressed bricks that expand 6 to 8 times in contact with water.

Advantages: Good aeration and water retention capacity.
Disadvantages: limited number of uses. Low drainage capacity.

Expanded clay (or LECA)

It is made from overheated clay, which causes the grains to swell like popcorn, forming “balls” up to 2 cm in diameter. It should be washed before use to remove any red powder that is usually present.

 Advantages: relatively light but possesses some water retention capacity; may be sterilized and reused. It is not compressible.
 Disadvantages: little water retention capacity (compared with coco peat, for instance ). It is obtained through mining.
Tip: it can be placed in a thin layer on the bottom of pots for better drainage and to prevent the escape of thinner substrates.

PERLITE

 

It is made of the fusion of volcanic rocks, overheated into expanded glassy grains. It is a culture medium commonly used to increase the aeration capacity of other substrates, being an extremely light and porous material. Its light weight may also be a disadvantage in certain systems as it moves very easily and may vanish or cause some kind of clogging. It is not used alone but mixed with coco peat or vermiculite .

Advantages: Very light, high aeration capacity.
 Disadvantages: Too light for certain systems. It is obtained through mining.

ROCK WOOL

  It is an inert substrate suitable for both closed and recirculating systems. Rock wool is made from molten rock, basalt, “dross” or “slag”, which is converted into bundles of single filament fibers and glued together in a medium with capillarity, and gets protected from the most common microbiological degradation. Despite being widely used in hydroponics (mainly in business exploitation), we do not recommend its use.

 Advantages: Good water retention and aeration. It can take many shapes and forms, being a consistent substrate that do not scatter about.
Disadvantages: It is pollutant, as it does not degrade. The resulting powder is considered hazardous to health, particularly to the lungs. Limitations in terms of the influence on the pH of the solution