Keep Your Nutrients in Your Plants!

Anthony LeBude Associate Professor, Nursery Crops Extension, NC State University

Anelle Ammons Program Manager – Duke Energy Carolinas TVM Regional Program

James S. Owen Jr. PhD Research Horticulturist, USDA-ARS Application Technology Research Unit

With fertilizer costs contributing a significant amount to overhead, an important goal of growers is to ensure that as much of the applied fertilizer ends up in their crop, getting them the most “bloom for their buck.” As well, the nursery industry is under increasing pressure to reduce environmental impacts from fertilizer use, with a continued focus toward nutrient runoff from containers via irrigation water. When these nutrients flow out of the nursery, after all, growers both see their dollars go down the drain and add to pollution issues in their surrounding communities. To be both economically sustainable and friendly to the environment, nurseries need to take steps to reduce runoff.

One important component of reducing runoff is keeping more water in containers, so it is actually utilized for growth. Factors such as the amount of irrigation, the amount of fertilizer, the placement of fertilizer and the type of potting mix all play roles in how plants access the nutrients present in containers. Learning how all these factors interact can offer insights into how to keep nutrients in containers, maximize plant growth and reduce pollution runoff.

Researchers at the North Carolina State University Mountain Horticultural Crops Research and Extension Center in Mills River, NC, potted Screaming Neon Red™ roses and Endless Summer® Bloomstruck® hydrangeas from Bailey Nurseries Inc. of Yamhill, OR, into Nursery Supplies trade #2 containers (C900) filled with either Aeration+ Potting Mix (AS40) or All-purpose Potting Mix (PM2) from Pacific Organics in Henderson, NC. AS40 provides 5% more air space, making it drier than PM2, which holds 7% more water by volume zAeration+ Potting Mix (AS40) and All-purpose Potting Mix (PM2) from Pacific Organics in Henderson, NC. yEC is electrical conductivity measured 4, 8, 12 and 18 weeks after potting, averaged over

(Table 1). The high rate (45g) of Harrell’s 18-4-8, 8-9 month controlled release fertilizer was top dressed, incorporated, or the medium rate (34g) (25% less than the high rate) was placed in the top half of the container to reduce leaching and maintain similar quality growth using fewer nutrients (Figure 1). Plants were irrigated overhead with two different low rates of water daily, resulting in plants that were grown dry in two diverse mixes, with different nutrient placements to slow leaching.

After 18 weeks, plants grown in the PM2 substrate were larger than those grown in AS40, regardless of irrigation rate or fertilizer placement (Table 2 and Figure 2), most likely due to the PM2’s finer texture and greater water-holding capacity. More water held by the potting mix meant more nutrients were available to the roots for a longer time between irrigation, and more available fertilizer usually results in more plant. Hydrangea plants grown with 25% less of the standard rate of fertilizer in the top half of the container were unsalable; however, rose plants were still salable if you “squinted,” since growth was only reduced by 14%, compared to the control (Figure 2). Reducing nutrients too much meant less growth in such relatively dry substrates. Recent research utilizing varied mixes in the same container, where fine mix was placed atop coarse mix, yielded shrub roses of comparable size with a 25% reduction in controlled-release fertilizer, regardless of irritation rate (https://www.mdpi.com/1197842). Future research may use engineered mixes with optimized water-holding capacity to further determine whether reduced fertilizer rates or irrigation volumes provide economic and/or environmental benefits.

Figure 2. Growth of rose (top) and hydrangea (bottom) in two different substrates with specified fertilizer rate and placement treatments.

Future Insights

By changing to a finer-textured potting mix like PM2, or to one with even less airspace and more water-holding capacity, growers can maximize their fertilizer investment when they wet their plants by keeping nutrients in the plants and out of the ecosystem. More information on this study can be found in a Journal of Environmental Horticulture article, “Effect of Irrigation, Fertilizer Rate and Placement, and Two Substrates on Growth of Rose and Hydrangea” (https://meridian.allenpress.com/ jeh/article/40/3/123/487303/ Effect-of-Irrigation-FertilizerRate-and-Placement).