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Title: Solute transport through a pine-bark based substrate under saturated and unsaturated conditions

Author
item HOSKINS, TYLER - Virginia Polytechnic Institution & State University
item OWENS, JAMES - Virginia Polytechnic Institution & State University
item FIELDS, JEB - Virginia Polytechnic Institution & State University
item Altland, James
item EASTON, ZACHARY - Virginia Polytechnic Institution & State University
item NIEMIERA, ALEX - Virginia Polytechnic Institution & State University

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2014
Publication Date: 11/1/2014
Citation: Hoskins, T., Owens, J., Fields, J., Altland, J.E., Easton, Z., Niemiera, A. 2014. Solute transport through a pine-bark based substrate under saturated and unsaturated conditions. Journal of the American Society for Horticultural Science. 139:634-641.

Interpretive Summary: Fertilizer nutrients as solutes are dissolved into, and move with, the flow of water through container substrates. Understanding the rate and nature of solute movement through substrates will improve our understanding of how fertilizers are dispersed and leached under typical irrigation events in production of container plants, and will ultimately improve fertilizer application technology. An experiment was conducted to document the speed at which cation and anion solutes move through a pine bark substrate. The behavior of individual ion species, during active solute transport through a pine-bark substrate, differed with the ion species (cation or anion) and the method of analysis (saturated or unsaturated pores). Anions were passed through the substrate relatively unobstructed, where cations were retained by the pine-bark to an extent that constituted a surprisingly small portion of the substrates measured Cation Exchange Chromatography (CEC). The approach employed in this study may be used in horticultural research to better understand how nutrients move through and leach from soilless substrates during irrigation. Expanding this knowledge base may lead to the refinement of crop models and production practices that improve nutrient and water use efficiency in container nurseries.

Technical Abstract: 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 solutes transport through pine-bark substrate by developing and analyzing breakthrough curves. Columns filled with pine-bark substrate were subjected to application of a nutrient solution (tracer) and deionized (DI) 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 via electrical conductivity (EC) and nitrate (NO3-), phosphate (PO43-) 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 occurred more abruptly (a quicker transition from C/Co = 0 to 1) under unsaturated than saturated conditions. Movement of the anions, NO3- and PO43-, through the substrate occurred 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. 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.