In high-tech greenhouse tomato and cucumber production, we carefully monitor EC, pH, radiation sums and irrigation strategy. However, one parameter that is often overlooked — yet can significantly influence plant performance — is dissolved oxygen (DO) in the nutrient solution.

Roots require oxygen to respire. Without adequate oxygen, even a perfectly balanced nutrient recipe will not perform as expected. When dissolved oxygen levels are low, roots cannot efficiently absorb water and nutrients. The result is often subtle at first: slightly dull roots, reduced vigour, inconsistent calcium uptake, or plants that shift vegetative when they should be generative. As stress increases, root tips brown, nutrient uptake becomes erratic, and the crop becomes more susceptible to pathogens such as Pythium.

This issue becomes particularly relevant during warm summer periods. The physical reality is simple: warm water holds less oxygen. At 20°C, water can hold significantly more oxygen than at 24°C. Yet summer conditions often push nutrient solution temperatures above 22°C — exactly when plant demand for oxygen is highest due to increased radiation and transpiration. In recirculating systems, this effect can be amplified if return water carries organic matter that consumes oxygen within the storage tank. (As well the return tanks generally smaller, will warm up faster)

As a guideline, dissolved oxygen levels above 6 mg/L are acceptable, while 7–9 mg/L is considered ideal. Once levels fall below 5 mg/L, root stress becomes increasingly likely. Many growers are surprised to discover how variable DO levels can be between the main tank and the slab drain return.

Measurement is straightforward using a portable dissolved oxygen meter. Companies such as Hanna Instruments and YSI produce reliable handheld meters suitable for greenhouse use. Optical probes are generally more stable and require less maintenance than older membrane-style sensors. Measuring both tank solution and return drain water provides valuable insight into system performance.

If dissolved oxygen levels are low, the first focus should be solution temperature. Maintaining nutrient solution between 18–20°C provides a strong foundation for oxygen availability. Insulating tanks, managing heat exchange systems effectively, and avoiding excessive daytime warming are practical starting points.

Increasing turbulence in the tank can also improve oxygen transfer. Simple adjustments such as allowing return lines to splash into the tank rather than discharging below the surface can make a measurable difference. Venturi injectors or air diffusers can further enhance aeration, particularly in larger recirculating systems.

Some growers consider hydrogen peroxide-based products to lift oxygen levels. While these products temporarily release oxygen as they break down, they should be viewed primarily as system hygiene tools rather than true oxygen management strategies. Long-term oxygen stability comes from temperature control and aeration, not chemical intervention.

In modern greenhouse production, where margins are tight and plant performance must be precise, dissolved oxygen should not be ignored. When roots are healthy and well oxygenated, nutrient uptake is more consistent, plants remain balanced, and disease pressure is reduced. As we continue to refine irrigation and climate strategies, it may be time to give dissolved oxygen the attention it deserves.

Article complied by Stefan Vogrincic.  To discuss this and more email: stefan@grower2grower.co.nz