Understanding phosphorus for containerized nursery crops

Fertility regimes for producing containerized nursery crops typically begin by amending the substrate (i.e., growing medium) with lime and/or micronutrients. The lime rate used depends on the desired pH that ensures mineral nutrients are readily available to the plant. If using a sulfate-based micronutrient fertilizer and dolomitic limestone, these routine amendments also supply plants with ample amounts of sulfur, calcium, and magnesium. The remaining macronutrients, [i.e., nitrogen (N), phosphorus (P) and potassium (K)], and possibly micronutrients, are commonly delivered as controlled-release fertilizer. A complete or incomplete liquid fertilizer is sometimes used to supplement controlled-release fertilizer when substrate electrical conductivity values—a proxy for nutrient levels—are low.

Numerous controlled-release fertilizer products are available for containerized crop production. These products offer a variety of fertilizer coating technologies, nutrient sources (e.g., N as urea vs. ammonium nitrate), longevities, and N-P-K formulations. Choosing an appropriate fertilizer for your production system will ultimately ensure healthy plants reach saleable grade as quickly as possible. Fortunately, many of the available controlled-release fertilizer formulations can achieve this goal. As long as all macro and micronutrient levels remain at or above the sufficiency threshold (“A” in Fig. 1) during active crop growth, plants will thrive. Nutrient toxicity or salt burn (“B” in Fig. 1) is uncommon if plants are fertilized according to the product label and is easily avoidable with proper monitoring of substrate electrical conductivity.

Although many controlled-release fertilizers can produce a salable crop, not all result in a high nutrient use efficiency (i.e., the percent of applied nutrients used by the plant). This is particularly true for P; often, less than half of the P we apply to a container-grown plant is actually used by the plant. Over the past five years we set out to better understand where P goes and how much is needed to produce a marketable crop.

A closer look at phosphorus

Pine bark- and peat-based substrates have little ability to retain P, causing P fertilizer to leach from containers during irrigation. In terms of plant needs, conventional controlled-release fertilizers often provide P at levels well beyond the minimum necessary amount to maximize crop development. While these excess P levels result in maximum growth, a consequence is that much of the fertilizer is wasted—it leaches from the container before being absorbed by the plant. Nursery research has repeatedly shown that healthy containerized woody crops fertilized with a conventional controlled-release fertilizer formulation (i.e., 6% P₂O₅) absorb between only 7% and 57% of the P applied. The proportion of applied P that is used by the plant can be increased by decreasing the P supply within the “adequate” range depicted in Fig. 1. Doing so not only improves fertilizer use efficiency and reduces the amount of bought P wasted, it can also help minimize P runoff from nursery sites to surface water. Excess P in surface water from non-point sources is a serious issue in the US. Proliferation of toxic algae and cyanobacteria species induced by elevated nutrient levels in aquatic ecosystems causes species-biodiversity loss, contamination of drinking water, and widespread fish kills. Improving fertilization management to minimize P leaching from containers could help the nursery industry avoid potential future restrictions on P fertilization and keep the green industry “green”.

Over the past five years, the Horticultural Research Institute, Virginia Nursery and Landscape Association, Virginia Agricultural Council, Center for Applied Nursery Research, and a USDA-NIFA funded grant (CleanWateR3), have provided the means to support both basic and applied research to yield answers to two questions: 1) how low can we go when applying phosphorus (i.e., the minimum amount of phosphorus applied that produces a salable plant) and 2) how do routine lime and micronutrient amendments influence phosphorus availability and leaching.

In our first experiment, we utilized various low-P liquid fertilizers to determine the minimum P concentration needed to maintain maximal growth of containerized (#1 gal.) ‘Limelight’ hydrangea, ‘Helleri’ holly and ‘Karen’ azalea. Current best management practices suggest 5-15 ppm P be maintained in substrate solution when producing nursery crops; however, the majority of previous research did not adequately investigate plant response to P concentrations less than 5 ppm. Therefore, plants in our research were constant-liquid-fed with five liquid fertilizers that contained a range of 0.5 to 6 ppm P and non-growth-limiting levels of N and K. Plants were potted in a lime- and micronutrient-amended pine bark substrate and grown for 80 days. Although P needs depended on growth stage, minimum P fertigation concentrations that sustained maximal growth were 5 ppm for ‘Limelight’ hydrangea, 3 ppm for ‘Karen’ azalea, and 1 ppm for ‘Helleri’ holly. Foliar P concentration increased (i.e., luxury consumption) when applied phosphorus exceeded the minimally-sufficient amount for maximal growth.

