Whether growing hydroponically or using a container system, choosing the right growing substrate for the application is critical. There are many different options available for growing substrates. What works best depends on the system used, the types of plants grown, and to some extent, personal preference.
The growing substrate is critical for the following reasons.
- Serves as a reservoir for moisture and nutrients around the plants’ roots, keeping nutrients in solution for uptake and utilization.
- Creates pore space around the roots for oxygen, allowing them to breathe.
- Anchors the roots and provides upright support to the plants.
Peat Moss
Peat moss has been used in the gardening industry for the better part of the last century and is popular as a substrate in hydroponics growing systems. It forms as plant materials decay and subsequently accumulates in poorly drained areas such as bogs or marshes.
Peat forms in peatlands of the Northern hemisphere — marshy, wet areas containing slow-draining soils. As vegetation dies, cooler temperatures and low oxygen rates slow bacterial decomposition, causing the partially decayed vegetation to accumulate in layers. This slow decomposition rate results in the formation of densely fibrous, homogenous material, known as peat.
Peat moss is a specific peat type, with a high percentage of sphagnum mosses within the vegetative layers.
Pros: lightweight, sterile, retains 10 to 20 times its weight in water, doesn’t compact, drains freely, displays fungistatic properties.
Cons: low pH needs adjusting before use, decaying organic matter can cause chemical instability in hydroponic systems.
Best used as a primary hydroponics medium and in container gardening for starting seeds, propagating cuttings, or as a component of potting mixes.
Coconut Coir
Coconut coir is a renewable material similar to peat moss. It is made from the brown and white fibers between the outer coating and shell of a coconut seed. To create coconut coir, the husks are soaked in water to soften and remove the fibers. The fibers are then dried and formed into bales.
Depending on the manufacturing process, the end product varies in size and shape, each type with specific uses.
- Pith: The most similar to peat moss, coco pith looks like finely ground coconut husks. The small particles are typically used as an ingredient in potting soils or as a soil conditioner. It is aged before use and rinsed well to prevent the release of salts. It isn’t recommended for use as a sole growing substrate as it holds too much water.
- Fiber: Once completely dried, the long fibers are removed from the husks without further processing. The unprocessed fibers don’t have the same high water retention as processed coco coir but still help increase the potting mixes’ drainage and porosity. The long fibers are often shaped into the liners used for hanging baskets or containers.
- Chips: Between coco pith and unprocessed fibers are chips — chunks large enough to create air pockets in a growing media or soil and help retain moisture. They are similar in function to expandable clay pellets but are a natural product.
Pros: resists compaction and decomposition, pH close to neutral, promotes the growth of beneficial bacteria due to lignins, rich in hormones and biostimulants.
Cons: low nutrient profile, high cation exchange capacity may tie-up nutrients, low-quality products are high in salts.
Best used as a primary hydroponics medium and in container gardening for starting seeds, propagating cuttings, or as a component of potting mixes.
Perlite
A silica-based product, perlite originates from volcanic obsidian. Gray or black rock is mined, crushed to create small fragments, and then heated to extremely high temperatures. Consequently, internal moisture vaporizes and expands the thin plates to create elongated particles with an accordion-like structure during the process. The resulting expanded vermiculite is golden-brown and graded based upon the particle sizes.
The resulting product is a light, airy substance known to increase aeration and drainage. Chemically, the composition is approximately seventy to seventy-five perfect silica or silicon dioxide. The remaining percentage is a mix of aluminum oxide, sodium oxides, and minerals such as potassium, calcium, magnesium, and iron.
Following the heating process, horticultural perlite is graded based upon its particle size.
- Super coarse perlite (1/4″ – 3/32″) has a 19% water holding capacity. The large particle size best increases the porosity to improve drainage and aeration. Yet, it isn’t as popular as the other sizes because of its diameter.
- Coarse perlite (3/16″ – 3/64”) has a 34% water holding capacity. It is a good balance of water-holding capacity and drainage. This grade works best as a sole media for plants that prefer quickly draining substrates (succulents, orchids, etc.).
- Medium grade perlite ( 1/8″ – 1/32″) has a water holding capacity of 46%. It is best used alone as growing media for potted plants or as an ingredient in potting mixes.
