Hydroponics Grow Media
The term ‘growing medium’ is used to describe the material used in a container to grow a plant.
The growing media is the substance over which the roots of the growing plants are supported. The plats can grow in either solid support media or simply over water. The function of the growing media in hydroponic crops is totally different from the one achieve by soil in traditional cultivation, because in this case the growing media is just the plants’ mechanical support and it’s not involved in any other growing process. Sometimes the term ‘substrate’ is also used and means the same thing.
Requirement of good Grow Media –
The ideal growing media is the one that can supply the plant’s necessities of air, water and support; the media has to have a favorable interaction with water in order to maintain the humidity for a long time and it also has to have particles big enough to let the air flow and therefore, allow the oxygen to dissolve in the nutrient solution. The ideal growing media has to be chemically inert for both the nutrient solution and the plant, and it shouldn’t modify neither the pH nor the solution’s nutrient balance. Additionally, the media shouldn’t have a significant reaction with any of the substances excreted by the plant. The media should also be biologically inert, which means that it shouldn’t contain any organism that might alter the solution’s composition (like algae) or damage the plant (like pathogen microorganisms)..
- To provide physical support for the plant
- To provide adequate air spaces for root respiration
- To hold sufficient available water
- To hold sufficient available nutrients & provide free passage to surplus nutrients
- To be free of plant pathogens, pests and weeds
- To be safe when handled by people
- Manufacturers also need growing media to be physically and chemically stable from the time of production until the time of use (this can be many months for retail products). The bulk density (weight) of the ingredients used is also important because this affects transport costs, a major part of the total cost of production and delivery to the end customer.
Different Growing Media –
Growing media is not entirely necessary in hydroponics, but utilizing grow medium maintains a reserve of nutrient solution in the root zone along with a percentage of air pore space. This can act as a buffer and save crops from failure
There are many different ingredients that can be used to make a growing medium; different parts of the world have developed media based on local availability of various raw materials. Such materials can be inorganic (eg rockwool, perlite) or organic (such as peat, bark). Growing media are often formulated from a blend of different raw materials in order to achieve the correct balance of air and water holding capacity for the plants to be grown. One of the most obvious decisions hydroponic farmers have to make is which medium they should use. Different media are appropriate for different growing techniques.
Expanded clay aggregate –
Baked clay pellets, are suitable for hydroponic systems in which all nutrients are carefully controlled in water solution. The clay pellets are inert, pH neutral and do not contain any nutrient value.
The clay is formed into round pellets and fired in rotary kilns at 1,200 °C (2,190 °F). This causes the clay to expand, like popcorn, and become porous. It is light in weight, and does not compact over time. The shape of an individual pellet can be irregular or uniform depending on brand and manufacturing process. The manufacturers consider expanded clay to be an ecologically sustainable and re-usable growing medium because of its ability to be cleaned and sterilized, typically by washing in solutions of white vinegar, chlorine bleach, or hydrogen peroxide and rinsing completely.
Another view is that clay pebbles are best not re-used even when they are cleaned, due to root growth that may enter the medium. Breaking open a clay pebble after a crop has been grown will reveal this growth.
Growstones, made from glass waste, have both more air and water retention space than perlite and peat. This aggregate holds more water than parboiled rice hulls.
Coco Peat, also known as coir or coco, is the leftover material after the fibres have been removed from the outermost shell (bolster) of the coconut. Coir is a 100% natural grow and flowering medium. Coconut Coir is colonized with trichoderma Fungi, which protects roots and stimulates root growth. It is extremely difficult to over water coir due to its perfect air-to-water ratio, plant roots thrive in this environment, coir has a high cation exchange, meaning it can store unused minerals to be released to the plant as and when it requires it. Coir is available in many forms, most common is coco peat, which has the appearance and texture of soil but contains no mineral content.
Parboiled rice hulls (PBH) decay over time. Rice hulls allow drainage, and even retain less water than growstones. A study showed that rice hulls didn’t affect the effects of plant growth regulators. Rice hulls are an agricultural byproduct that would otherwise have little use.
