The pour-through procedure preferentially extracts substrate solution from the bottom of the container in conventional and stratified substrates

Authors: Altland, James; Owen Jr, James - Jim

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2023
Publication Date: 1/16/2024
Citation: Altland, J.E., Owen Jr, J.S. 2024. The pour-through procedure preferentially extracts substrate solution from the bottom of the container in conventional and stratified substrates. HortScience. 59(2):201–208. https://doi.org/10.21273/HORTSCI17425-23.
DOI: https://doi.org/10.21273/HORTSCI17425-23

Interpretive Summary: The pour-through procedure is widely used in commercial horticulture production and research to monitor the fertility of container-grown plants and to infer mineral nutrient availability. Substrate stratification occurs unintentionally in virtually all container systems. Following every irrigation event, opposing forces from gravitational potential gradient and capillary water retention create a zone of saturation called the perched water table at the container bottom with decreasing volumetric water content with increasing vertical container height. This volumetric water gradient affects chemical and biological properties including decomposition processes, mineral nutrient release from controlled release fertilizers, and gas emissions from the substrate. Due to the widespread use of the pour-through procedure, the objective of this research was to determine if the pour-through procedure is biased by preferentially sampling substrate solution near the bottom. Our results demonstrates conclusively that the pour-through procedure is more reflective of the lower half of the container than the upper half. The pour-through procedure still yields useful information, but that information should be considered in the context of how the sample was collected and that the results may not reflect conditions throughout the entire container.

Technical Abstract: Due to the widespread use of the pour-through extraction procedure in horticultural production and research, the objective of this study was to determine if the method is biased by preferentially extracting substrate solution near the bottom of the container in both conventionally filled containers as well as intentionally stratified containers. Eight treatments were created. The first four treatments were created by layering a conventional pine bark substrate (CONV) that was either amended (+A) or nonamended (-A) with fertilizer and lime with the following layers: amended substrate throughout the entire container profile (+A/+A); amended substrate in the top half (top 8.5 cm) over nonamended substrate in the bottom half of the container profile (+A/-A); nonamended substrate in the top half over amended substrate in the bottom half (-A/+A); and nonamended substrate throughout the profile (-A/-A). An additional four treatments were created by intentionally stratifying a fine pine bark substrate (FINE) over a coarse pine bark substrate (CRSE) with the same combinations of +A or -A. On 0 and 42 days after potting, substrate pH and EC were determined on samples collected by the pour-through procedure and 1:1 water extracts of the top and bottom layers in the container. At 42 days after potting, nutrient ions (NO3-, PO42-, K, Ca, Mg, and SO42-) were also measured in both pour-through and 1:1 water extracts of the top and bottom layers. At both dates and in both CONV and intentionally stratified containers, substrate pH and EC as determined by the pour-through procedure more closely reflected pH and EC the bottom half of the container as determined by the 1:1 water extract. At 42 days after potting, nutrient ions determined by the pour-through procedure were more highly correlated to the 1:1 water extracts from the bottom half of the container compared to the top half of the container in both CONV and stratified substrates. Evidence herein demonstrates that the pour-through procedure is more reflective of the lower half of the container than the upper half for both CONV and stratified substrates.