The water retention of soilless substrates is an often overlooked yet important factor when developing a cultivation plan. Modern gardeners choose their cultivation media based on several factors, including operational logistics, fertigation programs, and environmental constraints. To illustrate, organic soils are attractive in a logistical sense because the same soil can be reused each growing season. However, when it comes to accuracy with fertigation programs, soils inhibit accurate feeding schedules as they are often heavily amended with fertilizer products. Conversely, soilless substrates generally don’t perform well outdoors in hot, arid conditions as they don’t retain water well enough to support plant growth in such extreme climates. When planning a garden operation, these factors should be considered before deciding to purchase a specific substrate in bulk.

Soilless Media Gaining Ground

For controlled environment agriculture (CEA), the use of soilless cultivation media is consistently growing in popularity. Most CEA horticulturists favor the use of these substrates because they are inert, meaning that they contain nearly no additional fertilizers or amendments. Point being, most soilless mediums allow growers exact control over what they are feeding their plants, which is an obvious extension of CEA methodology. The most popular types of soilless cultivation media are coco coir, stone wool, clay pebbles, perlite, and some proprietary products.

Most hobbyist gardeners are not aware of how horticulture is nuanced by different cultivation media. The exact same species and strain of a plant will grow differently in a slightly different substrate. This notion is important because if a grower decides to switch their substrate, they must pivot their entire feeding and watering methodology to accommodate for this infrastructural change. As different soilless media retain water at different capacities, these irregularities must be accounted for in the development of successful irrigation and fertigation regimes. Yet, most horticulturists don’t have the luxury of implementing trial-and-error runs to decipher how water retention in soilless mediums will affect their garden’s overall performance.

To shed some insight onto the complexities of water retention in soilless substrate growing, Maximum Yield reached out to highly accomplished horticulturist Doug Millar. Over the past 20 years, Millar has worked in various capacities related to modern gardening, including commercial floriculture cultivation, greenhouse equipment design, hydroponic product development, and vertical farming. For the last 10 years, he has studied and worked for Symbi Biological, a research group in the San Francisco Bay area that specializes in sustainable food systems development. Millar’s latest project at Simbi Biological is a closed loop aquaponics system. This cutting-edge research combines elements of traditional biology and modern gardening technology with the aim of helping solve food production issues worldwide.

Of the soilless mediums, which is the most difficult to use and why?

Each medium has its own learning curve. There is a difference between surviving and thriving—finding that sweet spot is really a matter of trial and error for most growers when using a new medium. I’d say clay pebbles are likely the most difficult simply because they often look dry on the top but hold moisture on the bottom. This issue often leads to problems with overwatering and pests. Materials handling is also a factor with pebbles as all the other options are much lighter to lift and move.

Which type retains the most water?

My initial answer is stone wool, but I would love to see a trial between the choices. While water holding capacity is important, I think a good adaptive zone and attentive irrigation strategy is more vital. Oxygen getting to the roots is crucial to healthy plants. Coco coir or stone wool are great at providing a good balance of air and water to the root zone. As with any of the medium choices, I think it comes down to choosing the right distribution systems for the crop with appropriately sized containers.

Which type retains the least water? Would this be the best choice for growers in carefully controlled indoor settings?

It’s hard to say which of the media hold the least water due to the different shapes and particle sizes we can get nowadays. More surface area in the medium typically means more water holding capacity. Container size also has a lot to do with how much water it holds. That said, in a carefully controlled environment, I tend to not want a big water holding capacity simply due to how easily you can overwater plants, which stresses the root zone.

How does water retention in soilless media affect nutrient uptake for plants?

Too much retention [due to poor choices concerning media, containers, and irrigation] leads to waterlogged containers and root damage. Maintaining aerobic conditions is critical to a healthy root system for optimal nutrient uptake.

Do you utilize soilless mediums with your aquaponics projects? How is your choice of medium related to water retention?

My primary method is deep water culture troughs with a dissolved oxygen level at eight parts per million. The plants are started in small (stone wool) cubes then transplanted into two-inch net pots that float on rafts. I use (stone wool to start) simply because it’s a good place to sprout seeds and it holds together well. Obviously, the deep water culture troughs have excellent water holding capacity.

You are based in California, a state with obvious issues concerning water shortages. Looking at horticulture from a conservationist’s stance, would you say that soilless cultivation practices waste more water than traditional soil growing?

In my aquaponics operation, I was using about 10 percent of the water it takes to grow a head of lettuce in the field. I think aquaponics may be the most efficient way to grow plants, but a well-designed hydroponic system is probably similar. Soilless growing is a great tool for society. Water-wise farming can be utilized locally in places where traditional farming cannot, like cities, deserts, and even Mars. I hope that we as a society consider all the options we have available to ensure food and water system security and equity for all people. There has been some pushback about whether or not hydroponics and aquaponics can be organic. I realize field-grown producers are not excited about having more competition in the organic market, but we have a world to feed and if all organic inputs are used to grow a crop, it’s organic to me.

Soilless cultivators face a plethora of choices when deciding which growing medium is right for them, and many of these choices can be boiled down to water retention issues. Judging by the advice of Millar, a good starting point in understanding how the water retention of a soilless medium impacts plant growth is to consider both the environment and container size as those two factors directly influence the performance of any substrate. Also, different plant species and phenotypes react differently to wet and dry root zones. All things considered, it seems that soilless horticulturists must account for all these unique operational factors when understanding exactly how water retention effects overall garden performance.