Air management

Defying Gravity – Nursery Management

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We have come a long way from using “egg” boxes filled with a “UC Soil Mix” made from soil or sterilized sand. Container production using blow or injection molded plastic containers, bark-based media, and controlled release fertilizers has eased production constraints and improved crop quality for many growers. Specialized media and fertility mixes have expanded the number of species produced and sold throughout the year. There is now less disease pressure thanks to media with good drainage and greater air space, and the harvesting process has been streamlined. However, the standard has remained static for nearly four decades, with only small advances in container design, micro-irrigation, and agrochemical dispensing technologies. We propose to reinvent media and believe that layering soilless media is an inexpensive, non-invasive technology that can save water and agrochemicals while maintaining or improving growth, quality and turnaround time. sale of crops. It starts with the use of specially designed substrates which are laminated in a container.

What is stratification?

Lamination is the layering of multiple carriers (synonymous with substrates) in a single container to advantageously manipulate the system. This approach mimics natural soil horizons and has also been used effectively in many industrial and environmental applications. Contrary to previous research on the use of “mulch” in containers, the layers are within normal rooting volume. Two main layering techniques are currently being evaluated in the United States: 1) placing fine or fibrous pine bark over a coarse substrate to manage water and fertility; and 2) place a coarse substrate on top of the conventional substrate (C. Marble at UFL in collaboration with J. Altland) to manage weeds. This article will focus on the first described technique of substrate lamination.

Fig. 1 Conceptual visualization of moisture distribution after irrigation in conventional, stratified substrate as a function of container height.

Why stratify?

Because we believe the solution is in the container. Over the past decade, new technologies that promised greater nutrient and water use efficiency, such as clay amendments or in situ moisture sensors, have yet to be adopted. due to complexity, cost, commercial availability or lack of robustness for industrial applications. These approaches have focused on unknown substrate amendments or the use of computer algorithms to control irrigation using remote sensor networks. We propose that the simplest and most straightforward solution is to conserve resources in the container using conventional substrate components, modified by layering two or more substrates in a single container. From the early days of container layering, placing a fine or fibrous, fertilizer-amended substrate on top of an unfertilized, coarse, well-drained substrate, our goal was to improve water retention and fertilizer in the container.

Fig. 2 Illustration of a fallow (left) and image of a rooted stratified substrate (right) in which a fine bark substrate has been placed on a coarse substrate. The image on the right is of a rooted, salable #15 maple that received a 20% reduction in water and top-applied time-release fertilizer.

This approach allows us to overcome the problem of water channeling, in which water moves directly through the pot when irrigated, as if it were in a straw. This and lack of root development also lead to leaching, causing unused fertilizer to be wasted from the bottom of the container when the crop is established. In addition, substrate stratification can address the undesirable moisture gradient (dry top and wet bottom) and potentially reduce or eliminate “perched water table”, a result of container shape and even substrate filling. not stratified. The adoption of this technology is made possible by the fact that individual substrate components can be selected, modified and mixed to create specific physical and chemical properties. Many growers already fill containers in two steps to make potting easier and ensure correct placement or height of the liner: fill the bottom of the container first, place the liner, then add the substrate around the liner at the top. of the container. Thus, three main topics informed our decisions when designing stratified substrates to improve container crop production: the substrate itself, fertility practices, and the type and timing of irrigation.

Substrate: The substrate profile of a given container has a water gradient, being the wettest on the bottom and the driest on the top. Currently, a substrate or mix is ​​chosen to retain enough water to quickly establish liners while minimizing disease pressure (about 25% to 30% minimum airspace); often receiving frequent and excess water to ensure the shallow planted plug or liner does not dry out. This leads to prolonged waterlogging or saturation of the substrate at the bottom of the container; often this problem manifests as a sour, rotten egg smell that indicates hypoxic conditions (i.e. lack of oxygen). Layering thin (¼ inch) barks can lessen the saturation zone at the bottom of the container and result in a more even, “gravity defying” distribution of water across the entire profile of the container. This in turn ensures water is available at the top of the substrate and an adequate amount of oxygen throughout the container profile for rapid root exploration and development. As the roots grow, the water-holding capacity of the lower coarse layers will increase, ensuring that water does not become limiting.

