Understanding Growing Media

Jeb S. Fields, Julie Brindley, Jim Owen and James Altland

To produce a desirable, salable containerized plant, we cannot separate the soilless substrate, water management or fertility practices because of their entangled relationship. Though the production system is robust, altering any one of these aforementioned factors without consideration to all parts of the root environment could result in unsightly consequences, or worse yet, a lost crop.

The growing media provides structure and support for the plant during growth; however, it must be engineered properly to ensure adequate air and water are present during and between irrigation. In a perfect world, we would also like the substrate to retain or provide nutrients and have ample amounts of available water to reduce frequency of irrigation events, without impeding on the needed air exchange for the crop. However, our understanding of the intricacies of these relationships during the wetting and drying of the substrate remain poorly understood. First, we need to change our thought process to focus more on pore space and less on the substrate surrounding said space.

Static vs. dynamic
There’s a shift ongoing in science today, from the study of “static” substrate physical properties to “dynamic” physical properties. Historically, research and recommendations have focused on the static physical properties (air space, water-holding capacity, total porosity and bulk density) of the soilless substrate at a given point of time for a given container geometry. This information had been successfully used to formulate substrates with seemingly ideal conditions for specific production scenarios.

More recently, a case was made by Dr. Jean Caron from the University of Laval in Quebec and his colleagues for the need to investigate and understand the dynamic parameters of the substrate that describe water, air and nutrient flux, transport gradients, and consumption to assess and select soilless substrates for the 21st century and beyond to deal with increasing ecological and economic challenges facing producers.

There’s an emerging need to quickly produce a salable, quality plant with fewer resources than in previous years, especially considering water scarcity issues and increasing potential of agrichemical regulation. Substrate properties that deserve more attention in our attempt to achieve greater production efficiency include, but are not limited to: how air diffuses in and out of a substrate; the ease of water movement from one point in the substrate to the root (i.e., hydraulic conductivity); and how fertilizer, whether applied with irrigation water or from a controlled-release fertilizer, is transported to the root of the plant (i.e., solute transport) to provide sufficient mineral nutrients.

These parameters will provide inferences into infiltration of oxygen and replacement of carbon dioxide from root respiration, water movement and connectivity, mineral nutrient and water availability, as well as leaching during crop production. Much of the phenomena can be better understood via accurately understanding the space between substrate particles, specifically pore size and structure, and their interconnection. More precisely, which pores are “allowable” (i.e., water transport isn’t hindered by capillary forces within the pore) or “accessible” (i.e., water and air can move to or from the pore), as described by Dr. Allen Hunt and his colleagues at Wright State University.

Engineering a substrate to alter static physical properties alone won’t always equate to a benefit during production. For example, increasing the proportion of available water of a substrate can result in decreased irrigation events, thereby decreasing costs. However, this differs from increasing total amount of water in the substrate, with the latter coming at the expense of air space. Therefore, altering substrates to hold more water doesn’t necessarily help producers, and may in fact, hinder growers by keeping the substrates too wet and reducing air exchange during production. For this reason, we at Virginia Tech are investigating “dynamic” properties using existing knowledge and models from soil science and physics to engineer substrates with optimal properties.

Benefits of dynamic substrates
The substrate hydraulic conductivity can lead to higher proportions of water in the substrate being available to the plant. Thus, looking closer at hydraulic conductivity we see that this property is associated with how the pores within a substrate are connected. By increasing the ability for water to move from pore to pore, the hydraulic conductivity and subsequent proportion of available water of the substrate has, in essence, been increased. This allows the root to obtain water from further away in the container, thus increasing the distance in the substrate that each drop of water can travel to be intercepted by the roots.

The increased available water in container substrates may help achieve higher levels of water use efficiency in container production. Higher percentages of plant-available water in containers results in growing a salable plant with less water. Therefore, the ratio of water needed to produce the same quality crop will shrink. This is essentially crop water use efficiency or the ratio of water used to biomass (carbon) accumulated.

Our understanding of dynamic processes in the substrate also has a profound impact on plant fertilizer use. Solutes (fertilizers dissolved in water) follow the path of water in substrates and move in two ways—either by mass flow or by diffusion (passive transport). Regardless of the path, nutrients are only available to plants in regions of the substrate where water is present, allowing said nutrients to be in the solution. Considering this fact alone, methods of nutrient delivery can be made increasingly efficient. Conventionally, containers are over-irrigated; meaning water leaches and carries mineral nutrients away.

Recently, in pine bark, we’ve found this to be a result of channeling and incomplete lateral distribution of water. However, if irrigated when closer to container capacity, with a smaller amount of water, a much higher percentage of that water will spread laterally through the container and avoid leaching vertically. The application in using liquid fertilizer is the same. If, when using liquid fertilizer, containers are pre-irrigated with clear water, we hypothesize a smaller proportion of nutrients will leach through the container and a larger portion of the substrate will retain applied nutrients in the pore water and on the substrate surface.

Another phenomenon observed is that mass flow drives much more water and nutrient movement than diffusion. Since the majority of the water is moving downwards in a container, the majority of the nutrients in the substrate move downwards as well. For this reason, we now hypothesize that fertilizer delivered via controlled-release fertilizers located in the lower portion of the container contribute less to the nutrition of the crop and instead readily leach during irrigation. Therefore, we propose that by applying fertilizer to only the upper half of the substrate, the nutrient runoff may be dramatically reduced.

The substrate in the bottom half of the container will still possess key nutrients deposited from the fertilizer in the upper half of the container via the downward movement of water or solute that occurs during each irrigation. Preliminary research has shown that crops grown in substrates with controlled-release fertilizers strategically placed in the upper half of the container show no difference in nutrition compared to crops grown with fertilizer spread throughout the entire substrate. Research on this aspect of fertilizer application technology is continuing at Virginia Tech and North Carolina State Universities.

These are only a few of the recent advances in substrate science. With the ecological and economic state of the world moving to more conservative approaches, more research is needed to help reduce the quantity of water and fertilizers applied to container crops. GT

Jeb S. Fields is a doctorate student, Julie Brindley is a research specialist and Jim Owen is an assistant professor stationed at the Virginia Tech Hampton Roads Agricultural Research and Extension Center in Virginia Beach, Virginia (nsy.prod.vt@gmail.com). James Altland is a USDA-ARS research scientist stationed at the Application Technology Research Unit in Wooster, Ohio.