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Nitrate in leafy greens: why hydroponics runs high, and the plain-water fix
A Thai study swapped nutrient solution for tap water 48-72 hours before harvest and cut lettuce nitrate by 61-63%, at some cost in vitamin C. Plus light and timing.
- Leafy greens store nitrate in stalks, veins and the core when they get plenty of nitrogen but not enough light, so hydroponic greens sitting in nutrient solution often run high.
- In a Prince of Songkla University trial, 48 hours on tap water before harvest cut nitrate by 61% and 72 hours by 63%; the two were not statistically different.
- The cost was vitamin C, down 16% at 48 hours and 30% at 72 hours, which makes 48 hours the better trade.
- Good daylight before harvest helps, but in one trial dim light at night did not lower nitrate any further.
Leafy greens such as lettuce, spinach and mustard greens can build up a lot of nitrate, and hydroponic greens, whose roots sit in a nitrogen-rich solution all the time, are the usual suspects. A study from Prince of Songkla University’s Pattani campus in southern Thailand found that simply swapping the nutrient solution for tap water 48 hours before harvest cut lettuce nitrate by about 61%, and 72 hours cut it by 63%. This lesson covers why nitrate builds up, the numbers from that trial, how much light and harvest timing help, and how to try it at home.
Here nitrate is treated as a quality trait of the crop, like crispness or sweetness, something the grower can steer. It is not about effects on the body.
Where the nitrate comes from
Plants take up most of their nitrogen as nitrate through the roots, move it up the water-conducting vessels into the stem, leaf stalks and veins, and gradually turn it into protein and chlorophyll. That conversion runs on energy from light, and the enzyme that starts it slows down when light is low.
So when a plant takes in more nitrate than light lets it convert, the surplus sits in the cells and the vessels, mostly in stalks, veins and the core. A grower’s guide from the Maine Organic Farmers and Gardeners Association (MOFGA) boils the risk down to three things: low light, lots of soluble nitrogen, and leafy crops.
Hydroponics easily ticks the first two boxes. Roots are bathed in nitrate around the clock, and many farms grow under shade netting. No wonder buyers ask about it first.
The Pattani trial: tap water before harvest
The team grew green oak lettuce in an NFT channel system under 50% shade net, at 24-32 °C, with a nutrient solution at EC 1.8 mS/cm. Thirty days after transplanting, they replaced the solution with plain tap water and harvested at 0, 24, 48 and 72 hours. The results were published in the ASEAN Journal of Scientific and Technological Reports in 2023.
| Time on tap water before harvest | Nitrate (mg/g dry weight) | Nitrate vs control | Vitamin C vs control | Soluble sugar vs control |
|---|---|---|---|---|
| 0 h (control) | 51 | - | - | - |
| 24 h | 41 | -19% (not significant) | -4% | +86% |
| 48 h | 20 | -61% | -16% | +105% |
| 72 h | 19 | -63% | -30% | +98% |
Three things stand out.
- 24 hours is not enough. Nitrate dipped, but not significantly.
- 48 and 72 hours did equally well statistically, and the authors concluded 48 hours is enough.
- Something is lost. Vitamin C fell steadily the longer they waited. Phenolics dipped slightly, not significantly. Soluble sugar roughly doubled, so the leaves were likely sweeter.
The authors’ explanation is that plain water flushes the nitrate left in the vessels of root, stem and leaf, while the plant keeps using light to convert what remains.
48 hours is the sweet spot: nearly the same nitrate drop as 72 hours, for about half the vitamin C loss.
Worked example: four lettuces in a Kratky tub
Say you grow lettuce by the Kratky method (how-to here) in a 20-litre tub with four plants, and plan to harvest on Saturday morning.
- Thursday morning. Count back 48 hours from harvest.
- Note the current level. Say about 8 litres of solution remain, 6 cm below the lid. Write it down.
