Water and air relations in propagation substrates

Authors: YAFUSO, ERIN - UNIVERSITY OF FLORIDA; FISHER, PAUL - UNIVERSITY OF FLORIDA; BOHORQUEZ, ANA - UNIVERSITY OF FLORIDA; Altland, James

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2019
Publication Date: 11/1/2019
Citation: Yafuso, E., Fisher, P., Bohorquez, A., Altland, J.E. 2019. Water and air relations in propagation substrates. HortScience. 54(11):2024–2030. https://doi.org/10.21273/HORTSCI14145-19.
DOI: https://doi.org/10.21273/HORTSCI14145-19

Interpretive Summary: During propagation of plant cuttings, high humidity and frequent mist irrigation are provided to hydrate unrooted cuttings; however, overwatering can potentially delay rooting and increase disease risk. The combination of short container height and fine substrate particles in propagation increases risk of inadequate gas exchange in the substrate. Because of the small container size, specialized materials, and the high moisture conditions in propagation, standard testing protocols need to be modified for quality control testing in propagation substrates. The objective of this study was to quantify and compare substrate water and air relations of three propagation substrates (peat, rockwool, and phenolic foam) that varied widely in physical characteristics by using four methods: 1) moisture retention curves (MRCs) by evaporation, 2) frozen column, 3) gravimetric analysis, and 4) CT analysis. In propagation cells, all substrates held high water content, and most pores were filled with water at container capacity, meaning that waterlogging would be possible with all substrates under poor control of mist irrigation. Peat had an even distribution of water as it dried from container capacity to 51% VWC. It would be useful to quantify water distribution in the three substrates under a range in moisture conditions. Peat had higher water-buffering capacity than rockwool or foam, which on the one hand would aid in cutting hydration because the substrate would dry slowly, but also increases the risk of over watering. Irrigation strategy based on substrate water potential is necessary to ensure adequate balance of water and air appropriate to the production phase.

Technical Abstract: 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.