Soilless Grow & Potting Mix

Why a "Chunky Mix" Isn't Enough.
Reading Time: 12 - 16 minutes (3545 words)

A Soilless Potting or Grow Mix Introduction

Soilless grow media, often referred to as a soilless potting mix represent a revolutionary approach in the world of gardening and plant care, offering an innovative alternative to traditional soil-based cultivation. These media, comprised of various organic and inorganic materials, provide a sterile, controlled environment for plant growth, optimizing water retention, aeration, and nutrient delivery. If you're wanting to understand more than the majority of plant owners, keep reading. By the end you'll have enough knowledge to reduce the biggest headaches you will likely face as a plant parent, overwatering.

Confusion Between Soil & Soilless

Let's start this unlikely article by clearing up some of the confusion gardeners have between soil and soilless substrates. Many people mistakenly use the terms interchangeably in reference to whatever a plant is anchored in as long as it isn't a liquid nutrient solution. While not the biggest of issues, knowing the difference is important to overall plant care. Soil, by definition, is a medium, composed of sand, silt, clay, organic matter, water, and air, that supports plant growth by providing nutrients, moisture, and structural support. In contrast, a soilless substrate is comprised of media elements designed to replace soil in various horticultural systems.

The inception of this innovative gardening method dates back to the 1930s when William Frederick Gericke coined the term 'hydroponics'. This term originally described the technique of using mineral nutrient solutions (fertilizer) in water to support plant growth in the absence of soil. Over time, the terminology evolved to 'soilless medium cultivation' to more accurately reflect the diversity of the mediums used, which can be either solid or liquid and made from organic or inorganic substrates like peat, coir, LECA, PON, Perlite, Pumice, rice hulls, rockwool, sawdust, etc. This diversity of potential materials has enabled the adaptation of horticulture across the globe, facilitating the production of both food and ornamental plants with enhanced yields and nutrient quality.

Colloquially, as noted already, we often refer to whatever a plant is growing in as soil, but for clarity, gardeners should use terms like grow mix, or soilless substrate, to help clear up any misconception of what a plant is actually relying on for its grow medium and nutrient needs. The other thing that many gardeners need to realize, is that when you need to provide nutrients to your grow media because it has no ability to provide for the plant on its own, it is by definition hydroponics. There is no such thing as "semi-hydro" or the variety of terms people associate with PON or LECA use. If you're growing in some combo of peat, bark, coir, perlite, etc., you're growing hydroponically.

If its not actually soil, it's most likely some derivative of hydroponics; aeroponics and aquaponics are also hydroponic derivatives.

What Makes an Ideal Soilless Medium?

An ideal soilless medium should possess certain physical and chemical attributes to support optimal plant growth whether its for commercial farming, backyard hobby farms, or for those of us gardeners with an overabundance of ornamental plants taking over our homes.

  • It should have a consistent texture that not only drains efficiently but also retains sufficient nutrients and moisture for the root system.
  • The bulk density should be low, ranging from 190 to 700 kg/m³, to ensure ease of handling and transport.
  • A high level of porosity, between 50% and 85%, is crucial for maintaining adequate air and water within the medium.
  • The particle size should be balanced, ideally between 0.25 and 0.5 mm, to ensure an optimal mix of air and water retention.
  • The pH value should fall within 5.0 to 6.5, and it should be adjustable as needed to meet specific plant requirements.
  • The medium should contain a low amount of soluble salts to avoid potential toxicity to plants.
  • Chemically inert, meaning it should not release inorganic ions into the nutrient solution or immobilize nutrients, thus maintaining the nutrient balance.
  • Durable enough to preserve its original properties over multiple cultivation cycles, ensuring longevity and cost-effectiveness.
  • It must be devoid of pathogens and pests to reduce the risk of disease, though it does not need to be sterile. It should also be free from any plant-toxic compounds.
  • The ability to be produced in consistent batches is important to facilitate uniform fertilization programs and ensure predictable plant growth outcomes.

