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Plant Health: What’s in the Pot?

Exploring Stratified Substrates and Soil Hydraulics in Agricultural Science

Lloyd Nackley

Nursery science researchers have embarked on a journey to harness the principles of soil hydraulics to reshape container production practices. In the past couple of years, Dr. Chris Criscione and Dr. Jeb Fields and others released a series of articles that showed that stratifying pine bark can serve as a substitute for peat-based media in floriculture and bark-based woody plant production. Through layering premium floriculture media over cost-effective pine bark within containers, they reduced reliance on peat. In their study focused on Petunia hybrid ‘Supertunia Honey’, the stratified substrates yielded crops of comparable size and quality, along with enhanced root productivity. Their work also showed superior performance of bark:coir substrates in a stratified setup, even when subjected to reduced irrigation when producing Loropetalum chinense ‘Ruby’ liners. Positive microbial communities in stratified systems further aided in mitigating water stress. Furthermore, Red Drift® rose plants grown in stratified substrates exhibited equal or superior crop growth despite receiving less controlled-release fertilizer. This suggests potential for reducing fertilizer and irrigation rates while upholding crop
quality, offering a sustainable avenue for containerized crops. This blog post aims to shed light on stratified substrates and also provide insights into ongoing projects at the Nackley Lab that are delving into this innovative frontier of nursery science.

image shows containers of growing medium of different textures
Image: mixing coarse and fine substrates in containers to address soil stratification

Soil hydraulics delves into the intricate world of water movement within soils—how water traverses the soil matrix, its distribution, and its intricate interactions with soil particles and structure. It’s a field dedicated to understanding the physics governing water’s journey through soils, and this knowledge carries significance for agriculture, environmental science, hydrology, and civil engineering. Soil hydraulics guides irrigation strategies, shapes drainage designs, and fuels sustainable land management practices.

image shows saturated and unsaturated soils in comparison
Image: water in saturated and unsaturated soils

Central to soil hydraulics is the concept of matric potential, a critical factor in irrigation management and plant-available water. Matric potential captures the degree to which soil particles retain water due to molecular attractions—it’s essentially the “stickiness” of water to soil particles. When soil isn’t completely saturated, minuscule air-filled spaces exist between particles. Water molecules adhere to these particles, creating capillary forces that coax water upwards. The strength of the bond between water and soil particles determines the matric potential, effectively influencing water availability to plants.

Image shows layering of the 3 soil phases: gaseous, liquid and solid
Image: the gaseous, liquid and solid phases of soils

Two other essential players in the realm of soil hydraulics are soil texture and structure. Envision soil as an intricate mosaic, with particles coming together to form aggregates, shaping distinct pathways and chambers. This arrangement, referred to as soil structure, creates macropores—
akin to express highways—through which water can flow rapidly. At the same time, micropores, reminiscent of narrow alleyways, gently retain water against gravity’s pull, acting as reservoirs
for plant roots during dry spells. Additionally, soil structure influences permeability, determining how efficiently water infiltrates and the risk of surface runoff.

Soil texture, on the other hand, hinges on the proportions of sand, silt, and clay particles in the soil. Each particle type comes with distinct traits that shape water dynamics. Coarser-textured soils with more sand have ample space between particles, allowing water to move quickly with
less retention. Finer-textured soils, rich in silt and clay, boast smaller gaps between particles, leading to slower water movement and higher water-holding capacity.

image shows the soil texture triangle
Image: The Soil Texture Triangle

However, when soil is confined to pots or containers, it undergoes transformations in its natural structure due to various container-related factors. The restricted space of a pot, contrasting with the expanse of natural soil, can lead to the erosion of soil structure. Aggregates—clusters of particles forming pores and pathways—can deteriorate over time due to limited expansion room. This confined space often results in compaction, where soil particles compress closely, reducing essential air-filled pores needed for root growth and water movement. The watering practices specific to potted plants can contribute further by compacting soil particles as water occupies gaps. The absence of natural soil organisms and plant roots within containers hampers the
maintenance of soil structure. Repeated disturbances, like transplanting or repotting, can exacerbate these structural changes. To counter these effects, selecting appropriate potting mixes that retain structure, incorporating organic matter for improved aeration and water retention, and being mindful of compaction during planting and watering are recommended.