Fig. 3 Comparison of Hydrangea macrophylla root and shoot growth in standard nursery mix (CRF incorporated throughout) versus stratified substrate (CRF containing fine bark on only coarse, lime-amended Douglas fir bark) about 1 year after the start of production.

Fertility: Controlled Release (CRF) fertilizer granules are usually placed on top of the container (i.e., top-dress) or incorporated into the entire profile of the substrate. Our research has shown that traditional fertilizer incorporation provides a consistent release of a consistently wet CRF granule that is not easily dislodged from the container if spilled; however, the mineral nutrients from the CRF in the lower part of the container are susceptible to leaching when irrigated. This is most apparent early in production when the roots of small plants have not explored the bottom of the container and are unable to intercept mineral nutrients released from the CRF lower in the substrate profile. Layered incorporation of fertilizer only in the top half of the container places the slowly released mineral nutrients close to the initial roots of the liner, where they are easily accessible. Later in the production cycle, lower roots intercept mineral nutrients pushed down during irrigation events from higher levels of the substrate profile. This provides an opportunity to reduce the amount of CRF applied and increase crop utilization of applied mineral nutrients.

Irrigation: During irrigation, water enters the dry substrate, channels (i.e., preferential flow) directly through the profile, and drains from the bottom of the container throughout the watering event. irrigation. This is aggravated by the use of single or cyclic irrigation (i.e. the water is divided into several rounds per day), as well as by the use of spray stakes in which a large amount of water is applied quickly (eg, a second or a few minutes). The ability to add a finer textured substrate, amended with peat or coir, to the top half of the container to encourage the development of smaller, not entirely dry substrate pores, decreases the rate at which water penetrates the substrate (i.e. infiltration rate) and subsequently slows the movement of water along the profile. Slowing the movement of water through the substrate improves water uptake and reduces leaching of water and subsequent agrochemicals.

Fig. 4 Nursery research comparing substrate stratification techniques and fertilizer placement.

Lessons learned so far

We continue to evaluate several ornamental taxa (e.g. arborvitae, crape myrtle, hydrangea, loropetal, gardenia, petunia, red maple, rose and zinnia) in different container sizes (e.g.

There have been minimal setbacks when implementing laminate substrates; however, a noticeable change in root growth of the PJM azalea at one of the two locations and a decrease in rose shoot growth with 20% deficit irrigation demonstrates the need for further research to optimize the system. : substrate – fertilizer – irrigation. Our next steps in layered substrates are to better understand the movement of water and fertilizer within the container, the physical interaction of the two strata, how best to modify the pouring method to monitor fertility. This could include adjusting strata depth and optimizing fertility and water by determining “how far can we go” while producing an equivalent or larger plant. Additionally, the potential reduction in material costs should be evaluated to determine if an inexpensive substrate combined with a lower amount of FRC per container can provide significant savings. Finally, the evaluation of this concept in the production of seedlings and in floriculture is attracting growing interest. In each case, the basic science will be explored in our respective research facilities and validated in incubators across the United States. To participate in laminate substrate trials, contact us directly to learn more about emerging opportunities.

About the Authors: James “Jim” Owen Jr. ([email protected]) and James Altland [email protected]) are horticulturalist and research director, respectively, in the Technology Research Unit of USDA-ARS app in Wooster, Ohio. Jeb Fields ([email protected]) is an assistant professor, extension specialist, and director of the LSU AgCenter Hammond Research Station in Louisiana. The research was funded in part by the Horticultural Research Institute, Oregon Department of Agriculture, USDA-ARS, University of Florida, Louisiana State University, and Virginia Tech,