Perlite is a volcanic rock that has been superheated into very lightweight expanded glass pebbles. It is used loose or in plastic sleeves immersed in the water. It is also used in potting soil mixes to decrease soil density. Perlite has similar properties and uses to vermiculite but, in general, holds more air and less water. If not contained, it can float if flood and drain feeding is used. It is a fusion of granite, obsidian, pumice and basalt. This volcanic rock is naturally fused at high temperatures undergoing what is called “Fusionic Metamorphosis”.
Like perlite, pumice is a lightweight, mined volcanic rock that finds application in hydroponics.
Like perlite, vermiculite is a mineral that has been superheated until it has expanded into light pebbles. Vermiculite holds more water than perlite and has a natural “wicking” property that can draw water and nutrients in a passive hydroponic system. If too much water and not enough air surround the plants roots, it is possible to gradually lower the medium’s water-retention capability by mixing in increasing quantities of perlite.
Sand is cheap and easily available. However, it is heavy, does not hold water very well, and it must be sterilized between use.
The same type that is used in aquariums, though any small gravel can be used, provided it is washed first. Indeed, plants growing in a typical traditional gravel filter bed, with water circulated using electric powerhead pumps, are in effect being grown using gravel hydroponics. Gravel is inexpensive, easy to keep clean, drains well and will not become waterlogged. However, it is also heavy, and, if the system does not provide continuous water, the plant roots may dry out.
Wood fibre, produced from steam friction of wood, is a very efficient organic substrate for hydroponics. It has the advantage that it keeps its structure for a very long time. Wood fibre has been shown to reduce the effects of “plant growth regulators.”
Wool from shearing sheep is a little-used yet promising renewable growing medium. In a study comparing wool with peat slabs, coconut fibre slabs, perlite and rockwool slabs to grow cucumber plants, sheep wool had a greater air capacity of 70%, which decreased with use to a comparable 43%, and water capacity that increased from 23% to 44% with use. Using sheep wool resulted in the greatest yield out of the tested substrates, while application of a biostimulator consisting of humic acid, lactic acid and Bacillus subtilis improved yields in all substrates.
Rock wool (mineral wool) is the most widely used medium in hydroponics. Rock wool is an inert substrate suitable for both run to waste and recirculating systems. Rock wool is made from molten rock, basalt or ‘slag’ that is spun into bundles of single filament fibres, and bonded into a medium capable of capillary action, and is, in effect, protected from most common microbiological degradation. Rock wool has many advantages and some disadvantages. The latter being the possible skin irritancy (mechanical) whilst handling (1:1000). Flushing with cold water usually brings relief. Advantages include its proven efficiency and effectiveness as a commercial hydroponic substrate. Most of the rock wool sold to date is a non-hazardous, non-carcinogenic material, falling under Note Q of the European Union Classification Packaging and Labeling Regulation (CLP).
Brick shards have similar properties to gravel. They have the added disadvantages of possibly altering the pH and requiring extra cleaning before reuse.
Polystyrene packing peanuts
Polystyrene packing peanuts are inexpensive, readily available, and have excellent drainage. However, they can be too lightweight for some uses. They are used mainly in closed-tube systems. Note that polystyrene peanuts must be used; biodegradable packing peanuts will decompose into a sludge. Plants may absorb styrene and pass it to their consumers; this is a possible health risk.
How do I choose a growing media? Choosing the growing media mostly depends on the particular experience. For drop irrigation systems I recommend to use a mixture of rise husk and sand, or to use perlite. For NFT systems I recommend gravel or vermiculite. When choosing a growing media it’s important to take into account the necessities of the plant for it to have the best possible development. For how long can I use the growing media? This depends on the nature of the media. Non-organic media such as perlite or gravel can be used many times, while organic media such as rice husk need to be renovated once or twice a year. What treatment should the growing media receive between different crops? Between crops the media should be washed with disinfectant. Personally I prefer to use hydrogen peroxide because it can be lately removed by the own plants. The system must be irrigated during a whole day with the hydrogen peroxide solution at 3%. After this, the system must be irrigated for two more days with common water and it will be ready to use again.