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Jeb S. Fields, James S. Owen Jr., James E. Altland, Marc W. van Iersel, and Brian E. Jackson

Water-efficient soilless substrates need to be engineered to address diminishing water resources. Therefore, we investigated soilless substrates with varying hydrologies to determine their influence on crop growth and plant water status. Aged loblolly pine (Pinus taeda) bark was graded into four particle size fractions. The coarsest fraction was also blended with either sphagnum peat or coir at rates that mimic static physical properties of the unfractionated bark or conventional substrate used by specialty crop producers within the eastern United States. Hibiscus rosa-sinensis ‘Fort Myers’ plugs were established in each of the seven substrates and maintained at optimal substrate water potentials (−50 to −100 hPa). After a salable crop was produced 93 days after transplanting, substrate was allowed to dry until plants completely wilted. Crop morphology and water use was affected by substrate hydrology. Increased substrate unsaturated hydraulic conductivity (K) allowed for plants to access higher proportions of water and therefore increased crop growth. Maintaining optimal substrate water potential allowed plants to be produced with <18 L water. Measurements of plant water availability showed that the substrate water potential at which the crop ceases to withdraw water varied among substrates. Pore uniformity and connectivity could be increased by both fibrous additions and particle fractionation, which resulted in increased substrate hydraulic conductivity (K s). Plants grown in substrates with higher hydraulic conductivities were able to use more water. Soilless substrate hydrology can be modified and used in concert with more efficient irrigation systems to provide more water sustainability in container crop systems.

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Jacob H. Shreckhise, James S. Owen Jr., Matthew J. Eick, Alexander X. Niemiera, James E. Altland, and Brian E. Jackson

Soilless substrates are routinely amended with dolomite and sulfate-based micronutrients to improve fertility, but the effect of these amendments on phosphorous (P) in substrate pore-water during containerized crop production is poorly understood. The objectives of this research were as follows: compare the effects of dolomite and sulfate-based micronutrient amendments on total P (TP), total dissolved P (TDP), orthophosphate P (OP), and particulate P (PP; TP − TDP) concentrations in pour-through extracts; to model saturated solid phases in substrate pore-water using Visual MINTEQ; and to assess the effects of dolomite and micronutrient amendments on growth and subsequent P uptake efficiency (PUE) of Lagerstroemia L. ‘Natchez’ (crape myrtle) potted in pine bark. Containerized crape myrtle were grown in a greenhouse for 93 days in a 100% pine bark substrate containing a polymer-coated 19N–2.6P–10.8K controlled-release fertilizer (CRF) and one of four substrate amendment treatments: no dolomite or micronutrients (control), 2.97 kg·m−3 dolomite (FL); 0.89 kg·m−3 micronutrients (FM); or both dolomite and micronutrients (FLM). Pour-through extracts were collected approximately weekly and fractioned to measure pore-water TP, TDP, and OP and to calculate PP. Particulate P concentrations in pour-through extracts were generally unaffected by amendments. Relative to the control, amending pine bark with FLM reduced water-extractable OP, TDP, and TP concentrations by ≈56%, had no effect on P uptake efficiency, and resulted in 34% higher total dry weight (TDW) of crape myrtle. The FM substrate had effects similar to those of FLM on plant TDW and PUE, and FM reduced pore-water OP, TDP, and TP concentrations by 32% to 36% compared with the control. Crape myrtle grown in FL had 28% lower TDW but pour-through OP, TDP, and TP concentrations were similar to those of the control. Chemical conditions in FLM were favorable for precipitation of manganese hydrogen phosphate (MnHPO4), which may have contributed to lower water-extractable P concentrations in this treatment. This research suggests that amending pine bark substrate with dolomite and a sulfate-based micronutrient fertilizer should be considered a best management practice for nursery crop production.

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James E. Altland, Leslie Morris, Jennifer Boldt, Paul Fisher, and Rosa Raudales

Paclobutrazol is a plant growth retardant commonly used on greenhouse crops. Residues from paclobutrazol applications can accumulate in recirculated irrigation water. Given that paclobutrazol has a long half-life and potential biological activity in parts per billion concentrations, it would be desirable to measure paclobutrazol concentration in captured irrigation supplies. However, there are no standard protocols for collecting this type of sample. The objective of this research was to determine if sample container material or storage temperature affect paclobutrazol stability over time. In two experiments, paclobutrazol was mixed in concentrations ranging from 0.04 to 0.2 mg·L−1 and stored in polyethylene, clear glass, or amber glass containers at temperatures of either 4 or 20 °C. Paclobutrazol concentration was measured at 3, 14, and 30 days after the start of each experiment. Across the two experiments, there were no consistent trends in reduction of paclobutrazol concentration with respect to container material or storage temperature. In the first experiment, there was an average of 5% reduction across all treatments from day 0 to 30, whereas in the second experiment, concentration did not decrease over the 30-day time period. These data suggest that paclobutrazol is stable in collected water samples for at least 30 days, and that either glass or polyethylene containers are suitable for collecting greenhouse water samples for analysis of paclobutrazol concentration. A minimum volume of 100 mL was determined to be the optimum to analyze water samples with diverse paclobutrazol concentrations.