These characteristics are essential for a soilless medium to effectively support plant growth, ensuring that plants receive the right balance of nutrients, water, and aeration while minimizing risks of disease and nutrient imbalances.

wdt_ID Material Advantages Disadvantages
1 Sand Relatively low cost, Good drainage ability Low nutrient and water-holding capacity
2 Rockwool Light weight and ease of handling, Totally inert media, Possibility to carefully control nutrition Disposal problem, Energy consumption during manufacture
3 Vermiculite Light weight, High nutritional-holding capacity, Good water holding ability, Good pH-buffering capacity, Good aeration Tendency to compact when too wet, High cost, High energy consumption
4 Perlite Low density, Sterility, Neutral pH, No decay, Local product in some regions, Excellent aeration Low nutrient and water holding capacity, High cost, High energy consumption
5 Peat Physically stability, Good air- and water holding capacity, Low microbial activity, Light weight, Low pH easy to adjust Finite resources, Environmental concern and contribution to CO2 release, Increase cost due to energy crises, possibly strong acidity
6 Coconut coir Physically stability, Good air content and water holding capacity, Low density, Low pH easy to adjust Potentially high salt level, Energy consumption during transport
7 Bark (well manured) Good air content and water holding capacity Increase costs – since used as an alternative to fuel and in landscaping
8 Green compost Good source of potassium and micronutrient, suppression of disease, good moisture content Variability in composition, high salt and heavy metals, high CEC, tendency to become easy waterlogged
9 Biochar Enhances soil fertility, Improves moisture retention, Reduces soil acidity Initial cost, Variable quality
10 Rice Hulls Biodegradable, Improves aeration and drainage, Low cost Decomposes over time, May contain seeds
11 Foam Materials Lightweight, Excellent aeration and drainage Non-biodegradable, Can be expensive
12 Glass Aggregates Inert, Good for drainage and aeration Heavy, Can be sharp and dangerous
13 Wool (Sheep's) Sustainable, Good water retention Decomposes, Can shrink and compact
14 Pumice Lightweight, Porous for good aeration and water retention Can be difficult to source in some areas, Variable quality
15 LECA Lightweight, porous, and provides excellent drainage and aeration. Reusable and pH neutral. Insulates root systems against temperature fluctuations. Initial cost may be higher than some other media. Can float in water-based systems, requiring proper management. May require pre-soaking before use to improve water retention.
16 Polyurethane Grow Slabs Controlled water and nutrient delivery, Reusable to some extent Environmental concerns due to disposal, Cost
17 Flax and Hemp Fibers Sustainable, Good moisture retention May decompose over time, Can vary in quality
18 Aerated Concrete Lightweight, Good insulation Alkaline nature may require pH adjustments, Brittle
19 Foam Cubes Excellent aeration, Lightweight Chemical degradation over time, Environmental concerns
20 PON (Porous Organic Network) High porosity for excellent air and water retention, conducive to microbial colonization, promotes efficient nutrient exchange Possible higher cost due to advanced manufacturing, may require specific conditions to optimize performance
21 Horticultural Charcoal Improves soil aeration and drainage, Helps remove toxins and odors, Promotes beneficial bacteria growth. Can be relatively expensive, Not a nutrient source by itself
22 Biochar Enhances soil fertility, Increases water retention, Encourages soil microbial life, Sequesters carbon, reducing greenhouse gases. Initial cost, Requires proper charging or inoculation with nutrients before use, Quality and characteristics can vary.

Understanding Pore Space in Soilless Substrates

Definition and Significance of Pore Spaces

Pore spaces in soilless substrates refers to the voids or gaps between particles within the medium. These spaces are crucial for the substrate's ability to manage water and air, which directly impacts your plant's health. These spaces play a pivotal role in ensuring that plants receive adequate oxygen and moisture, essential components for photosynthesis, respiration, and nutrient uptake. The significance of pore spaces cannot be overstated, as they fundamentally influence the physical properties of the substrate, including its porosity, aeration capacity, and water retention ability.

The Role of Pore Spaces in Air Exchange and Water Retention

Pore spaces facilitate air exchange by allowing oxygen to penetrate the substrate and reach plant roots. This oxygen is vital for root respiration, a process that releases energy for plant growth and nutrient absorption. Simultaneously, these spaces retain water, creating a reservoir that plants can draw upon between watering cycles. The balance of air and water within these pores is critical; too much water can displace air and suffocate roots, while too little can lead to dehydration.