- Pour out the solution. Lift the lid with the plants slowly and rest it on another tub. The old solution can go on soil-grown plants.
- Refill with about 8 litres of tap water to the same level. The upper roots must stay in the air, because that part of the root does the breathing. Flood them all and the plants may wilt.
- Move the tub to the sunniest spot you have. Light matters most in these two days, because the plant needs it to convert the leftover nitrate.
- Saturday morning, harvest whole plants. If the trial holds, nitrate should be down by around half or more, though at home it depends on variety, sun and weather, and won’t match the trial exactly.
In an NFT or other pumped system it is the same idea: swap the reservoir for plain water and let the pump run as usual.
Light and harvest time help, but not simply
More daylight does help. MOFGA notes that leaf nitrate rises overnight and falls during sunny daytime hours. New Alchemy trials cited there found lettuce harvested in the afternoon had about 15-20% less nitrate than lettuce cut in the morning, and the darkest season was when levels peaked. The Pattani paper also cites work where continuous light before harvest lowered nitrate in plant factories.
Dim night lighting did not always help. A 1993 Dutch study lit lettuce at night at about 27 µmol per square metre per second for up to 8 nights before harvest. Nitrate per fresh weight did not drop further; the authors put it down to a rise in dry matter.
The best hour is unclear. A Brazilian study cut hydroponic lettuce at five times from 6 am to 6 pm and found the plant parts moved differently: the stalk peaked at noon, at about 2,702 ppm fresh weight, the inner leaves peaked at 9 am, at about 1,879 ppm, and the outer leaves did not change at all.
In practice: give the crop full sun in the days before harvest and pick after some good daylight, rather than after several overcast or rainy days. Don’t expect changing the harvest hour alone to match the plain-water swap.
Nitrogen fertiliser: where it all starts
Soil or water, leaf nitrate begins with the nitrogen you feed. MOFGA reports that soluble nitrate fertilisers gave higher levels than composted manure, and in one greenhouse survey the only low-nitrate house was one whose grower had not fertilised for months.
If you grow in soil, the rule from the water spinach lesson applies: don’t push heavy nitrogen close to harvest. If you grow in water, don’t run the EC above the recommendation hoping for faster growth.
Common mistakes
- Swapping to plain water for one night only. In the trial, 24 hours was no different from no swap.
- Leaving plants in plain water for a week. Vitamin C keeps dropping and the plants start to starve. 48 hours is enough.
- Flooding the roots in a Kratky setup. The upper roots lose air and the plant wilts.
- Moving the tub into shade for the last two days. The opposite of what helps; this is when the plant needs light.
- Assuming washing removes nitrate. Nitrate is inside the tissue, not on the surface. Washing is for soil and germs (how to wash here).
Sources
- Effect of Pre-harvest Periods by Replacing Nutrient Solution with Tap Water on Nitrate and Quality in Hydroponic Lettuce — ASEAN J. Sci. Tech. Report (ThaiJO)
- Influence of the harvesting time in the level of nitrate in hydroponic lettuce — Semina: Ciências Agrárias (UEL)
- Nitrate Accumulation — MOFGA
- Effects of low light intensities at night on nitrate accumulation in lettuce grown on a recirculating nutrient solution — NJAS (Wageningen)
FAQ
I grow in soil with compost. Do I need to worry about this?
Less so. A Maine organic growers' guide reports that soluble nitrate fertilisers gave higher nitrate than composted manure. But heavy nitrogen feeding or a very shady spot can push soil-grown greens up too.
Can I do the plain-water swap in a Kratky bottle?
Yes. Pour out the solution and refill with tap water to the same level, so the upper roots stay in the air as before, then harvest about 48 hours later. Do not fill it up so the whole root mass is under water.
So hydroponic vegetables are unsafe?
This article treats nitrate as a produce-quality measure a grower can manage. It does not rank vegetables by health effect. If you have questions about diet and a medical condition, ask a doctor or dietitian.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.