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
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
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.
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
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