Balance Between Air and Water Space for Root Health

Achieving the right balance between air and water space within a substrate is essential for maintaining healthy root systems. This balance ensures that roots have access to sufficient oxygen while remaining hydrated. It's a delicate equilibrium: excessive drainage can result in dry substrates and stressed plants, whereas inadequate drainage can cause water-logging, root rot, and other oxygen-deprivation issues. Optimal substrate management involves selecting a medium with the appropriate pore space distribution to support the specific water and air requirements of the plant species being cultivated.

As hinted at already above, the optimal ratio of air to water in a soilless substrate varies depending on the type of plants being grown and their specific needs. Generally, a healthy balance ensures that there's enough water for plant roots to absorb moisture and nutrients while maintaining sufficient air space to allow roots to breathe and prevent "over watering". A commonly recommended balance for many horticultural substrates is around 30% air space to 70% water-holding capacity after irrigation, ensuring that plants receive adequate oxygen and moisture. However, this ratio can be adjusted based on plant requirements, substrate composition, and environmental conditions.

Water-Holding Capacity and Its Implications

Definition of Maximum Water-Holding Capacity & Its Importance for Plant Health

The maximum water-holding capacity (WHC) of a substrate refers to the total amount of water it can retain after excess water has drained away. This characteristic is crucial for plant health as it determines how much water is available to the plant's roots between watering cycles. Ensuring that a substrate has an adequate water-holding capacity is essential for maintaining moisture levels that support plant growth, nutrient uptake, and physiological processes.

Influence of Substrate Type, Compaction, & Container Height on WHC

  • Substrate Type: Different substrates have varying water-holding capacities. For instance, peat-based substrates tend hold as much as 30x their dry weight. Perlite by comparison only holds .3x its dry weight. The choice of substrate directly impacts the moisture availability for plants.
  • Compaction: Compacted substrates have reduced pore space, affecting their ability to hold water and air. Overly compacted media can lead to poor drainage, reduced aeration, and ultimately, root related problems, like root rot.
  • Container Height: The height of the container influences the water-holding capacity due to gravity's effect on water drainage. Taller containers may exhibit faster drainage, reducing water retention time and necessitating adjustments in watering frequency or substrate choice. They also affect a substrate's wicking (capillary action) ability.

Understanding these factors is key to selecting the right substrate and container combination for specific plant requirements, ensuring optimal growth conditions and plant health.

Pore Classification

In soilless substrates, the classification of pores—ranging from macro-pores to ultra-micro-pores—plays a critical role in determining water availability for plants. Macro-pores ensure proper drainage and aeration, preventing water-logging, while meso-pores balance moisture retention and airflow, essential for plant health. Micro-pores hold water that plants can utilize, ensuring moisture is available during dry periods. Ultra-micro-pores, however, retain water too tightly for plant use, underscoring the importance of an optimal pore size distribution for effective water management in horticulture. Here is some more detailed info so you can get lost down the rabbit hole of Pore-adise City 🤘🏻.


These are the largest pores, generally larger than 0.08 inches (2 mm). They facilitate air movement and drainage but hold little water. They are essential for ensuring adequate drainage and aeration within the substrate, preventing waterlogging and facilitating gas exchange necessary for root respiration. Their size allows for quick water movement, which is crucial in preventing root diseases associated with excessive moisture.


Sized between macro-pores and micro-pores, meso-pores are crucial for water retention while still allowing some air flow. These pores play a pivotal role in water management by retaining moisture after excess water has drained away. They strike a balance between providing enough water for plant uptake and maintaining sufficient air space for gas exchange, making them critical for sustained plant growth.


These small pores, often less than 0.03 inches (0.75 mm), retain water that plants can use, providing moisture for root absorption. They are vital for holding water within the substrate, making it available to plants over extended periods. This water is crucial for plants during dry conditions, ensuring they have access to moisture necessary for their physiological processes.


