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There are many choices to make when starting a hydroponic system, and one of the most important is the growing medium. The medium is the material in which the plants’ roots will grow, so it must be sterile, absorbent, and have good drainage. There are many types of growing mediums available, so it is important to choose one that is best for your particular system.
Each type of medium has advantages and disadvantages and the choice that you make will weigh these up and see how the hydroponic system you are using and the needs of the type of plant that you are growing. It is more than likely you will use several mediums across different systems and needs.
We will look in detail at the properties that are required of a growing medium and assess how many of these are present in the most commonly used media used.
What Factors Should Influence Your Choice of Hydroponics Growing Medium
There are so many different factors that will affect your choice of growing medium, and your choice will depend on the plants that you want to cultivate, as well as the design, size, and placement of your hydroponic system, which will all have an impact on the medium that you choose.
As you consider the many alternative growing mediums there are several things that you need to have at the front of your mind to ensure you make the best choice.
The first aspects to consider are physical aspects:
- Is it simple to handle and prepare in the pot?
- Is it within your means, and can you get it where you live?
- If it is used for germination, does it create a good contact between the seed and the medium?
- Will it provide plants that are developing any degree of physical stability?
- What capacity does it have to insulate a plant’s roots to shield it from possible temperature variations?
- Is it devoid and resistant to pathogens and pests that may prevent germination, slow development, or spread disease?
- Does its physical appearance coincide with any visual effect you are trying to achieve?
Beyond these basic questions about the physical nature of the medium, you also have to consider the plant and its needs and the hydroponic system that you are employing, and whether the medium has the capacity to meet these needs.
Water Holding Capacity (WHC)
Another important factor that needs to be taken into account is the growing medium’s water retention capacity. The importance of its ability to retain water will depend on the type of hydroponic system being used. Therefore the way a system will store water in between cycles of irrigation will impact on the medium’s capacity for holding water.
The air-filled porosity (AFP) of a medium also needs to be assessed. This refers to the proportion of the grow medium’s gaps available that are not currently submerged by water/nutrient solution at any given moment in time. The medium shouldn’t inhibit aeration, with there needing to be a decent level of aeration going to the plant roots.
How the growing medium you choose affects the nutrient delivery can also have an impact. This is determined by the level of its cation exchange capacity. This is the ability of a growing medium to deliver nutritional cations to the soil solution for plant uptake. The plant nutrition cations potassium (K+), magnesium (Mg2+), and calcium (Ca2+) should be absorbed into the medium through the cation exchange process. This means that the growing medium needs an equivalent amount of negative charges present for the process to be at its most efficient. The vast majority of the micronutrients and even some of the macronutrients included in fertilizer have a positive charge of +1 or +2.
If you are looking to avoid using pesticides and instead want to use an organic defense or integrated pest management methods then this will also impact your choice of growing medium. While not necessarily the easiest to implement in home garden environments, if you want to follow this path you should try to use natural organic media. This can be a medium such as coco coir that can be infected with bacteria that are helpful to the IPM process.
Ability to be recycled?
A further consideration is the ability to recycle or reuse the medium again. This will depend on a number of factors from the amount the material degrades to the ease with which it can be washed and sterilized to avoid any possible pathogen or disease transfer.
How effective and efficient the growing medium that you choose will be will largely depend on all of these traits and attributes. Before you begin scaling up your operations, you should make it a priority to conduct tests using a variety of media in order to identify the approach that will provide the most satisfactory results for your expanding requirements. It’s possible that various crops will do well in a variety of different environments, circumstances, and media arrangements.
The Most Common Hydroponic growing Mediums
Almost any material that fits the criteria that we have discussed above can be used as a growing medium, including the absence of one altogether (aeroponics). There are however a number of mediums that have become commonly used in hydroponic systems.
Expanded Clay Pellets or LECA
LECA is an abbreviation for lightweight expanded clay aggregate, often known as hydroton stones or clay pebbles. They are balls/pebbles that range in size from 0.3′′ to 0.6′′ and are formed of porous clay that has been fired. They are used in hydroponics and in combination with, or as an alternative to, growth media like as soil or potting mix.
