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Downtown farming: Substrate Selection for Hydroponics

Selecting the correct substrate for anchoring the plant’s roots in hydroponic production is of the utmost importance. The substrate must ensure optimal root growth in an environment where the proper balance between available water, available nutrients, and oxygen can contribute freely to optimal root and plant development. Furthermore, the substrate’s biological and chemical characteristics must meet specific criteria to facilitate optimal root and plant growth.



Hydroponic production is possible without using a substrate under certain conditions and growing specific crops. These methods have been highlighted in a previous article and will be addressed in a forthcoming article.


However, if we are going to use a substrate, we must ensure that it serves the purpose for which the criteria of substrate selection. Furthermore, the chosen substrate should adhere to the requirements of the ideal substrate. Moreover, it is preferred to use substrates of reliable and transparent suppliers to ensure optimum yield.


Requirements of The Ideal Substrate


Selecting the ideal substrate will be different for the type of crop, the hydroponic system used, and the cost of the substrate. However, in general, the following requirements need to be met by the selected substrate.


Optimized Rootzone Environment Needed for Optimal Growth


The ideal root zone environment is crucial for optimal growth and production of hydroponic crops. Conversely, should the rootzone environment be less than perfect, less optimized growth will result in a less optimized crop yield. For this reason, it is imperative to capitalize on the physical and chemical properties of the substrate to ensure maximum yield.


Physical Properties Needed for The Ideal Substrate


Oxygen is needed in the root zone to supply roots with oxygen for other growth processes. Therefore, a well-aerated substrate is preferred. Oxygen is not transported from the leaves to the roots and must therefore be available for uptake in the root zone. Just like humans, roots can’t operate without oxygen.


The uptake of nutrients and water by plant roots requires energy. The energy for nutrient and water uptake starts with the leaves supplying the roots with sugars derived from photosynthesis in the leaves. Then, the sugars and oxygen taken up by the roots are utilized in respiration to provide the energy for nutrient and water uptake. Therefore, inadequate oxygen in the root zone will lead to a deficiency in nutrient uptake that influences yield.


Plant roots thrive in aerobic environments where oxygen is prevalent. A lack of oxygen will induce an anaerobic situation where roots die without oxygen. The smaller root area will influence nutrient and water uptake, reducing growth, with a diminished yield as a result.


A well-aerated substrate with elevated amounts of oxygen is essential to crop growth and production. Aeration is accomplished through the substrate’s loose structure that allows oxygen trapped in a multitude of micropores of the substrate. Water is trapped in the macropores of the substrate and an optimal relationship between macro= and micro-pores is needed for the optimal growth of a plant.


Water retention of the substrate is needed to supply the roots with water and nutrients dissolved even when no free water is available in the root zone. This is indicative of the substance’s Water Holding Capacity (WHC).


Between irrigation occurrences, a water deficit might occur for short periods. However, due to the water retention of the substrate, the retained water is still available to the roots. Therefore, a substrate with higher water retention will give you a larger buffer between irrigation events. 


A substrate with a porous structure will allow optimal root expansion in an atmosphere where they can thrive. As a result, producers can optimize water and nutrient absorption in these ideal conditions contributing to optimal plant growth and yield.


Biological Properties of The Ideal Substance


Some substrates might be inert and contain no nutrients in their raw form, while others might contain metals in excess that can be harmful to plant growth. Coco coir has a pathogen reduction ability, and the infestation of pathogens is therefore significantly eliminated even under conditions used in multi-season hydroponic production.


Chemical Properties of The Ideal Substrate


The chemical properties of the substrate will directly influence the plant roots and will have an indirect influence on the solution added through irrigation. Therefore, the chemical properties should adhere to the plant’s requirements for optimal plant growth.


1, The pH of The Substrate (pH)


The pH of the substrate is essential because that will affect the pH of the solution in the root zone. Therefore, the substrate’s high or low pH will negatively influence the availability of essential elements in the solution. Hence, a pH that ranges from 5.3 to 6.3 is ideal for a substrate.


While some substrates are within the required pH range, others will require buffering or the addition of minerals to set the pH right.


2. Electrical Conductivity of The Substrate (EC)


The electrical conductivity, as addressed previously, directly relate to the number of ions in a substrate. Ions can be nutrients or other salts that are not a benefit to plant growth. Specific substrates like coconut coir initially have a high salt content which is removed and buffered to your needs with certain suppliers


You can request substance with a buffered EC that you prefer with certain suppliers. For example, an EC lower than two is optimal for a new substrate.


3. The Cation Exchange capacity of The Substrate (CEC)


Substrates have negatively charged sites on their particles to which positively charged nutrients can adhere. Therefore, the substrate can serve as a reservoir for essential nutrients needed by the plant for optimized growth. In addition, these negatively loaded cites can also adhere to protons (H+) Wich serves as a buffer against pH changes. Therefore, the pH is buffered to stay neutral, ensuring that all essential elements are available for plants.


CEC can serve as a buffer if any essential nutrient deficiencies occur. However, it is up to the producer to decide whether that important nutrient backup is needed. If not required, a substrate with a low CEC can be selected and vice versa. The type of substrate with accompanying CEC used will be influenced by the irrigation system used in the hydroponic system.


The CEC of a substrate is measured as milliequivalents per 100g of the substrate (meq/100g). Because mineral ions could occur in large quantities and cause a toxic situation, care should always be taken to select a substance from a trusted supplier with a trusted and known CEC.