The smallest pores, which hold water very tightly, making it less available to plants. They contain water that is often too tightly bound to be readily available to plants. While they contribute to the substrate's overall water-holding capacity, the water in these pores is largely inaccessible, highlighting the importance of a balanced distribution of pore sizes for optimal plant hydration.

Understanding the distribution and function of your substrate's pore sizes helps in selecting or engineering substrates that meet the specific water and air requirements of different plant species, ultimately influencing plant health and productivity. "Chunky" is about as ambiguous a term as anything else when it comes to randomly throwing a variety of ingredients into a mix.

How to Ensure Optimal Water Availability

Managing water availability in soilless substrates is crucial for optimal plant growth. Effective strategies involve selecting the right substrate, precisely managing irrigation, and continuously monitoring moisture levels. Additionally, employing techniques like analyzing water retention curves and adjusting substrate composition can significantly enhance water availability. Through these methods, gardeners can ensure that their plants receive the ideal balance of water and air, promoting healthy growth and maximizing yield in soilless horticultural systems.

Substrate Selection

Selecting the right substrate involves understanding how different materials' pore sizes affect water retention and drainage. A substrate with a balanced distribution of macro-pores and micro-pores ensures that while excess water drains away efficiently, sufficient moisture remains accessible to the plant roots. This balance is key to avoiding waterlogged conditions that can lead to root diseases, while also preventing the substrate from drying out too quickly. Adjusting the mixture of substrates, such as combining perlite for increased drainage with peat moss for water retention, can create an optimal environment for root growth and plant health.

Irrigation Management

Irrigation management in soilless substrates requires aligning watering techniques with both the plant's needs and the substrate's characteristics. Drip irrigation, which delivers water directly to the base of each plant, can be tailored to match the precise water requirements, minimizing waste and reducing the risk of overwatering. Capillary mats, on the other hand, rely on the substrate's ability to draw water upwards, ensuring a consistent moisture level beneficial for seedlings and smaller plants. Both methods enable careful control of water application, ensuring plants receive the right amount of water at the right time.

Monitoring Moisture Levels

Monitoring moisture levels in soilless substrates is essential for optimal water management. For commercial growers, using tensiometers or moisture sensors allows for precise monitoring of the substrate's moisture content. Tensiometers measure soil tension, indicating how much effort plants need to exert to extract water, while moisture sensors provide real-time data on the moisture level.

For home users and growers, regular checks with your finger is often the best way to proactively check for moisture levels. The cheap moisture meters available online and at most garden stores are not recommended for soilless substrates because they too often lack accuracy and reliability, especially in substrates with lots of voids. These meters do not typically provide precise moisture readings due to the different physical properties of soilless media compared to actual soil, leading to incorrect watering decisions. For optimal plant health and efficient water use, investing in a higher-quality moisture meter that is calibrated for soilless substrates or uses a method known to work well with them is advisable. My finger, has never failed me yet, however.

How to Accurately Measure and Improve Water Availability

Measuring Pore Space

To accurately calculate the porosity, along with the air and water content within your soilless substrate, proceed through the following sequence of steps:

Step 1 - Measure the Container Volume

Figure out the total volume of the container you will be using for plant cultivation, This might be listed on the container. If not then you will need to plug the holes and fill it with water. The easiest way to do this is simply to tape off the holes on the outside of the pot with waterproof duct tape or by placing a cheap thin bag inside the pot. Fill the pot to the brim with water and then before any seeps out, pour the water into a measuring cup to get an accurate reading of the volume.

Step 2 - Fill With Substrate

With the container empty once again, keep the container plugged, or replace the bag/tape if necessary, and fill the pot/container with the substrate of your choice, compacting it like you would if you were planting a plant in the media. This is now effectively the same volume of dry material as you had with the water in Step 1.

Step 3 - Substrate Saturation

Now, leaving the media in the pot, slowly fill it with water until it reaches the very brim. Keep track of how much water is required. Carefully set it aside for about 1 hour allowing it to absorb as much of the water as possible. If the water level drops during this time, add a little more to top it up, adding this to your previous measurement. This volume of water is the available pore space volume.

