“Oxygen-poor nutrient solution disperses in the substrate from the top, filling up the pores and pushing air out. The oxygen in substrate pores and root tissues will dissolve in the oxygen-poor nutrient solution, until oxygen equilibrium is reached, creating near hypoxic conditions in the root zone,” says the R&D team at Atami. How long that lasts depends on growth stage, substrate type, temperature, humidity and drainage. “In substrates with a higher air percentage, recovery can occur within hours, if the substrate remains aerated and overwatering is avoided. In low air percentage substrates, oxygen can stay depleted for days,” the team says.

The Dutch method measures air-filled porosity at 10 centimeters of pressure height, which compacts the material as part of the measurement. “I would put the minimum air percentage at 10, preferably at 15, for general root health. Below 10% will almost always get problems with waterlogging and anoxic spots in the substrate,” the team says. Above that floor, the number becomes a steering decision. “The higher the air percentage, the easier drain will be and less water buffer you have, this will impact the way the crop is steered with irrigation,” the team explains. Whether the requirement moves between veg and flower, the answer is root colonization, when air percentage and oxygen levels need to be at their highest.

Going with the flow
Oxygen carried in the nutrient solution contributes only around 5% of what the root gets from a well managed substrate. Aerating a tank shortens the initial shock of an irrigation event, since root tissue reaches equilibrium with the water sooner, and roots and microbes consume the extra quickly. “Aerating a tank will not help with a structural problem with a substrate. The primary oxygen source for roots remains the substrate air, not the water,” the team points out.
Growers have answered the problem by irrigating in smaller and more frequent shots, and the team puts a ceiling on that. Holding a substrate exactly balanced against plant uptake is theoretically possible, particularly in vegetative growth, and it needs highly accurate moisture sensors and extremely precise irrigation equipment, potentially measuring individual plants. “In general, the more precise the grower wants to be, the more precise the system and equipment need to be,” the team says. “For generative phases larger dry backs are preferred, therefore smaller and more frequent shots can counteract generative signals. The extra investment and effort might be partly wasted,” the team adds.

Porosity
Starting figures depend on ingredients, their quality and their particle sizes, so a peat mix with 15% perlite reads differently from one with 25, and coarser coco raises air percentage while lowering water buffer. “A standard peat-based mix with 10% perlite and more than 50% peat will have an air percentage between 10 and 15. A standard washed and buffered coco with a particle distribution favoring smaller particles will have an air percentage between 20 and 25,” the team says.

Around half the air porosity is lost after four to five weeks, depending on the substrate. Settling and compaction is the first and main cause, since material fresh out of the bag sits much higher. Root growth comes second. “Roots take up pore space, but more importantly, roots secrete mucilage or exudates, which create dense regions. These dense regions block air and water movement,” the team says. Microbes are third, eating organic matter and producing biofilm that clogs pores. Fourth is bad water management, where frequent irrigation without enough drain creates permanent waterlogging. Fifth is degradation, through microbes breaking down organic matter and the structure of peat, or through ingredients degrading naturally, such as certain binding agents in stone wool. Last is buildup of salt or organic material, which clogs or coats pores. “A substrate is not simply something that holds the plant upright, it’s the most important support structure the plant has throughout its life, balancing water, nutrients and air,” the team says.

Reading the drain
Drain timing is the first signal. An irrigation event that should produce drain but only starts draining 30 minutes later indicates a waterlogged substrate, and drain pH is the second check. “If the pH is under 5.5 when nutrient solution is between 5.8 and 6.2, this means you might have anaerobic bacterial activity. If the drain smells foul, like rotten eggs, is a heavy indicator,” the team says. In other words, a substrate with good dry backs is much less likely to have oxygenation problems at all.

Temperature moves the same variable three ways, since warm water holds less oxygen, roots and microbes deplete it faster when active, and it diffuses more slowly through warmer substrate. “Keeping substrate temperature between 18 and 24 degrees Celsius is the best balance between root activity and oxygen levels. Substrate temperatures of 30 degrees Celsius or even higher can trigger anoxic events in low air percentage substrates,” the team says.

Container shape decides where the trouble appears. “Tall and narrow containers have better vertical drainage and have less compaction in the top and middle part of the container, where the roots are most active,” the team says, with the trade being impeded diffusion through closed sides, anoxic pockets at the base in low porosity substrates, and a temperature difference between bottom and top. Short wide containers get better diffusion from all sides and more uniform temperature, and they “tend to dry out faster on top, because of relatively large surface ratio,” along with degrading faster as that surface wets and dries.
None of it is what growers usually call about. “Overwatering, often after transplantation. Growers think the seedling or clone is fragile and coddle the young plant. This is not correct and will create weaker plants. Let the young roots search for water and nutrients,” the team says. Depending on cultivar, substrate size and conditions, that can mean no irrigation for the first five to ten days, until a 40 to 50% dry back, since a small plant grows slowly, carries few leaves and transpires little, and a drier substrate before full root colonization lowers infection risk. “Do not spoil young plants,” the team says.

“There isn’t one ideal substrate for every cultivation system, because irrigation frequency, container geometry, container size and environmental conditions all influence what the root zone needs,” the team says. Atami’s Cocos Substrate carries water-holding and water-distribution characteristics with an airy porous structure and a starting nutrition, High Porosity Cocos adds 30% perlite for more aeration and drainage, and Washed and Buffered Cocos comes in at a very low initial EC for growers wanting control of nutrient ratios from the start. “The physical properties of the substrate and the irrigation strategy must work together, you cannot optimize one without considering the other,” the team says.