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M. Gabriela Buamscha, James E. Altland, Dan M. Sullivan, Donald A. Horneck, and John P.G. McQueen

The objective of this study was to determine if there are growth differences in geranium (Pelargonium ×hortorum ‘Maverick Red’) produced in fresh or aged douglas fir (Pseudotsuga menziesii) bark (DFB). A second objective was to document nitrogen (N) immobilization and decomposition rates of fresh and aged DFB to better understand the cause of growth differences. A series of experiments to measure plant response, N draw-down index (NDI), and percentage of cumulative carbon (C) loss were conducted on fresh and aged DFB. Geranium plugs were transplanted to containers filled with fresh or aged DFB. Treatments were arranged in a 2 × 3 factorial with two DFB ages (fresh and aged) and three N fertilizer rates (200, 300, and 400 mg·L−1). Plant growth was affected by DFB age in that geraniums were smaller when grown in fresh DFB. N draw-down analysis demonstrated that a large fraction of N in solution was immobilized in fresh and aged DFB. Carbon loss, measured as a gauge of bark decomposition, was not affected by N rate or bark type. Similarities in C loss between fresh and aged DFB agree with the similar N immobilization potential (NDI) in the two materials.

Open access

Erin J. Yafuso, Paul R. Fisher, Ana C. Bohórquez, and James E. Altland

Greenhouse propagation of unrooted plant cuttings is characterized by short container cell height and high irrigation frequency. These conditions can result in high moisture level and low air content in soilless container substrates (“substrates”), causing delayed growth of adventitious roots and favoring root disease. The objective of this study was to quantify and compare substrate water and air relations for three propagation substrates (peat, rockwool, and phenolic foam) that varied widely in physical characteristics using four methods: 1) evaporation method with a tensiometer, 2) frozen column method, 3) gravimetric analysis, and 4) X-ray computed tomography (CT) analysis. Moisture retention curves based on evaporation (1) and the frozen column (2) resulted in differences for peat, but similar curves for rockwool and foam. The frozen column method was simple and low cost, but was constrained by column height for peat, which had a higher water potential compared with the other two substrates. Substrate porosity analysis at container capacity by gravimetric or CT methods were similar for volumetric water and air content (VWC and VAC) in rockwool and foam, but differed for peat for VWC and VAC. Gravimetric analysis was simple, rapid, and low cost for whole-cell analysis, but CT further quantified spatial water and air relations within the cell and allowed visualization of complex water and air relations in an image. All substrates had high water content at container capacity ranging from 67% to 91% VWC with 5% to 11% VAC in the short propagation cells, emphasizing the need for careful irrigation management.

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M. Gabriela Buamscha, James E. Altland, Daniel M. Sullivan, and Donald A. Horneck

Annual vinca [Catharanthus roseus (L.) G. Don ‘Peppermint Cooler’] plugs were transplanted to containers filled with Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] bark (DFB) in May and June 2005 (Expts. 1 and 2, respectively). Treatments were arranged in a 2 × 3 factorial with two DFB ages (fresh and aged) and three micronutrient sources (DFB alone, 10% by volume yard debris compost, or 0.9 kg·m−3 Micromax fertilizer). Plants were measured for shoot dry weight and foliar color. Substrate and foliar samples of each plant were analyzed for 13 essential macro- and micronutrients plus substrate pH and EC. Douglas fir bark alone appears to provide sufficient micronutrients for annual vinca grown at pH 4.7 to 5.7 over a 2-month period. In Expt. 1 there were no differences in shoot dry weight or foliar color regardless of DFB age or micronutrient source. At the end of Expt. 2, plants in aged DFB were larger than those in fresh DFB, but differences were primarily the result of nitrogen availability. None of the treatments developed color symptoms that could be associated with micronutrient deficiency. Micronutrient availability in DFB should be considered in container fertilizer management plans.

