OFA Bulletin July/August 2012 : Page 2

OFA Mission Statement To support and advance professional horticulture. OFA – The Association of Horticulture Professionals Grower Quality Control in Growing Media: Porosity for Plug and Liner Mixes By Jinsheng Huang, Paul R. Fisher, and Bill R. Argo 2130 Stella Court Columbus, Ohio 43215-1033 USA 614-487-1117 Fax: 614-487-1216 ofa@ofa.org www.ofa.org OFA Bulletin July/August 2012 NUMBER 934 Editor’s Note: This is the second in a series of articles about substrate quality control procedures for young plant production. Editorial Staff Stephen A. Carver, Ph.D. Scott Leyshon Laura Kunkle Editor Introduction A great deal of the art and science of growing plugs and liners is in managing the balance between water and air in propagation trays. Aeration is essential for healthy root growth. Mild water stress leads to toned and compact growth with increased root-to-shoot ratio, whereas overwatering encourages soft growth, poor rooting, and disease. While much of the control of the air/water ratio is in the hands of the grower, the porosity of the substrate (how much of cell volume is made up of water, air, or solid at saturation) also plays a large role. In this article, we discuss factors that influence porosity, and how to measure and interpret substrate porosity onsite in plug trays. Contributors Bill R. Argo Bridget K. Behe Jennifer Boldt Raymond A. Cloyd Margery Daughtrey John Erwin Paul R. Fisher Joy Gendusa Ken Harr Jinsheng Huang Heiner Lieth Carol Reese Al Zylstra What is Substrate Porosity? A cell is filled with solid particles, such as peat and perlite, as well as the holes (“pore spaces”) between and within these particles. Whether those pores are filled with air or water is determined by several factors: • The size of the pores. Larger pores tend to be filled with air, whereas small pores fill with water. • The height of the container. The combination of gravity and capillary action result in a saturated zone at the bottom of the container where pores are filled with water, with little air space. As you increase the height of the container, some of the water will drain from the pores, increasing the air-filled porosity. • Substrate particle size. Small particles of peat, perlite, and other components lead to small pores and, therefore, more water and less air. Substrates used in plug or propagation trays tend to have low air-filled porosity compared to larger pots because short containers filled with fine substrates maximize water-holding capacity and minimize aeration. Management practices can also alter the air/water ratio: • The amount of water a “dry” plug grower applies can mean the substrate rarely reaches saturation and many pores remain air-filled. With other crop types and stages (for example, mist propagation of unrooted cuttings for the first two to five days) the substrate may remain near saturation for an extended period. • Damaging the substrate with mixing and filling equipment can decrease particle size and aeration. • Compaction in the tray by overfilling or forceful irrigation reduces air porosity. The container capacity (or water holding capacity) is the total amount of water present in the container after the substrate has been saturated and allowed to drain. The percent of container volume composed of pore space is referred to as total porosity . The percent volume filled with air is called air porosity , and the percent volume filled with water is called water holding porosity . The percent of volume taken up by solid ( percent solid ) can be calculated by subtraction (equal to 100 minus the percent of total porosity). Another way to measure the amount of solid in a tray is by drying the substrate down and measuring dry bulk density (weight of dried substrate in grams per liter or tray). OF A Bulletin Published Bimonthly Copyright © OFA 2012. Permission is hereby given to reprint articles appearing in this OFA Bulletin provided the following reference statement appears with the reprinted article: “Reprinted from the OFA Bulletin, (phone: 614-487-1117) July/August 2012, Number 934.” No endorsement is intended for products mentioned in this OFA Bulletin, nor is criticism meant for products not mentioned. The authors and OFA assume no liability resulting from the use of practices printed in this OFA Bulletin. 2

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