In the next experiment, 9-month controlled-release fertilizer formulations (blended by Harrell’s) with 1 to 4 percent P₂O₅ were applied to #1 gal. ‘Helleri’ holly and Bloomstruck hydrangea to compare growth response to plants fertilized with a conventional nine-month, controlled-release product (15-6-12). This experiment was conducted in USDA Hardiness Zones 6 (Blacksburg, VA) and 8 (Virginia Beach, VA). Our results for ‘Helleri’ holly were inconclusive since holly growth increased with increasing P application rate in hardiness zone 8, but responded minimally in hardiness zone 6. Conversely, Bloomstruck hydrangea responded similarly in both Zones 6 and 8, with maximal growth attained when fertilized with 18-3-12, a 50% reduction in P compared to a conventional 15-6-12 controlled-release fertilizer. The prospect of a 50% reduction in P fertilization could have major implications since hydrangea is the second leading deciduous shrub produced in the U.S.

Concurrent with our applied research, we conducted several laboratory experiments to better understand the effect of dolomitic limestone and a sulfated micronutrient fertilizer on P leaching and plant-availability when applied as controlled-release fertilizer. This was accomplished in two studies, first in fallow containers, then in substrate of containerized crape myrtle. In both studies, pine bark substrate was either non-amended or amended with dolomitic limestone, micronutrients, or both. Results of both studies indicated that amending pine bark with both dolomitic limestone and micronutrients can reduce P availability and leaching by over 60%. Phosphorus reductions were attributed primarily to the presence of dolomitic limestone; however, the addition of a micronutrient package incorporated at the time of potting provided some short-term P retention and was necessary to maximize growth and P uptake of crape myrtle. The short-term P reduction by the micronutrient fertilizer is attributed to the fact that it contained a small amount of dolomitic limestone in addition to P-complexing micronutrient cations (i.e., Fe and Mn). Although dolomitic limestone and micronutrient amendments reduced the immediate plant-availability of P in the pine bark substrate, total P uptake by crape myrtle was unaffected by these amendments. Hence, when growing containerized crape myrtle, amending the substrate with dolomitic limestone and micronutrients can maximize growth and P uptake while reducing P leaching from containers to the environment. Further investigation is needed to determine if the P associated with these amendments can serve as a slow-release P supply for plant uptake.

In summary, P fertility should be targeted to a particular species’ needs and can be affected by production location, substrate, fertilizer source, and watering practices (which was not discussed herein). Additionally, greater amounts of P may be absorbed by the plant than is needed to improve crop growth. Hence, current foliar P sufficiency ranges for many ornamental plants may be anecdotal if determined when luxury consumption was occurring. When fertilizing with liquid alone, applied P concentration can be less than or equal to 5 ppm for hydrangea, holly and azalea. When supplementing controlled-release fertilizer with liquid feed, additional P is most likely unnecessary; therefore, consider a nitrogen-only or incomplete (nitrogen and potassium) supplemental liquid fertilizer. Our experiments on using low-P controlled-release fertilizer formulations suggest 4% P₂O₅ can be used across containerized ornamental crops. However, P content may be further reduced to 3% P₂O₅ when growing some shrub rose and Hydrangea macrophylla taxa. Additionally, amending the substrate with lime and/or micronutrients reduces P availability and subsequent P leaching. We strongly urge growers to experiment with low-P fertilizers to ensure these fertilizers can be successfully integrated in their unique production systems before widely adopting a low-P regime. The benefit of not putting dollars or P down the “drain” can keep our industry proactive to possible future regulatory pressure and preserve our industry’s title as “green.”

Dr. Jake Shreckhise (jshreck@vt.edu) is a recent graduate and technical writer in the Department of Horticulture at Virginia Tech. Dr. Jim Owen (jim.owen@vt.edu) is an Associate Professor of Horticulture and Nursery Crops Extension Specialist located at the Virginia Tech Hampton Roads Agricultural Research and Extension Center in Virginia Beach. Dr. Alex Niemiera (niemiera@vt.edu) is a Professor of Horticulture, Horticulture Undergraduate Program Director, and Assistant Dean of Student Programs in the College of Agriculture and Life Sciences at Virginia Tech. This research was made possible by the agencies, associations, and allied suppliers mentioned in the article and the many nurseries who graciously donated plants: Bailey Nursery, Bennett’s Creek Nursery, Hermitage Farms, Lancaster Farms, and Saunders Brothers Nursery.