image shows a hydrangea in a pot with drip irrigation
Image: Hydrangea in pot with drip emitter irrigation

Traditionally, the nursery field has focused on creating homogeneous potting mixes that maintain structure while offering suitable hydraulic properties. Classic blends often comprise bark, coir, peat, perlite, vermiculite, and pumice. However, a recent shift in focus has led scientists to explore how layering media can simulate natural soil hydraulics—an approach known as stratified substrates.

coarse substrate
Coarse substrate
fine substrate
Fine substrate

Stratified substrates involve arranging potting media of varying textures in layers within a single container. This structured layering entails placing coarser-textured substrates at the bottom and finer-textured ones on top, mimicking natural soil layers. This technique aims to influence water
movement, nutrient distribution, and hydraulic behavior within the confined environment of a container. By borrowing from the stratification seen in the ground, stratified substrates strive to
optimize resource efficiency, plant growth, and root development in controlled settings like potted plants.

Some may draw parallels between stratified substrates and the practice of placing rocks or gravel at the bottom of larger plant pots. While both concepts involve layering materials, there are distinctions. The practice of adding gravel or rocks to enhance drainage in larger pots shares a
kinship with stratified substrates. However, it doesn’t replicate the comprehensive layering dynamics seen in stratified substrates. Adding gravel mainly addresses drainage concerns without fully incorporating the layered hydraulic principles inherent to stratified substrates.

image shows 2 piles of substrates by texture
Image: fine (l) and coarse (r) substrate, side by side

Dr. Chris Criscione, in collaboration with the Dr. Jeb Fields group at Louisiana State University, has been at the forefront of investigating stratified substrates in containerized plant growth. Their research delves into how layering different potting media textures can enhance water retention, nutrient availability, and overall plant performance. The studies highlight promising outcomes, such as heightened root productivity, improved growth, and enhanced quality under stratified
conditions compared to conventional substrates. This technique holds potential for bolstering sustainable crop cultivation within controlled environments.

gravel pad with pots of hydrangea growing
Image: potted hydrangeas on the gravel pad at NWREC

Nevertheless, it’s crucial to acknowledge that findings from studies conducted in one geographic region—such as the Southeastern USA—may not seamlessly extrapolate to other areas with distinct climates and environmental conditions, like the Pacific Northwest. Climate, temperature, humidity, and other factors can significantly influence plant growth and water dynamics. Given this variation, research in regions like the Pacific Northwest, such as Oregon, is crucial. The
unique environmental factors there, including cooler temperatures and higher rainfall, can impact water movement, nutrient availability, and plant response to stratified substrates. Bark-based substrates, common in some areas, may behave differently in terms of water retention and
drainage in regions with distinct soil compositions. To address this, the Nackley Lab initiated a collaboration with Dr. Fields and others in 2022, planning to explore the impacts and benefits of stratifying substrates in Nursery production. The first stratified substrate experiment was
launched in 2023 at Oregon State University North Willamette Research and Extension Center (NWREC), aiming to reduce resource demand and provide insights into the effectiveness of stratified substrates in that context, contributing to more informed decision-making for nursery production and horticulture practices in the region.

More Information:

Evaluating Stratified Substrates Effect on Containerized Crop Growth under Varied Irrigation Strategies

Root Exploration, Initial Moisture Conditions, and Irrigation Scheduling Influence Hydration of Stratified and Non-Stratified Substrates

Single-screen Bark Particle Separation Can be Used to Engineer Stratified Substrate Systems

Stratified Substrates Can Reduce Peat Use and Improve Root Productivity in Container Crop Production

Meet the Team: Summer Update

The Gravel Pad update you’ve been waiting for, and more!

There’s so much going on in the season of plenty around NWREC! Enjoy this virtual tour of a few projects around the nursery.

Dalyn has been continuing her work with mini-lysimeters that control irrigation in shade trees – these tiny scales weigh the potted plants and use the change in weight as they dry to determine when to turn on the water. The lysimeters are gathering data on plant weight along with an on-site weather station to better understand the relationship between heat and irrigation in gravel pad production. Read more about this project here.

View of gravel pad with potted maple trees
Young red maple trees on the gravel pad are using lysimeters to monitor water loss in conjunction with a dedicated weather station (left).