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James E. Altland, Charles H. Gilliam, Gary J. Keever, James H. Edwards, Jeff L. Sibley, and Donna C. Fare

Two experiments were conducted with pansy (Viola ×wittrockiana Gams `Bingo Yello') to determine the relationship between foliar nitrogen (% of dry weight) (FN) and either sap nitrate concentration (SN) in petioles or SPAD readings of foliage. FN was highly correlated to SN throughout both experiments (r = 0.80 to 0.91). FN was poorly correlated to SPAD readings early in both experiments (r = 0.54 to 0.65), but more highly correlated later when visual symptoms of N deficiency were apparent (r = 0.84 to 0.90). SN determined with the Cardy sap nitrate meter was a reliable predictor of FN in pansy, while SPAD readings were only reliable after symptoms of N deficiency were visually evident. FN can be predicted with SN using the following equation: log(SN) = 0.47*FN + 1.6 [r 2 = 0.80, n = 132]. Growers and landscape professionals can use SN readings to predict FN levels in pansy, and thus rapidly and accurately diagnose the N status of their crop.

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Tyler C. Hoskins, James S. Owen Jr., Jeb S. Fields, James E. Altland, Zachary M. Easton, and Alex X. Niemiera

An understanding of how dissolved mineral nutrient ions (solutes) move through pine bark substrates during the application of irrigation water is vital to better understand nutrient transport and leaching from containerized crops during an irrigation event. However, current theories on solute transport processes in soilless systems are largely based on research in mineral soils and thus do not necessarily explain solute transport in soilless substrates. A study was conducted to characterize solute transport through a 9 pine bark:1 sand (by volume) substrate by developing and analyzing breakthrough curves (BTCs). Columns filled with pine bark substrate were subjected to the application of a nutrient solution (tracer) and deionized water under saturated and unsaturated conditions. Effluent drained from the columns during these applications was collected and analyzed to determine the effluent concentration (C) of the bulk ions in solution through electrical conductivity (EC) and nitrate (NO3 ), phosphate, and potassium (K+) concentrations. The BTCs were developed by plotting C relative to the concentration of the input solution (Co) (i.e., relative concentration = C/Co) as a function of the cumulative effluent volume. Solutes broke through the column earlier (i.e., with less cumulative effluent) and the transition from C/Co = 0 to 1 occurred more abruptly under unsaturated than saturated conditions. Movement of the anion, NO3 , through the substrate was observed to occur more quickly than the cation K+. Throughout the experiment, 37% of the applied K+ was retained by the pine bark. The adsorption of K+ to pine bark cation exchange sites displaced calcium (Ca2+) and magnesium (Mg2+), of which the combined equivalent charge accounted for 43.1% of the retained K+. These results demonstrate the relative ease that negatively charged fertilizer ions could move through a pine bark substrate while solution is actively flowing through substrate pores such as during irrigation events. This approach to evaluating solute transport may be used in horticultural research to better understand how mineral nutrients move through and subsequently leach from soilless substrates during irrigation. Expanding this knowledge base may lead to the refinement of production practices that improve nutrient and water use efficiency in container nurseries.

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Adam F. Newby, James E. Altland, Daniel K. Struve, Claudio C. Pasian, Peter P. Ling, Pablo S. Jourdan, J. Raymond Kessler, and Mark Carpenter

Greenhouse growers must use water more efficiently. One way to achieve this goal is to monitor substrate moisture content to decrease leaching. A systems approach to irrigation management would include knowledge of substrate matric potentials and air-filled pore space (AS) in addition to substrate moisture content. To study the relationship between substrate moisture and plant growth, annual vinca (Catharanthus roseus L.) was subject to a 2 × 2 factorial combination of two irrigation treatments and two substrates with differing moisture characteristic curves (MCCs). A gravimetric on-demand irrigation system was used to return substrate moisture content to matric potentials of −2 or −10 kPa at each irrigation via injected drippers inserted into each container. Moisture characteristic curves were used to determine gravimetric water content (GWC), volumetric water content (VWC), and AS at target substrate matric potential values for a potting mix consisting of sphagnum moss and perlite and a potting mix consisting of sphagnum moss, pine bark, perlite, and vermiculite. At each irrigation event, irrigation automatically shut off when the substrate-specific weight of the potted plants associated with the target matric potential was reached. Irrigation was triggered when the associated weight for a given treatment dropped 10% from the target weight. VWC and AS differed between substrates at similar matric potential values. Irrigating substrates to −2 kPa increased the irrigation volume applied, evapotranspiration, plant size, leaf area, shoot and root dry weight, and flower number per plant relative to irrigating to −10 kPa. Fafard 3B had less AS than Sunshine LB2 at target matric potential values. Plants grown in Fafard 3B had greater leaf area, shoot dry weight, and root dry weight. Leachate fraction ranged from 0.05 to 0.08 and was similar across all treatment combinations. Using data from an MCC in conjunction with gravimetric monitoring of the container–substrate–plant system allowed AS to be determined in real time based on the current weight of the substrate. Closely managing substrate matric potential and AS in addition to substrate water content can reduce irrigation and leachate volume while maintaining plant quality and reducing the environmental impacts of greenhouse crop production.