- Fine grade perlite (1/16″ – 1/128″) has a water holding capacity of 52%. It works best for starting seeds and rooting propagation cuttings.
Pros: neutral pH, doesn’t decompose, non-toxic, inexpensive.
Cons: nonrenewable resource, dust is an irritant, very little water holding capacity.
Best used as a primary medium in drip or bucket hydroponics systems. As a primary medium in container gardening for starting seeds and cutting, and growing succulents or orchids. Also used as an ingredient in potting mixes.
Vermiculite
Similar to perlite, vermiculite is formed by heating biotite or phlogopite, two types of phyllosilicate rocks. These rocks contain high levels of silicon oxide, magnesium oxide, and aluminum oxide. The particles are arranged in layers of thin plates, similar in appearance to mica.
Once mined, the ores are separated from other minerals and superheated in a process known as exfoliation. During the process, internal moisture vaporizes and expands the thin plates to create elongated particles with an accordion-like structure. The resulting expanded vermiculite is golden-brown in color and graded based upon the particle sizes.
Pros: sterile, pH close to neutral, doesn’t decompose, high water holding capacity.
Cons: nonrenewable resource, fosters alkaline reactions to raise pH in the root zone, high water retention can create anaerobic conditions.
Best used in container gardening, either as an ingredient in potting mixes or a primary medium for seed starting and rooting cuttings. Often mixed with perlite for use in hydroponics.
Expanded Clay Pebbles
Expanded clay pebbles, commonly known as LECA (lightweight expanded clay aggregates), is a manufactured product used to increase large pore space in the root zone. Clay is heated to over 2000℉ in a rotary kiln, resulting in marble-sized round pebbles filled with air bubbles.
Clay pebbles are used in many hydroponics systems and for growing various plants. Large pore space between the roots improves oxygenation and allows for more frequent irrigation scheduling. Plants develop a healthy, vigorous root system and grow quickly.
Pros: Doesn’t decompose, holds onto water and nutrients, pH neutral, large pore space improves drainage and aeration.
Cons: Need to be washed and soaked to remove dust before use, much heavier than other substrates, expensive to ship, dry out quickly.
Best used in hydroponics but is gaining popularity in container gardening for succulents and orchids.
Rockwool Cubes
Rockwool cubes are formed by spinning melted basalt rock into dense mats of long, intertwined fibers. The resulting structure is inert and perfectly suited to retain a higher amount of moisture and oxygen.
Rockwool is a popular medium used in hydroponics growing systems because it works across all of a plant’s growing stages, from starting seeds to harvest.
Pros: chemically inert, sterile, high water holding capacity, reusable.
Cons: fibers are known as a skin and eye irritant, pH needs to be adjusted before using, little carbon for microbial populations.
Best used for hydroponics, primarily to germinate seeds and root cuttings.
Potting Soil
Also known as potting mix, potting soil is a “soilless” mix of components used as a growing medium in greenhouses and container gardening. The exact formulation varies depending upon the intended plants, but it typically contains a mixture of peat moss or coconut coir, organic material (compost, pine bark, etc.), perlite, and vermiculite.
Pros: Inexpensive, resists compaction, lightweight, widely available, good nutrient and water retention.
Cons: Messy to work with, may not be suitable for organic growing operations.
Best used for container gardening.
Wood Residues
Wood residues such as sawdust and wood chips are often used for growing media because of their high availability and low cost. They have similar characteristics to peat moss, but as a by-product of the lumber industry, they are considered a sustainable, renewable material.
Pros: renewable resource, inexpensive, good water retention.
Cons: breaks down over time, may contain pests or chemicals, may tie up nitrogen, sawdust compacts quickly in hydroponics systems.
Best used for container gardening as an ingredient in potting mixes or a sole growing medium for orchids.
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Amanda Shiffler
Most comfortable with soil under my fingernails, my enthusiasm is sparked by gardening, agriculture, and all things plant-related. With a master's degree in agriculture and more than a decade of experience gardening and tending to my yard, I combine my plant knowledge and a knack for writing to share what I know and love.
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