@article{fields_fonteno_jackson_heitman_owen_2020, title={The Use of Dewpoint Hygrometry to Measure Low Water Potentials in Soilless Substrate Components and Composites}, volume={10}, ISSN={["2073-4395"]}, DOI={10.3390/agronomy10091393}, abstractNote={Plant water availability in soilless substrates is an important management consideration to maximize water efficiency for containerized crops. Changes in the characteristics (i.e., shrink) of these substrates at low water potential (<−1.0 MPa) when using a conventional pressure plate-base can reduce hydraulic connectivity between the plate and the substrate sample resulting in inaccurate measures of water retention. Soilless substrate components Sphagnum peatmoss, coconut coir, aged pine bark, shredded pine wood, pine wood chips, and two substrate composites were tested to determine the range of volumetric water content (VWC) of surface-bound water at water potentials between −1.0 to −2.0 MPa. Substrate water potentials were measured utilizing dewpoint hygrometry. The VWC for all components or composites was between 5% and 14%. These results were considerably lower compared to previous research (25% to 35% VWC) utilizing conventional pressure plate extraction techniques. This suggests that pressure plate measurements may overestimate this surface-bound water which is generally considered unavailable for plant uptake. This would result in underestimating available water by as much as 50%.}, number={9}, journal={AGRONOMY-BASEL}, author={Fields, Jeb S. and Fonteno, William C. and Jackson, Brian E. and Heitman, Joshua L. and Owen, James S., Jr.}, year={2020}, month={Sep} }
 @article{altland_owen_jackson_fields_2018, title={Physical and Hydraulic Properties of Commercial Pine-bark Substrate Products Used in Production of Containerized Crops}, volume={53}, ISSN={["2327-9834"]}, DOI={10.21273/HORTSCI13497-18}, abstractNote={Pine bark is the primary constituent of nursery container media (i.e., soilless substrate) in the eastern United States. Pine bark physical and hydraulic properties vary depending on the supplier due to source (e.g., lumber mill type) or methods of additional processing or aging. Pine bark can be processed via hammer milling or grinding before or after being aged from ≤1 month (fresh) to ≥6 month (aged). Additionally, bark is commonly amended with sand to alter physical properties and increase bulk density (Db). Information is limited on physical or hydraulic differences of bark between varying sources or the effect of sand amendments. Pine bark physical and hydraulic properties from six commercial sources were compared as a function of age and amendment with sand. Aging bark, alone, had little effect on total porosity (TP), which remained at ≈80.5% (by volume). However, aging pine bark from ≤1 to ≥6 months shifted particle size from the coarse (>2 mm) to fine fraction (<0.5 mm), which increased container capacity (CC) 21.4% and decreased air space (AS) by 17.2% (by volume) regardless of source. The addition of sand to the substrate had a similar effect on particle size distribution to that of aging, increasing CC and Db while decreasing AS. Total porosity decreased with the addition of sand. The magnitude of change in TP, AS, CC, and Db from a nonamended pine bark substrate was greater with fine vs. coarse sand and varied by bark source. When comparing hydrological properties across three pine bark sources, readily available water content was unaffected; however, moisture characteristic curves (MCC) differed due to particle size distribution affecting the residual water content and subsequent shift from gravitational to either capillary or hygroscopic water. Similarly, hydraulic conductivity (i.e., ability to transfer water within the container) decreased with increasing particle size.}, number={12}, journal={HORTSCIENCE}, author={Altland, James E. and Owen, James S., Jr. and Jackson, Brian E. and Fields, Jeb S.}, year={2018}, month={Dec}, pages={1883–1890} }
 @inproceedings{fields_fonteno_jackson_2014, title={Plant available and unavailable water in greenhouse substrates: Assessment and considerations}, volume={1034}, DOI={10.17660/actahortic.2014.1034.42}, abstractNote={Accurate assessment of available water in substrates usually includes a measurement of water unavailable to plants. Plant roots have an ability to pull suctions up to 1.0 to 2.0 MPa, depending on species, with the classic value for unavailability measured at 1.5 MPa. Five samples each of peat moss, pine bark and perlite and a clay soil were placed in a 1.5 MPa porous plate system for 48 hours. The samples were then removed and run in a dewpoint potentiometer then dried for 24 hours at 105°C. The mineral soil potentials averaged 1.39 MPa, but the others were much smaller: peat = 0.38, bark = 0.21 and perlite = 0.28 MPa. Peat and bark were re-tested at 0.3 MPa on the porous plate system then placed in the potentiometer. The peat water potential was 0.33 MPa while the bark was 0.34 MPa, showing good agreement with the porous plate pressures. The samples of highly porous materials of peat, bark and perlite possibly seemed to lose hydraulic continuity between the samples and porous plate above 0.3 MPa of pressure which stopped the flow of water from the samples. This resulted in artificially high values. In a second study, substrate samples (3 peat: 1 perlite: 1 vermiculite, v/v/v) were taken from mature marigold plants in three stages of wilt: Stage 1: light wilt (initial leaf flagging), Stage 2: moderate wilt (leaves wilted to ~ 45°of vertical) and Stage 3: heavy wilt (leaves wilted and curled to main stem). Water potentials were measured at each stage using the potentiometer. After substrate sampling, each plant was re-watered and level of recovery was noted. Plants at Stage 1 wilt had soil potentials of ~ 0.6 MPa. Stage 2 wilt was at ~ 1.55 MPa and Stage 3 wilt was ~ 2.2 MPa. All plants visually recovered from wilt at all stages. The potentiometer may be useful in determining actual soil water potentials