They are an inorganic substance that is clean, can be reused, and are good to the environment. Additionally, they have a pleasant appearance. Because they are used as a growth medium, they make it possible to cultivate plants without the need to worry about watering them and also assist avoid overwatering.
Rockwool as a Hydroponics Growing Medium?
Also known as rock wool, this product is composed of a fine fiber that is woven into a matrix of glass fibers. It comes in various sizes and has several uses, including hydroponics, horticulture, and aquaculture. It can be used on its own or as part of a soilless mix. It retains moisture well, does not float in water, and does not have cation exchange capacity (so it provides no buffering), but it can be easily saturated with water if the top layer is removed from the container. Rockwool has excellent aeration properties for rooting plants; however, it may still retain too much water to use for ebb-and-flow systems.
If washed and sterilized Rockwool can be reused. However, it is not biodegradable and when it comes to the end of its life there are environmental implications as it normally ends up in a landfill.
Coconut coir is an inert material and is made from the husk-coir, the husks of coconuts. Coconut coir is processed and can be left as long fiber or more usually ground down into a granular form and compressed into cubes or bricks that are expanded using water and broken up for use at which point it has a resemblance to soil.
As a material, moisture-rich material with high porosity and good water retention abilities. It absorbs water quickly and can hold up to four times its weight in water, making it an excellent growing medium for hydroponic systems. Coconut coir also provides aeration to the roots. It is often used as an alternative to peat moss as it has many of its properties.
However, it can be expensive because the pellets must be hand-bagged and then cut into small sizes before loading them onto trucks or barges for transport. Its quality depends on how the coir is flushed; how it is cleaned, soaked, and dried.
Coco coir can be used in a number of different ways but it can also be mixed with other growing mediums such as perlite (generally 70- 30%) to help with drainage as sometimes there can be too much water retention in the coir.
Coco Coil can come in various different forms from coco peat (ground down husks), coco fiber and coco chips (often used as a mulch)
A lightweight, permeable, and porous volcanic rock that is the main ingredient in a number of concrete products. Perlite has been used as an alternative growing medium for many years by commercial growers. It is extremely porous and allows water to penetrate easily, which can lead to problems with algae buildup on the surface of your pots.
As such it is good for its aeration capabilities but is unable to perform cation exchange. Although it can absorb water it is unable to retain it and can therefore dry out between waterings.
Vermiculite is a mineral and is similar in nature to perlite, however, it is better at retaining water and can be used in hydroponic systems. It is lightweight – works well as an open-top reservoir or as the medium for a drip system (as opposed to growing on soil) and can prevent pests and bacterial fungi.
The downside is that it’s a fairly rare material and as such expensive in comparison to some other products, but still cheaper than Rockwool, which has similar benefits
When used in hydroponics it can be combined with perlite in order to prevent the growing medium from becoming excessively saturated with water. Better aeration and drainage result from this. Because it can retain more water than perlite, vermiculite is often utilized on its own in regions with higher average temperatures.
Oasis cubes are a great alternative to Rockwool. They’re also made from natural fibrous materials such as cotton and wood pulp and are available in various sizes.
The Oasis cube’s interlocking design provides a stable platform for roots to grow on. It is an excellent choice for hydroponics because it is lightweight, yet durable; its porous structure allows water and air to penetrate easily, and it will not retain any smell or contaminate the medium with harmful chemicals or microorganisms that can destroy plants.
One disadvantage is that you should use oasis cubes within 2-3 months after purchase because they become compacted when wetted and will break apart over time if you don’t use them often.
Starter plugs are growing medium plugs specifically designed for hydroponic system use. They come in various shapes and sizes to accommodate different hydroponic systems, they are lightweight and easy to transport, making them the perfect choice for beginning growers.
