In all crops under glass, drain water should be reused. This offers advantages such as saving water and fertilizers. However, there are also disadvantages such as spreading pathogens (think of viruses, bacteria and fungi), but one also has to deal with higher nitrite levels in the drain water. If no action is taken to reduce these nitrite levels in the water, this can result in growth problems. Especially in a nitrite sensitive crop such as cucumber this manifests itself very quickly. If measurements are taken in the water, values of 1 mg/l are already on the high side.

How is nitrite formed?

Nitrification is an important step in the nitrogen cycle of ecosystems, where nitrogen sequestered in dead organic matter becomes available again to living plants. Nitrification involves conversion of the nitrogen compound ammonium to the nitrogen compound nitrite, followed by oxidation of this nitrite to the nitrogen compound nitrate. This ammonium comes partly from nitrogen fixation by symbiotic root bacteria of the bacterial genus Rhizobium, and partly from biodegradation of proteins present in organic matter by free-living soil bacteria.

What is the effect of elevated nitrite concentrations on plant health?

In a test on tomato plants, increasing the concentration of nitrite nitrogen reduced dry matter yield, total acidity, concentration of nitrogen, phosphorus and potassium in tomato plants, and increased leaf chlorosis and root shedding.

In another study, vegetable crops were grown in solution culture with varying nitrite and pH levels.

• At pH 5, plants supplied with nitrite showed browning of the roots, wilting of the leaves, dark green leaves and growth retardation with increasing severity as the nitrite concentration increased. In some species nitrite at pH 5 also caused leaf burn, chlorosis or staining on the leaves.
• At pH 6 or 7, nitrite was less harmful than at pH 5, but chlorosis due to iron deficiency was caused by nitrite in some species, including tomato and bell pepper.
• Growth inhibition by nitrite was more evident at lower pH values. Nitrite tolerance varied greatly among species.
• Nitrite treatment mainly reduced the concentration of phosphorus, potassium, calcium and magnesium in leaves. The effect of nitrite on the concentration of heavy metal elements, Fe, Mn and Zn in leaves was variable depending on the crop species.

The solution to reduce nitrite concentration?

An easy solution to eliminate nitrite in the water is to oxidize it to nitrate. This can be done by injecting ozone into the water. Ozone is a highly oxidizing chemical compound and will oxidize virtually all nitrite to nitrate, eliminating the toxicity caused by nitrite. The oxidation of nitrite to nitrate with ozone is pH independent.

The accompanying table shows the results of Agrozone ozone treatment of drain water from a cucumber grower. Untreated drain water and drain water that was previously disinfected with UV (Decontaminated drain water) were tested. Nitrite (2.4 to 2.6 mg/l) was detected in both the untreated drain water and the decontaminated drain water. After treatment with ozone 10 -12 minutes at a redox potential of 800-850 mV, the levels of nitrite were reduced by 98.5%.

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Based on these results, the cucumber grower decided to install an Aqualine ozone sanitizer from Agrozone to treat drain water. This disinfector has the following advantages:

• Disinfection of drain water (bacteria, fungi, yeasts, cucumber fur virus);
• Conversion of nitrite to nitrate;
• Increasing oxygen levels in the water.

Uit herhaalde praktijkmetingen bij deze komkommerteler zijn de voordelen van de Aqualine ontsmetter bevestigd. Specifiek voor nitriet blijkt dat deze bij een ozon behandeling bij een redoxpotentiaal van 850 mV gedurende 5 minuten geheel wordt verwijderd tot beneden de detectiegrens van < 0,030 mg/l.

Why is there an increase in nitrite concentration when using UV treatment?

Op basis van onderzoek is bewezen dat nitraatbestraling met UV-licht (bij golflengten <240nm en 300-325 nm) genereert nitrietionen (NO2-), als bijproduct (3, 4 en 5). Dit gebeurt meestal bij hoge UV-straling doses.

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References:

1. Phipps, R.H., Cornforth, I.S. Factors effecting the toxicity of nitrite nitrogen to tomatoes. Plant Soil 33, 457-466 (1970).
2. OSAWA, T. (1971). Nitrite toxicities in vegetable crops I. Effect of nitrite and pH levels in nutrient solution on growth of vegetable crops. Journal of the Japanese Society for Horticultural Science, 40(4), 395-400.
3. Wageningen UW Horticulture (Oct 2008), Exploration of nitrite formation and toxicity of nitrite
4. Lester, Y., Dabash, A., & Eghbareya, D. (2018). UV sensitization of nitrate and sulfite: A powerful tool for groundwater remediation. Environments, 5(11), 117
5. Lu, N., Gao, N. Y., Deng, Y., & Li, Q. S. (2009). Nitrite formation during low pressure ultraviolet lamp irradiation of nitrate. Water Science and Technology, 60(6), 1393-1400