under dry conditions, not normally measurable using the traditional porous plate system. Unavailable water content for horticultural substrates may be overly high using the porous plate system as confirmed with the dew point potentiometer. Measuring water potentials during plant wilt may help to refine the nature of permanent wilt and more precisely determine water is truly unavailable to plants. INTRODUCTION The term available water capacity, first defined by Veihmeyer and Hendrickson (1927), describes water held in a soil between field capacity (or container capacity in horticultural substrates) and the permanent wilting point (PWP). Permanent wilt describes the condition where a plant has reached a low enough water potential that there can be no recovery (Taiz and Zeiger, 1996). Richards and Wadleigh (1952) found that the PWP for most agricultural crops is between -1.0 and -2.0 MPa, with the convention of -1.5 MPa to be PWP. Plants do not generally reach permanent wilt at the instant they reach this potential, but instead gradually reduce transpiration until available water is lost. Denmead and Shaw (1962) showed that many plants start to reduce transpiration rate at as low as 0.2 MPa. In order to determine available water content, container capacity and unavailable water must be measured. To measure UW, Bouyoucos (1929) described an apparatus which produces a suction equal to -1.5 MPa which draws upon a soil sample. This idea was refined by Richards and Fireman (1943) who applied 1.5 MPa of pressure, and Proc. IS on Growing Media & Soilless Cultivation Eds.: C. Blok et al. Acta Hort. 1034, ISHS 2014 342 employed the use of porous plates which soil samples are placed upon to allow water to be moved out of the samples until equilibrium is reached with the 1.5 MPa pressure that has been applied. A modified version of Richards and Fireman’s pressure plates is currently the most common method of measuring UW, along with the plant-based method using sunflower (Cassel and Nielsen, 1986). The sunflower method was first proposed by Furr and Reeve (1945) and involves growing sunflower seedlings and allowing them to wilt until PWP is reached, and measuring soil water content. The sunflower method can take long periods of time and due to the noninstantaneous wilt of plants, this method can lack in accuracy. However, inaccuracies have been reported with the use of pressure plates at tensions as high as -1.5 MPa (Stevenson, 1982; Fonteno and Bilderback, 1993; Gee et al., 2002). A possible explanation for the inaccuracies with pressure plates is the loss of hydraulic connectivity, or the lack of an unbroken water column throughout the sample. If the water column between the plate and the length of the sample is broken, pressure will be applied to either end of the sample, and thus result in no net flow of water. Recent research by Curtis and Claassen (2008) has shown the effectiveness of using dewpoint potentiometry to measure the water potential of inorganic amendments with higher precision. The objectives of this research were: 1) to determine the potentials reached when -1.5 MPa are applied to organic greenhouse substrate components, and 2) to determine soil water potentials of plants grown in container substrates during the wilting process. MATERIALS AND METHODS This experiment required the use of pressure plate extractors (PPE; Soilmoisture Equipment Corp.; Santa Barbara, CA) and a WP4C Dewpoint Potentiometer (Decagon; Pullman, WA). Traditional horticultural substrate components including, sphagnum peat moss (Premier Tech, Canada), aged pine bark, and perlite, were tested along with a clay mineral soil classified as Gerogiaville. Five rubber rings were placed on each moistened 1.5 MPa pressure plate, and each ring was filled with one of the materials being tested. In total 20 total samples were tested (4 materials, 5 replications). The samples were saturated for 24 h, and placed in PPEs. Flat circular lead weighs were placed on top of each sample, in order apply a slight downward force to ensure connectivity between the plate and the sample. Nitrogen gas (N2) was then slowly passed into the PPEs until the PPEs were pressurized to 1.5 MPa. The pressure in the PPEs was maintained for 48 h. The samples were then removed, sealed and measured in the WP4C dewpoint potentiometer. The dewpoint potentiometer uses a chilled-mirror dewpoint technique. Relative humidity is measured until equilibrium is attained between the air in the chamber and the sample. Water potentials were determined using repeated measures until successive readings were equal. Testing the samples from the pressure plate allowed the measurement of water potential rendered after pressures of 1.5 MPa. Experiment Two – Plant Wilt Plastic containers of 7.6 cm diameter and 7.6 cm height were filled with a substrate consisting of a mixture of peat: vermiculite: perlite (3:1:1, v/v/v) at a bulk density of 0.13 g/cm3 to ensure uniformity. Marigold (Tagetes erecta L.) seeds were sewn directly into the containers, placed into the greenhouse and irrigated as needed. Fertilization was with 200 ppm N (total) in liquid feed once every 2 to 4 days. Once these plants were mature and flowering the rooting environment (after approximately 8 weeks), each container was saturated, allowed to drain and to begin the wilting process. The plants were observed for wilting until the plant reached one of the three stages of visible wilt (Fig. 1): Stage 1 – initial flagging; Stage 2 – leaves wilted with stems drooping to an angle of 45°; and Stage 3 – all leaves completely wilted. Once at the appropriate wilt stage, plants were photographed and removed from the container. A soil sample approximately 2 cm wide was removed from the top to the bottom of the substrate. Any visible roots were removed and a portion of the sample was placed}, booktitle={International symposium on growing media and soilless cultivation}, author={Fields, J. S. and Fonteno, W. C. and Jackson, B. E.}, year={2014}, pages={341–346} }