Porosity Percentage Calculation

Step 4 - Substrate Drainage

Over a large bowl, or suitable container, poke holes in the tape or plastic to allow the water to drain out of the pot and be captured. Allow the media to drain for 15-20 minutes. Once complete measure the resulting volume of drained liquid. This volume of liquid is equivalent to the air space within the substrate.

Air Space Percentage Calculation
Water Holding Capacity Calculation

The moisture that is absorbed by the media, is the water space, or more accurately referred to as the maximum water holding capacity (WHC), or container capacity of the substrate. It is the percentage of total pore space that water occupies after excess water has been drained from the substrate (after air occupies pore space). After draining this the excess water, air fills the space once occupied. A substrate can hold water within the smaller pore sizes by capillary action which defies the force of gravity. This residual moisture is the WHC.

Water Holding Capacity needs to be understood for the health of your plants. It is often confused with the volume of water required to saturate the media, as demonstrated above, this is not the case since saturation does not provide any air space. This is how plant roots die — no air for them to breathe.

WHC is a dynamic value since it is affected by the substrate mix, substrate compaction, and the root volume of the plant which is continually changing.

Pot and Container Design

The relationship between your pot or container height and its capacity to retain water is a critical factor in soilless growth mediums. When comparing containers of the same diameter and media mix, the capacity of the substrate to hold water diminishes as the container's height increases, a phenomenon attributed to the incraesed gravitational pull on the water column.

Conversely, for containers of identical height, the ability to retain water improves with an increase in the container's diameter. Despite the presence of drainage holes at the bottom, a certain amount of the substrate remains saturated. This occurs because gravity's pull on the water in this lower section is comparatively weaker.

This saturated section (water space in diagram) is known as a perched water table, characterized by a lack of air space as the height of the container is reduced, the perched water table occupies a larger percentage of the container's total volume which forces a plant to sit in saturated media.

This is a clear example that should quell the ongoing and seemingly ceaseless myth that a plant likes to be stuck in smaller, root-bound pot, or that a large pot is worse for a plant's roots. The idea that a larger pot with an identical soilless substrate creates a more saturated environment is clearly incorrect.

Selecting containers with design features that promote even moisture distribution and adequate drainage is crucial in preventing water-logging and ensuring healthy plant growth. Containers with built-in drainage holes, raised bottoms, or porous materials help to avoid excess water retention, allowing roots to breathe and preventing root rot.

Additionally, some designs incorporate self-watering systems that provide a consistent moisture level, ideal for various plant types and growing conditions provided that your substrate delivers the exact porosity your plant needs. If not, then self-watering pots will fall victim to either over or under-watering since the WHC will be sustained until the available reservoir no longer wicks into the substrate.

The Last Drop

This unlikely article, like most, was a lot to soak in, and while it probably felt like drinking from a firehose, the need to understand soilless substrate porosity cannot be understated. While understanding light science is a necessity for healthy plants over the longer term, water management is a critical aspect that should be understood from day one if you want to avoid killing your plants from showering them with too much H2O love. Wrapping your head around the science of soil porosity will stop the guessing game of relying on some arbitrary, interpretive, and mystical voodoo plant influencer telling you to grab a variety of ingredients, mix them all together and simply hope you managed to get the concoction right.

There is no need to guess what a "Chunky Mix" actually means. There is no need to believe that a larger pot spells certain death for your prized green chlorophyll loving child, or that any plant you own wants its roots crammed into a pot that's too small. Bad plant advice abounds, regardless of how well intentioned, and when somewhere close to 80% of unintended plant mortality is due to a water management issues, understanding the simple science of substrate porosity will save you untold frustration with every plant you own.

PS - As much as the steps to measure your air and water above were done with an empty pot. You can also do the tests with pots already filled with plants. It's actually really helpful to know. Same steps as above, you just have to find an empty pot of the same size, to know the initial volume, and do your best to use a plant where the substrate is almost bone dry. It won't be perfect, but it will help give you an idea of where existing plants in your care might me. Make sure you plug the drain holes first though. Duct tape may be harder due to moisture, but waterproof silicone can be squeezed in, left to set, and then pulled out when required.

Good luck out there!

Life is busy. Give Us Your Name & Email and We'll Send You Content.

Got Something to Say?