The Willamette Valley has had a few HOT summers in a row, even though lately this one has been pretty mild. Nevertheless, we haven’t given up on finding solutions for heat mitigation – including growing ornamentals under drought conditions to see which are the most “climate-ready” to meet changing needs. We’ll be asking the public to evaluate those plants in the upcoming Climate-Ready Field Day, come along and see how the plants are progressing (click the link above for more info).

In addition, we’re evaluating different means of mitigating the heat and the resultant high rates of evapotranspiration (basically ways to reduce plant sweat), from misting the young plants to covering tissues with kaolin, introducing fungicides that may be beneficial in managing water loss, using white pots instead of the traditional black, and even growth inhibitors – it’s been a pretty amazing feat to monitor the effects as you can see- check out this monitoring station!

Jaiden, Lloyd and Dalyn at the ET monitoring station with shade trees
Dalyn, Lloyd and summer hire Jaiden show off the monitoring station in the heat stress/evapotranspiration mitigation study.

young flowering shrubs in alternating black and white pots
Does the color of the pot change the heat stress for these water-loving shrubs on the gravel pad?

A small project growing marigolds for festivals and holidays – like Dia de los Muertos – is also underway. Growing the marigolds has certainly brightened up the Nursery Zone at NWREC, and we’ve progressed into evaluating passive means to dry the flowers, saving energy and resources while preserving the gorgeous summer color.

fresh marigolds
Marigold blooms
dried marigolds

There’s even more in the works – stay tuned for information about fall workshops and PACE courses created specifically for nursery and greenhouse production for topics covering drone sprays, integrated pest management, and more.

Meet the Team: Unmanned Aerial Systems

Sadie Keller

Unmanned Aerial Systems applications in agriculture have interested me from the time I saw some trial flights at the World Ag Expo in Tulare, California in 2020. The possibilities of crop mapping, using multispectral imagery to create NDVI data, and optimizing resources seemed
endless. And while all of this is still true, I have learned over the last couple years that there is a lot more to it than just getting out the drone and flying it over a field.

Dalyn McCauley, captured in a 2022 drone image

In the Fall quarter of 2022, I was able to take a class on campus from Dr. Michael Wing called “Unmanned Aerial Systems Remote Sensing. In this course I was able to get some hands-on experience flying a DJI Phantom 4, a DJI Matrice 200, and a DJI Matrice 300. We also were able to take aerial imagery with a MicaSense Altum multispectral and thermal camera attached to the DJI Matrice 300 and analyze that data during the course of the class. To analyze the data we learned how to use AgiSoft Metashape photogrammetry software to make orthomosaics of our area of interest. With the orthomosaics, we were able to perform different sorts of analysis using ArcGIS Pro software and R. This class really gave me a solid introduction to the collection and analysis of aerial imagery.

DJI matrice 210 with a zen muse xt2 camera flying over red maples
The DJI matrice 210 with a zen muse xt2 camera flying over red maples

With some knowledge under my belt after taking that course, I decided to look into taking the Part 107 Certification exam to obtain my FAA administered Remote Pilot License. The purpose of this certification is to be able to fly a drone for commercial, government, or any other non-
recreational purposes. In the Nackley Lab, we have access to a DJI Matrice 210 quadcopter and a MicaSense Red-Edge M camera so I wanted to be able to open up some more avenues for research by being able to pilot this drone for our lab!

DJI matrice 300 with a micasense altum camera
DJI matrice 300 with a micasense altum camera

In our lab, a lot of our work centers around major challenges to the nursery production industry in Oregon. Working with a drone can allow you to survey a large area for early signs of drought stress, nutrient deficiencies, or pests to minimize a loss in yield. Now that I have the Remote Pilot Certification, my goal is to help our lab create more aerial imagery (data) that ties into our work which addresses major challenges to nursery production in Oregon like irrigation application, pest management, plant nutrition, and climate adaptations.

Thinking about getting a Remote Pilot Certification? Here are Sadie’s tips for Studying (click here).

For more information about the part 107 certification or UAS applications feel free to contact me
@ kellesad@oregonstate.edu.

Other great OSU resources:
BEAV UAS Program – Dr. Kristine Bucklin and Dr. Lloyd Nackley
Aerial Information Systems Laboratory – Dr. Michael Wing