OFA Bulletin July/August 2012 : Page 3

July/August 2012 • Number 934 Why is Porosity Important? Substrate with more air and less water per cell can be an advantage for slow-rooting crops that require a large quantity of mist, or a long duration under mist, because air is always available for root growth. For example, slow-rooting woody cuttings are often grown in nursery substrates with around 50 percent perlite. However, if a propagation substrate has too much air, often associated with a very coarse mix, the resulting root ball falls apart too rapidly at transplant. If the substrate does not hold enough water, it will rapidly dry out making it more difficult for your customers to hold on a bench before transplant. Conversely, a “waterlogged” substrate with less air and more water will often result in slow rooting, elevated levels of root diseases, “water roots” with few root hairs, and a low root:shoot ratio. A mix that is hard to dry down can result in soft and leggy growth. Measuring porosity is not complicated – it just requires some patience and care, especially in plug trays. A step-by-step protocol for either loose-filled substrates (Figure 1, page 4) or stabilized substrates (Figure 2, page 5) are provided here. These protocols are suitable for growers to test their growing substrates onsite. It is difficult to get accurate measurements on trays with more than 128 cells. We suggest that growers consider testing substrate porosity with each major shipment of substrate, or when mechanical adjustments are made to flat filling lines. At a minimum, we suggest irrigating trays thoroughly, allowing them to drain, and weighing the tray. If that saturated weight (which is mostly made up of water) increases, it may have resulted because the batch of media has finer particles and smaller pores. To help prepare interpretive guidelines for propagation substrates, we evaluated 15 plug and 22 liner propagation or potted plant substrates, and 18 stabilized substrates from growers and media companies in the Floriculture Research Alliance at the University of Florida. For each loose-filled Testing & Interpreting Substrate Porosity substrate, small 16-cell cut sections of Blackmore ® 128 trays were used. Trays were first top-filled to the surface, dropped down to the table 10 times from 6 inches high. Additional substrate was added to uniformly cover the top surface. These plug trays were then tested for porosity by sub-irrigation (Figure 1, page 4). Since the calculation of porosity is very dependent on the overall volume of the cell, great care was taken to measure cell volume as described in Figure 1 and Figure 2 on page 4 & 5– do not rush this stage of testing! Substrate samples were brought to full saturation by sub-irrigation, by placing the trays into a basin of water that wet the substrate from underneath by gradual elevation of the water level and immersion until full saturation, with the water level about 1 cm below the top of the substrate. The average dry bulk density for plug substrates or liner/ potting substrates was similar, at 108 g/L for plugs and 104 g/L for liners (Table 1), respectively. Stabilized media had more variability (the range from lightest to heaviest substrate) in dry bulk density than loose media (Table 1). For example, Oasis had very low dry bulk density (18 g/L). Compare the paper-wrapped pots in Table 1, which were filled with liner/potting media, against the loose-filled liner/ potting media. Dry bulk density was 32 percent higher on average in paper-wrapped pots compared with loose-filled cells. This indicates compaction in the paper-wrapped pots. Paper-wrapped pots, therefore, tend to have less air (4.6 percent) than the equivalent loose-filled substrates (6.9 percent). However, with air space around the paper-wrapped pot in the tray, the paper-wrapped pot tends to dry more quickly. Do not set the vacuum on an Ellepot machine any higher than needed to create a liner that holds together – otherwise you will lose air porosity and use more substrate, which increases cost. There was no difference between average water holding capacity between plug substrates and liner/potting substrates with a mean 19 to 20 mL per cell (i.e. 760 to 800 mL/L), and range from 14 to 21 mL/cell (Tables 1). Paper-wrapped pots (Ellepot® and Fertiss®) had slightly lower water holding Table 1. The average cell volume, dry bulk density, water holding capacity and porosity for several types of propagation substrate. Loose-filled substrates included 15 plug and 22 liner/ potting substrates in a 128 count Blackmore® plug tray at typial ompaction. Paper-wrapped pots (25-mm in diameter), including 8 examples of Ellepot® and Fertiss® liners with a comparable cell size to the loose-filled trays. Other stabilized substrates included 4 Preforma® pots, and one Oasis® form in a 10-count strip. Average Cell volume (mL) Mean 25 25 19 18 19 108 104 137 92 18 Dry bulk density (g/L) Range 82 to 141 73 to 157 96 to 166 89 to 95 Water holding capacity (mL/cell) Mean 19.7 19.3 15.2 7.5 16.7 Range 14.4 to 21.5 14.8 to 21.3 13.4 to 17.6 6.6 to 8.5 % Air by volume at container capacity Mean 6.8% 6.9% 4.6% 2.7% 3.3% Range % Water by volume at container capacity Mean Range Substrate Loose-filled plug seedling substrates Loose-filled liner/ potting substrates Paper-wrapped pots Preforma® (peat/polymer) Oasis® foam 5.3% to 9.0% 78.9% 57.5% to 86.1% 4.8% to 9.7% 77.1% 59.3% to 85.2% 3.3% to 5.9% 78.9% 69.5% to 91.4% 1.9% to 3.1% 42.2% 37.1% to 47.5% 86.9% Continued on page 4 OF A Bulletin 3

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