Ways to Balance Air and Water in Plant Propagation Cells
A challenge during propagation is to balance an adequate supply of both water and air for rapid rooting. Cuttings need water for hydration, but also enough oxygen for roots to respire and grow. Using a substrate with adequate air porosity, and using the 1 to 5 moisture scale can help to prevent overwatering that leads to slow rooting and increased risk of disease. At the University of Florida, as part of the Clean WateR3 project, we are providing training materials and research on this topic.
Many growers use the 1 to 5 moisture scale (Figure 1 in the slideshow above) to decide when to water, and to communicate the meaning of wet and dry. On this scale, 1 represents bone dry and 5 is saturated. During initial callus formation, cells are usually kept around 4 to 5 to hydrate the cutting. Once root initials form, moisture levels are reduced to 3 to 4 for most plants, and during hardening off, substrate usually goes through wet and dry cycles between 2 and 4. You can find training videos in English and Spanish at our website (backpocketgrower.com) under Training and Irrigation.
How Effective is Oxygenating Irrigation Water?
The first part of Erin Yafuso’s Ph.D. research focused on whether it was effective to provide oxygen to the plant root zone during propagation by super-oxygenating irrigation water. Injecting oxygen into irrigation water increased dissolved oxygen (DO) to 30 ppm, which is three to four times the saturation level of DO.
Aeration with bubblers or fountains is known to be effective in pond management for algae and turbidity control. Injecting oxygen or air into a hydroponic nutrient solution also provides DO to roots that are submerged in water.
However, we found that oxygenation of water did not benefit mist-irrigated plants grown in a container substrate. One reason is that DO in water quickly off-gasses into the air when small water droplets are delivered through a mist emitter or boom.
Air Still the Best Delivery Method for Oxygen
The other finding from Yafuso’s Ph.D. research was that the best oxygen delivery system when growing plants in containers is via air, rather than the DO in water. There are pore spaces between substrate particles that can fill with water or air. If you allow the substrate to dry down even a little to moisture level 3, then oxygen in a propagation cell increases greatly (Figure 2 in the slideshow above). Aeration is further increased when careful water management is combined with a substrate that has large pore spaces.
We can see the water and air content in substrates by x-ray analysis using nano-computed tomography (CT). Figure 3 in the photo gallery shows an example of a CT scan with fine peat substrate in 55 milliliter (72-cell count) propagation cells. This image allows us to look inside a propagation cell at two stages of moisture (level 5 or level 3), and clearly shows the increased air-filled pores as the soil dries.
In this article, we emphasized the importance of using the 1 to 5 moisture scale to balance water and air in root substrate. Use a root substrate with adequate air porosity. Large pores will quickly become filled with air as the substrate dries, delivering oxygen to the roots.