Jeb S. Fields and Kristopher Criscione

Over the past several decades, soilless culture, the practice of producing growing plants and crops in containers instead of in the field, has revolutionized global horticultural production practices through introducing new opportunities to produce crops in individual containers, offering growers complete control and increased resource use efficiency.

For the longest time, the nursery and ornamental industry relied on soilless culture, as the entire plant was the commodity, and the whole plant needed to be transplanted after sale. As time progressed, the production of many vegetable and small fruit crops began to transition to soilless culture for the increased control, particularly the enhanced environmental control that growing in a greenhouse provides.

However, throughout the globe we are witnessing a soilless substrate renaissance of sorts, where a variety of crops, ranging from the traditional ornamental and environmental plants to food crops encompassing vegetables, small fruit and large tree fruit are transitioning to soilless culture for the added benefits of precision control of environment, reduced disease pressure, more efficient resource use and indifference to arable soil.

Moreover, growers in some parts of the world are beginning to explore production of staple agricultural crops, like potato and soybean, in soilless culture systems. Not only do soilless systems allow for more control and efficiency in producing crops, they also remove the need for arable land from the equation. Thus, food can be produced near urban areas, mitigating food deserts and improving production sustainability, particularly with regards to timely transportation of harvested crops. In view of this transition, recent global modeling efforts have projected soilless substrate use and reliance to grow four-fold over the next 30 years.

While growing crops in soilless substrates can be more sustainable than field production, there are still limitations with soilless substrates, particularly with regards to source material sustainability. Substrate materials are often industrial byproducts like tree bark (from the lumber and paper industry), or natural resources like peat moss. Thus, cost and availability of these materials are subject to fluctuation.

From a production perspective, soilless substrate systems require more nuanced management practices, specifically surrounding water and nutrition conditions. Substrate systems were developed in a time where resource use and efficiency may not have been prioritized in producers’ minds. Substrates were developed to be forgiving, where excessive water and fertilizer applications would not necessarily induce the crop loss.

There have been limited efforts in reimagining the container-substrate system over the past 50 years. While the adage says, “if it isn’t broke, don’t fix it,” failure to innovate will indeed pose challenges. As we move forward into a global economy that demands precision use of resources to improve crop productivity, a more synergistic substrate system may help support this rapid onset of new growers and provide insurance for resource limitations that may occur, all while ensuring the present and future success of the horticultural industry.

Substrate stratification is a relatively novel soilless substrate technique being developed within the Fields Lab in conjunction with U.S. Department of Agriculture Agricultural Research Service scientists; it involves stacking or layering unique substrate materials within a single container system. Layering substrates was initially developed to allow for precision fertilizer placement within the container system, allowing a more targeted fertility approach where nutrients are more available and accessible to growing crops. This then evolved to the ability to easily manipulate the air and water balance within the container, which is a container filled uniformly with a substrate. The stratified system is layering two substrates within.

Gravity dictates moisture distribution within the substrate, typically resulting in saturated media at the bottom of a container and drier conditions in the top of the container. This results in the need for very regular irrigation applications to maintain a moisture balance optimal for crop growth. However, incorporating fine or fibrous particles, materials which increase water holding capacity, within the top of the container can modify the hydraulics of a substrate. By using coarse materials below the fine particles of the surface, air-filled porosity is increased at the bottom of the container. In essence, water will “defy gravity” and result in a more uniform and balanced substrate moisture profile.

In typical substrates, bark is blended with fibers like peat, wood or coir to create industry standard substrates. The proportions can be adjusted to shift the air and water balance holistically (i.e., make the entirety of the system wetter or drier); however, stacking two unique substrates in a stratified system can individually fine tune the moisture balance of the top or bottom of the container, allowing more control and more precision in applying water to specific regions of the container.

In stratified substrate systems, the finer particles in the top portion slow water infiltration rates. The slower water movement provides a more uniform wetting distribution, holding water where it is usually quickly depleted and needed by plants. The finer particles also help retain mineral nutrients where they are readily accessible to growing crops, rather than losing these crucial components to leaching. Larger particles in the lower portions increase pore space, gas diffusion and air storage, reducing the likelihood of the anerobic conditions that are often experienced by roots at the bottom of the container.

Stratified substrate research from our lab has indicated that water and fertilizer application rates can be decreased by 25% while maintaining or improving crop productivity and quality. Moreover, root growth has been shown to improve in stratified substrates, possibly due to quicker establishment and optimized rhizosphere conditions. The newest research findings in stratified substrates involve peat use.

We have identified that using stratified substrates where a peat-based substrate is layered atop a low-cost bark substrate not only cuts peat usage in half but may also provide increased drought resistance in tandem with the other previously described benefits of improved water and fertilizer efficiency and crop rooting. Financially incentivizing the use of this technology, peat substrates can be upwards of 10 times the cost of bark substrates. Replacing 50% of the container with cheaper materials without any sacrificing crop quality can provide substantial cost savings and increase profitability.

In summation, stratified substrates currently present numerous opportunities to help prepare for the surge of soilless substrate use in the coming decades and the demand to improve production sustainability with regards to resource use and efficiency. The added benefits in crop growth and root development are currently under investigation and may continue to provide added benefits to growers. This simplistic reimagining of the container substrate system has identified major benefits and improvements for growers, and we are just beginning to scratch the surface of this technology. As we move into the future, we must continue to evaluate and refine our traditional systems in order to explore new frontiers and opportunities.

Jeb S. Fields is an assistant professor and extension specialist whose research lab focuses on environmental nursery production and soilless substrate science at the Hammond Research Station. Kristopher Criscione is a doctoral student working with Fields in soilless substrate science.

(This article appears in the spring 2023 edition of Louisiana Agriculture.)

A stratified substrate system should consist of finer substrate material layered atop coarse material as can be seen in this photo of a stratified nursery system utilizing pine bark grades to generate the layers. Photo by Jeb S. Fields

Low-cost pine bark can be layered below high-performance substrates to improve root productivity while reducing the need for costly peat-based substrates. Above is a representation of the variation in particle sizes that can occur in a stratified system. Photo by Jeb S. Fields