The use of dewpoint hygrometry to measure low water potentials in soilless substrate components and composites

Authors: FIELDS, J - Louisiana State University; FONTENO, W - North Carolina State University; JACKSON, B - North Carolina State University; HEITMAN, J - North Carolina State University; Owen Jr, James - Jim

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2020
Publication Date: 9/15/2020
Publication URL: https://handle.nal.usda.gov/10113/7115890
Citation: Fields, J.S., Fonteno, W.C., Jackson, B.E., Heitman, J.L., Owen Jr, J.S. 2020. The use of dewpoint hygrometry to measure low water potentials in soilless substrate components and composites. Agronomy. 10(9):Article 1393. https://doi.org/10.3390/agronomy10091393.
DOI: https://doi.org/10.3390/agronomy10091393

Interpretive Summary: Understanding the amount of water available to a crop is essential for the producer to make sound water management decisions; especially for constrained container systems used in controlled environment agriculture (CEA). Scientists separate water in a crop production system, soil or growing media, into two categories: (1) available water that is readily used by the crop and (2) unavailable water that is tightly bound to surfaces and unable to be used by the crop. These measures are traditionally accomplished in the laboratory by using a pressure outflow apparatus in which growing media samples are set on a ceramic plate and water is pushed from the media using increasing levels of positive pressure. Soilless substrates are coarse, light, and typically comprised of organic materials allowing them to lift away from the plate during analysis; thus, halting the continuum of water loss across pressures that represent available and unavailable water and resulting in an erroneous determination. An alternate method, a dewpoint potentiometer, uses hygrometery to measure unavailable water. The hygrometry method has been reported to provide more precise measurements of unavailable water since the method requires neither a pressure apparatus nor a ceramic plate. In our research, we demonstrated the use of dewpoint hygrometry allowed for much lower estimates of unavailable water in Sphagnum peatmoss, coconut coir, aged pine bark, shredded pine wood, pine wood chips, and two growing media blends used in CEA. This finding would indicate that there may be up to 50% more crop available water than the historic convention, requiring a possible update to current guidance informing best management practices when producing CEA crops in soilless growing media.

Technical Abstract: 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%.