The growing medium plugs allow roots plenty of space to grow unrestricted as well as the ability to absorb water quickly which can prevent root rot and other moisture-related problems. Another benefit of starter plugs is the fact that they retain water, meaning you can water them less frequently and still achieve successful hydroponic growth.
While a relatively sustainable solution they are only useful when looking to grow from seed. An additional problem is that they can attract pests such as gnats.
It can be a title to describe various forms of growing mediums but here we are talking about those made from recycled glass. Glass growstones are a popular choice for hydroponics growing systems because, similar to LECA, they provide good drainage and aeration, which is essential to controlling moisture and airflow. They come in a variety of sizes and colors, so you can customize your system to match the rest of your plants or decor.
Growstones are used as a bottom layer in flood and drain systems, but they also make an excellent top layer for drip or mist systems. Their ability to draw up water in a capillary action makes them ideal for wick systems
Using Rice Hulls
Rice hulls are the cleaned husks from rice. They’re a great medium for starting seedlings because they contain some phosphorus, which is good for establishing root structure and strength). In addition, they can perform the function of a filter by collecting dirt and particles that are suspended in the water. This makes it easier to maintain a clean water supply and lowers the risk of the hydroponic system becoming clogged.
Besides its filtration properties, there are several other benefits to using rice hulls. These include a good air-to-water ratio, are sustainable, and also are inexpensive.
The downside of rice hulls is that they are not very absorbent and will tend to retain some moisture (although this can be an advantage if you’re growing in a humid environment). They also decay and cannot be reused, although they can be composted.
Gravel is one of the most popular growing mediums for hydroponics. It is inexpensive and easy to find. The main disadvantage of gravel as a growing medium is that it does not hold nutrients very well and needs frequent watering to prevent nutrient loss.
Gravel should be rinsed thoroughly before use and kept in a sealed container to prevent algae growth. It also can harbor bacteria if left wet too long after it has been washed. However, washing with bleach or other disinfectants should kill off any pathogens in the gravel.
The fact that it has no water retention capabilities means that it is not appropriate for use in hydroponic systems that need the growing medium to keep moisture for the roots. It can though be mixed with other mediums such as coconut coir or Rockwool which will provide these capabilities.
Wood Fiber/ Sawdust
Sawdust is made from sawdust and wood chips. It is available in a range of sizes, but the finer chips are more expensive. This material can be used to start seeds or to provide aeration in the root zone; it also makes a good growing medium for plants requiring extra water retention such as tropicals.
In order to use wood fiber correctly, you will need to presoak it in some form of pH buffering solution prior to adding it into your system. Sawdust should not be added to an already established hydroponic system, as it can cause problems with nutrient absorption.
Sand is the most commonly used growing medium in hydroponics, and has been since the very beginning of the use of hydroponics as a growing method.
Sand can be purchased as coarse or fine-grained material, with varying degrees of particle size, although you should use a particle size of at least 2mm. It is used in hydroponics because provides good drainage and supplies decent aeration.
On the downside, sand is a heavy material that needs to be sterilized regularly. You should also look to be aware of sand with limestone content as this can alter the pH balance of your nutrient solution, making it more alkaline.
Coarse sand is often mixed with other materials such as vermiculite or perlite to make a starter medium for new seedlings.
Other Growing Mediums
We have covered the most popular forms of growing mediums. There are many others such as phenolic foam, pumice, hemp fiber, and brick shards to name but a few. In truth, the list itself is almost endless and if the material that you want to use is able to provide a number of the properties and capabilities listed earlier then it is likely it can be used.
Summary: Hydroponics Growing Medium
There are many factors to consider when choosing the best growing medium for your hydroponic system. You should weigh the benefits and drawbacks of each medium before making a decision.
Prior to beginning to scale up your operations, make it a point to run testing utilizing a number of different media to determine the strategy that will deliver the most suitable outcomes for your increasing needs. It is likely that different crops may thrive in diverse settings, conditions, and media setups.
- The University of Georgia: Cation Exchange Capacity and Base Saturation