Oxygenation of Nutrient Reservoirs in Substrate-Based Soilless Crops
When growers new to hydroponics start working with soilless systems, one of the first questions that comes up is whether they should be actively aerating their nutrient solutions. Air stones bubbling away in reservoirs have become synonymous with hydroponics, particularly in deep water culture systems. However, when growing in substrates like coconut coir or rockwool, the situation is fundamentally different. Understanding where root oxygen comes from in substrate systems can help you avoid wasting resources on unnecessary equipment while also helping you understand the real limitations of these growing methods.
Where Roots Get Oxygen in Substrate Systems
In substrate-based growing systems, roots obtain nearly all their oxygen from air-filled pores within the growing medium, not from dissolved oxygen in the nutrient solution. Substrates like rockwool and coconut coir typically have total porosities exceeding 80%, compared to typical soil porosities below 40% (1). This high porosity ensures there are enough water-filled pores for nutrient transport as well as enough air-filled pores for oxygen transport.
The key parameter governing oxygen availability in substrates is air-filled porosity, which represents the percentage of air contained in a fixed volume of substrate after it has been saturated with water and the free water has drained (2). Research on growing media has shown that adequate air-filled porosity levels for optimal plant growth typically range from 10-20%, with some studies suggesting that values above 20% may be necessary immediately after irrigation to prevent hypoxia (3).
When you irrigate a substrate, the nutrient solution displaces air in the open pores. As the substrate drains, air is drawn back down into the root system. This cycle of wetting and drying is what supplies roots with fresh oxygen. The oxygen diffusion coefficient in air is approximately 10,000 times higher than in water, which means that gas-phase oxygen transport through substrate pores is far more efficient than dissolved oxygen transport through water (4).
| Substrate Type | Total Porosity (%) | Air-Filled Porosity at Field Capacity (%) | Water Holding Capacity (%) |
| Rockwool | 95-97 | 15-20 | 75-80 |
| Coconut Coir | 85-90 | 20-30 | 60-70 |
| Coco/Perlite (70:30) | 85-90 | 25-35 | 55-65 |
| Perlite | 50-70 | 30-40 | 30-40 |
Does Nutrient Solution Oxygenation Make Sense?
The short answer is that in properly managed substrate systems with adequate irrigation frequency, oxygenating the nutrient solution in your reservoir provides minimal benefit to plant growth. The reason is simple: the overwhelming majority of oxygen uptake occurs through gas-phase diffusion in the air-filled pores of the substrate, not through dissolved oxygen in the water phase.
Research comparing water-based and substrate-based cultivation systems has demonstrated that substrate-grown plants can thrive even when oxygen supply through irrigation is potentially growth limiting, as long as the substrate maintains adequate air-filled porosity (1). In contrast, water culture systems where roots are continuously submerged rely entirely on dissolved oxygen, making aeration critical in those applications.
The irrigation strategy you use has far more impact on root zone oxygen than dissolved oxygen levels in your reservoir. Allowing substrates to dry down between irrigations increases air-filled porosity and draws fresh air into the root zone. Over-irrigation is far more likely to cause oxygen deficiency problems than low dissolved oxygen in your nutrient tank. When substrates remain saturated, air-filled pores fill with water, creating anaerobic conditions regardless of how much you aerate your reservoir.
The exception to this general rule would be in situations where you have continuous or very frequent irrigation with minimal drainage, essentially converting your substrate system into something closer to a water culture system. In such cases, dissolved oxygen becomes more important, but this represents poor management of a substrate system rather than a reason to add aeration.
The Pathogen Risk of Solution Aeration
While aerating nutrient solutions might seem harmless even if unnecessary, there is a significant downside that growers should consider: the increased risk of introducing and spreading waterborne pathogens, particularly species of Pythium and Phytophthora.
These oomycete pathogens are among the most problematic diseases in hydroponic systems. They produce motile zoospores that can swim through nutrient solutions using flagella, allowing them to spread rapidly through recirculating systems (5). When closed hydroponic systems are used, pathogens can enter and then rapidly disseminate, particularly during periods of stress such as high temperatures or low dissolved oxygen levels (5).
Aeration systems create several opportunities for pathogen introduction and proliferation. Air stones and diffusers provide surfaces for biofilm formation where pathogens can colonize. The turbulence created by aeration helps distribute any pathogens present throughout the solution more effectively than they would spread by passive diffusion. The air being pumped into the system can carry airborne pathogen propagules, and unless you are using sterile filtration on your air intake, you are essentially inoculating your reservoir with whatever microorganisms happen to be in your growing environment.
Low dissolved oxygen has been reported to increase Pythium infection in hydroponic systems (6). However, in substrate systems where roots obtain oxygen primarily from air-filled porosity rather than dissolved oxygen, the relationship between solution aeration and disease suppression becomes less clear. The more relevant factors for disease prevention in substrate systems include maintaining proper irrigation frequency to ensure adequate substrate aeration, avoiding prolonged saturation, and keeping solution temperatures below 24°C where practical.
| Pathogen Risk Factor | Risk Level with Aeration | Risk Level without Aeration |
| Airborne contamination introduction | High | Low |
| Pathogen distribution through solution | High (turbulent mixing) | Moderate (passive diffusion) |
| Biofilm formation sites | High (air stones, tubing) | Low (tank surfaces only) |
| Solution temperature increase | Possible (pump heat) | Minimal |
Practical Recommendations
For growers using substrate-based systems, the evidence suggests that resources are better spent on proper irrigation management than on solution aeration. Focus on selecting substrates with adequate air-filled porosity, implementing irrigation schedules that allow periodic drying to refresh the air in the root zone, and maintaining appropriate solution temperatures.
If you are growing in pure water culture systems like deep water culture, NFT, or aeroponics, then maintaining adequate dissolved oxygen becomes critical and aeration or other oxygenation methods are necessary. But if you are growing in rockwool, coco coir, or similar substrates with good drainage, your plants are getting their oxygen from the air in the substrate pores, not from the water in your reservoir.
The key takeaway is this: in substrate systems, oxygen management happens at the substrate level through proper irrigation practices, not at the reservoir level through aeration. Understanding this fundamental difference can help you avoid unnecessary equipment costs while potentially reducing your risk of introducing waterborne pathogens into your growing system.
