In a five-year study, researchers Jake Shreckhise, Jim Owen and Alex Niemiera determined how much phosphorous is necessary to produce a marketable crop for containerised nurseries. Read an extract of the report here.
Fertility regimes for producing containerised 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 sulphate-based micronutrient fertiliser and dolomitic limestone, these routine amendments also supply plants with ample amounts of sulphur, calcium, and magnesium. The remaining macronutrients (nitrogen, phosphorous and potassium) and possibly micronutrients are commonly delivered as controlled-release fertiliser. A complete or incomplete liquid fertiliser is sometimes used to supplement controlled-release fertiliser when substrate electrical conductivity values—a proxy for nutrient levels—are low.
Numerous controlled-release fertiliser products are available for containerised crop production. These products offer a variety of fertiliser coating technologies, nutrient sources (e.g. N as urea vs. ammonium nitrate), longevities and N-P-K formulations. Choosing an appropriate fertiliser for your production system will ultimately ensure healthy plants reach saleable grade as quickly as possible.
Fortunately, many of the available controlled-release fertiliser formulations can achieve this goal. As long as all macro and micronutrient levels remain at or above the sufficiency threshold during active crop growth, plants will thrive. Nutrient toxicity or salt burn is uncommon if plants are fertilised according to the product label and is easily avoidable with proper monitoring of substrate electrical conductivity.
Although many controlled-release fertilisers can produce a saleable 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 phosphorous (P) as often, less than half of the phosphorous applied to a container-grown plant is actually used by the plant. Over the past five years we set out to better understand where phosphorous goes and how much is needed to produce a marketable crop.
A CLOSER LOOK AT PHOSPHOROUS
Pine bark- and peat-based substrates have little ability to retain phosphorous, causing phosphorous fertiliser to leach from containers during irrigation. In terms of plant needs, conventional controlled-release fertilisers often provide phosphorous at levels well beyond the minimum necessary amount to maximise crop development. While these excess phosphorous levels result in maximum growth, a consequence is that much of the fertiliser is wasted—it leaches from the container before being absorbed by the plant.
Nursery research has repeatedly shown that healthy containerised woody crops fertilised with a conventional controlled-release fertiliser formulation (i.e. six percent P2O5) absorb between only seven percent and 57 percent of the phosphorous applied. The proportion of applied phosphorous that is used by the plant can be increased by decreasing the phosphorous supply within the “adequate” range. Doing so not only improves fertiliser use efficiency and reduces the amount of bought phosphorous wasted, it can also help minimise phosphorous runoff from nursery sites to surface water.
Excess phosphorous 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 minimise phosphorous leaching from containers could help the nursery industry avoid potential future restrictions on phosphorous fertilization and keep the green industry “green”.
Over the past five years, the Horticultural Research Institute, Virginia Nursery and Landscape Association, Virginia Agricultural Council, Centre 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:
- How low can we go when applying phosphorous (i.e. the minimum amount of phosphorous applied that produces a saleable plant)?
- How do routine lime and micronutrient amendments influence phosphorous availability and leaching?
Growth response of holly, azalea, and hydrangea potted in a lime- and micronutrient-amended pine bark-only substrate and constant-liquid-fed for 80 days. The five liquid fertilisers used contained a range of 0.5 to 6 ppm phosphorous and non-growth-limiting levels of nitrogen and potassium.
In our first experiment, we utilised various low-P liquid fertilisers to determine the minimum phosphorous concentration needed to maintain maximal growth of containerised ‘Limelight’ hydrangea, ‘Helleri’ holly and ‘Karen’ azalea. Current best management practices suggest 5-15 ppm phosphorous be maintained in substrate solution when producing nursery crops. However, the majority of previous research did not adequately investigate plant response to phosphorous concentrations less than 5 ppm. Therefore, plants in our research were constant-liquid-fed with five liquid fertilisers that contained a range of 0.5 to 6 ppm phosphorous and non-growth-limiting levels of nitrogen and potassium.
Plants were potted in a lime- and micronutrient-amended pine bark substrate and grown for 80 days. Although phosphorous needs depended on growth stage, minimum phosphorous fertigation concentrations that sustained maximal growth were 5 ppm for ‘Limelight’ hydrangea, 3 ppm for ‘Karen’ azalea, and 1 ppm for ‘Helleri’ holly. Foliar phosphorous concentration increased (i.e. luxury consumption) when applied phosphorous exceeded the minimally-sufficient amount for maximal growth.
In the next experiment, nine-month controlled-release fertiliser formulations with one to four percent P2O5 were applied to ‘Helleri’ holly and Bloomstruck hydrangea to compare growth response to plants fertilised with a conventional nine-month, controlled-release product. 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 phosphorous 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 fertilised with 18-3-12, a 50 percent reduction in phosphorous compared to a conventional 15-6-12 controlled-release fertiliser. The prospect of a 50 percent reduction in phosphorous 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 sulphated micronutrient fertiliser on phosphorous leaching and plant-availability when applied as controlled-release fertiliser. This was accomplished in two studies, first in fallow containers, then in substrate of containerised 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 phosphorous availability and leaching by over 60 percent. Phosphorous 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 phosphorous retention and was necessary to maximise growth and phosphorous uptake of crape myrtle. The short term phosphorous reduction by the micronutrient fertiliser 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 phosphorous in the pine bark substrate, total phosphorous uptake by crape myrtle was unaffected by these amendments. Hence, when growing containerised crape myrtle, amending the substrate with dolomitic limestone and micronutrients can maximise growth and phosphorous uptake while reducing phosphorous leaching from containers to the environment. Further investigation is needed to determine if the phosphorous associated with these amendments can serve as a slow-release phosphorous supply for plant uptake.
In summary, phosphorous fertility should be targeted to a particular species’ needs and can be affected by production location, substrate, fertiliser source, and watering practices. Additionally, greater amounts of phosphorous may be absorbed by the plant than is needed to improve crop growth. Hence, current foliar phosphorous sufficiency ranges for many ornamental plants may be anecdotal if determined when luxury consumption was occurring. When fertilising with liquid alone, applied phosphorous concentration can be less than or equal to 5 ppm for hydrangea, holly and azalea.
When supplementing controlled-release fertiliser with liquid feed, additional phosphorous is most likely unnecessary; therefore, consider a nitrogen-only or incomplete (nitrogen and potassium) supplemental liquid fertiliser. Our experiments on using low-P controlled-release fertiliser formulations suggest 4 percent P2O5 can be used across containerised ornamental crops. However, phosphorous content may be further reduced to 3 percent P2O5 when growing some shrub rose and Hydrangea macrophylla taxa.
Additionally, amending the substrate with lime and/or micronutrients reduces phosphorous availability and subsequent phosphorous leaching. We strongly urge growers to experiment with low-P fertilisers to ensure these fertilisers can be successfully integrated in their unique production systems before widely adopting a low-P regime. The benefit of not putting dollars or phosphorous down the “drain” can keep our industry proactive to possible future regulatory pressure and preserve our industry’s title as “green.”
Article by Dr. Jake Shreckhise, a recent graduate and technical writer in the Department of Horticulture at Virginia Tech; Dr. Jim Owen, an Associate Professor of Horticulture and Nursery Crops Extension Specialist located at the Virginia Tech Hampton Roads Agricultural Research and Extension Centre in Virginia Beach; and Dr. Alex Niemiera, 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.