Other Requirements of a Substance in Hydroponic Production


Besides physical and chemical properties being a requirement of substrates in hydroponic production, other factors influence substrate selection.


1. Biodegradability of The Substrate

After a season or more of utilizing the substance, you will have to get rid of it. While substances are reusable, others are not biodegradable, and certain substances are not even allowed in landfills. Select, therefore, a substrate that caters to your needs.


2. Reusability of the Substrate


Some substrates might be reusable, while thorough cleaning will be needed. Some substrates are reusable for multiple years. Part of selecting the ideal substrate will be to select the substrate that meets your needs.


3. Length of Growing Season


The time a substrate can be utilized is different for different substrates, and therefore the appropriate substrate must be selected.


4 Will the Substance Stay Intact?


In a recirculating system, substances that stay intact are preferred. Substances that come apart might clog irrigation and create problems with solution supply.


5, Support Expected from Supplier


Be cautious of suppliers selling a substrate without after-sales support. Some suppliers are willing to supply agronomy support regarding all facets of your hydroponic production. 


Some General Substrates Utilized in Hydroponics


While most substrates can be used without mixing substrates, some growers prefer to mix substrates. Unfortunately, some producers apply the mixing formulas according to their understanding and not scientific findings. For this reason, we propagate the mixing of substrates based on research findings and thorough investigation.

We will only discuss three substrates, while you can find information on more substrates through links in the article.


Coconut Coir as Substrate.

Coconut coir is a byproduct of the Coconut Industry. The husks are ground into different-sized particles, and whereafter the long fibers are removed during sieving. During the sieving, particles are classified into different sizes using a process of coir segmentation. Then, the different sizes are combined to produce different substrate structures preferred by other crops, climates, and regions.



The structure of the segmented substance allows for a high-Water Holding capacity (WHC). The segmentation allows a high Air-Filled Porosity (AFP), and the coco coir substrate meets these two physical characteristics. Some producer’s recon that the AFP is too low, but they don’t take segmentation into account that allows the formation of an ideal balance between micro and macropores in the substance


Coco coir has a pH of 5.8 to 6.9, and no additives are necessary to correct the pH. The EC of coco coir range from 0.3 to 2.9 mmho/cm and can be further reduced through buffering, if required, with specific producers. As a result, coco coir has a negligible CEC, although it can facilitate enough essential nutrients in case of an emergency.


Coco coir can be used for two to three years and must be sterilized between consecutive plantings. Furthermore, the WHC will increase over time with the AFP getting less. Because it is organic, used coco coir can be reused for multiple seasons while biodegradable into compost and used in other applications.


Overall, by selecting the appropriate particle sizes with coco coir, producers can create the ideal substrate for plant root development. The ideal environment includes the substrate’s physical, biological, and chemical balance to enhance optimal root elongation and thus plant growth. 


Rockwool As Substrate


Rockwool is an artificial mineral fiber manufactured by spinning melted basaltic rock into fibers. Some Rockwool contains excessive minerals that might combine with nutrients and can negatively affect plant growth. In addition, because it is manufactured, the WHC, AFP, and drainage of Rockwool from different suppliers may vary.

The pH of Rockwool might be high at 8.0, but because it has no buffering capacity, it will assume the pH of the solution or the substrate it is mixed with. Although the slab’s density is influenced by initial compression, a relatively high WHC is recorded with a moderate AFP. Therefore, the EC or Rockwool may be higher than what is required for most crops, and a process of leaching should be conducted before use.


The micropores will increase with plant water extraction from the solution, leading to an increase in AFT. For this reason, the irrigation intervals with rock wool as a substrate are stretched to increase the AFT of the substrate.


Rockwool is a manmade substrate and inorganic in nature. Therefore, it is not biodegradable but can be reused after cleaning and sterilization without compression. Compression of the substrate will decrease WHC and AFT, as stated before.


Peat Moss


Peat is the partially decomposed part of plant material. Peat can be divided into three groups based on their origin and degree of decomposition. Sphagnum Peat Moss is usually used as a substrate in hydroponic production.

The pH of peat moss tends to be acidic, and additives are needed to correct the pH. Peat moss has a high-water holding capacity with a moderate to high EC. Variable amounts of minerals may be present in peat moss, contributing towards a high EC. Peat moss doesn’t contain many microorganisms. Furthermore, it has no weed seeds and possesses no disease-suppressing qualities.


Peat moss is usually combined with other substrates because of its high WHC. As a result, the easily decomposable parts of Peat Moss are decomposed, leaving only the less compostable parts. The effect on nitrogen inhibition is therefore low.


The following table compares the properties of the mentioned three substances.


Properties Rockwool Coco Coir Peat Moss
EC (dS m -1) 0.06 0.10 0.41
pH 6.5 6.1 6.1
C (%) 2.2 49.5 15.9
N (mg kg -1) 56 44 64
P (mg kg -1) 30 38 42
K (mg kg -1) 176 1500 246
Ca (mg kg -1) 279 58 1668
Mg (mg kg -1) 216 55 636
S (mg kg -1) 303  405 645
Porosity (%) 80.2 85.6 66.0
Air Porosity (%) 5.1 5.0 2.6
Water porosity (%) 84.1 80 64
Bulk Density (%) 0.06 0.2 0.4


This discussion is based on three different substrates and the properties the selected substrate should adhere to. There are a variety of substrates available, and it is required to study them in detail to choose the best substrate suitable to your hydroponic system, requirements, and personal preferences.