Library › Part E · Article 43 in the series · Level 2 · 10 min read

Water, energy and nutrients: what the evidence says

Standfirst: After reading this article you will know which study the “90% less water” claim comes from and what that study compared. You will be able to estimate the water a kilogram of lettuce must transpire and the electricity an indoor farm needs to light it, and you will know why the climate footprint of hydroponic food depends more on the power grid than on the growing method.

Before you read: What a plant needs, Water in the plant, Managing a recirculating solution, Light: PPFD, DLI, spectrum and photoperiod, Choosing and sizing grow lights.

Where “90% less water” comes from

The most quoted comparison is a 2015 study by Barbosa and colleagues at Arizona State University¹. They compared lettuce grown in fields around Yuma, Arizona, with lettuce grown in a hydroponic greenhouse of 815 m² that did not exist: it was a model built from engineering equations and values from the literature¹. The field figures came from crop budgets and ten years of agricultural statistics¹.

The modelled greenhouse used nutrient film technique, heating and cooling, supplementary lamps and circulation pumps¹. It needed 20 ± 3.8 litres of water per kilogram of lettuce, against 250 ± 25 litres for the field, and it yielded 41 ± 6.1 kg per square metre per year against 3.9 ± 0.21 kg¹. That is 13 times less water per kilogram¹, a saving of 92% by our arithmetic. This is the comparison behind the round “90% less water”^1,6.

The same study found an energy demand of 90,000 ± 11,000 kJ per kilogram for the greenhouse and 1,100 ± 75 kJ for the field, 82 ± 11 times more¹. Heating and cooling took 74,000 kJ of it, because Yuma averages 34.7 °C in summer and 14.1 °C in winter¹. The authors concluded that, at that time, commercial hydroponics was not a suitable alternative to conventional lettuce production in Yuma¹. They counted direct energy only, not the energy in materials or fertilisers¹.

Claims drift when they are repeated. A 2022 paper summarised the study as using “90% less water to produce 20 times more food”⁶, while the study itself reported 11 ± 1.7 times the yield¹.

Water: transpiration is the floor

A plant loses almost all the water it takes up through its leaves, a process called transpiration (Water in the plant). The transpiration ratio is the water transpired per unit of dry matter built. For C3 plants such as lettuce in open fields it is estimated at about 500 litres per kilogram of dry matter². Under cover, with less radiation and wind, crops transpire less water per kilogram produced², so the floor depends on the climate around the leaves. No growing system can go below what the leaves transpire unless it captures the vapour again.

A field loses more than transpiration. Water also evaporates from bare soil, drains below the roots and runs off. Barbosa and colleagues note that a hydroponic plant uses about as much water as a field plant, but the system delivers it more efficiently¹. Yuma lettuce is mostly flood-furrow irrigated, and water the shallow roots do not take up quickly is lost to percolation¹. The saving therefore depends on the field you compare with. A drip-irrigated field in a mild climate loses less than a furrow-irrigated field in a desert.

A sealed indoor room can go below the transpiration floor. Its air conditioner must remove the heat of the lamps, and in doing so it condenses much of the transpired vapour, which can be recycled⁴. Kozai’s review gives a water use efficiency of 0.95 to 0.98 for such rooms, meaning that almost all the water supplied is either held in the plants or recovered for reuse, against 0.02 to 0.03 in a ventilated greenhouse⁴. Graamans and colleagues assumed the same effect in their model: in a closed plant factory, water use approaches the water held in the harvested crop⁵. The life cycle study by Casey and colleagues lists 1.6 L of water per kilogram for a container farm and 8 to 60 times more for field supply chains⁶.

Worked example 1: the water in a kilogram of lettuce

Inputs. Transpiration ratio for C3 crops in open fields: about 500 L per kg of dry matter². Dry-matter content of lettuce: normally 4% to 6%, as Dediu and colleagues report, citing Seginer et al. (2004)³; we take 5%.

1. Dry matter in 1 kg of fresh lettuce: 1,000 g × 0.05 = 50 g = 0.05 kg.
2. Water transpired: 500 L kg⁻¹ × 0.05 kg = 25 L^2,3. At 4% and 6% dry matter it is 20 L and 30 L.

These are our estimates from the inputs in² and³. Barbosa’s 20 L per kilogram was built the same way, from 400 mL per gram of dry mass, a dry-matter ratio of 0.045 and an extra 10% for flushing the solution¹; we re-ran those inputs and obtain 19.8 L. So “20 L” is roughly the transpiration of the crop, our inference from^1,2,3. Field lettuce at 250 L¹ used about ten times its transpiration, by our estimate.

Open systems, drainage and the fertiliser behind the solution

A drain-to-waste system (Drip and substrate culture) adds drainage to transpiration. Growers apply more solution than the plants use so that salts do not build up in the medium. Under well-controlled conditions the leaching fraction, the share of applied water that drains out, ranges from 0% to 20%⁷; for rockwool slabs Resh advises irrigating long enough to get 20% to 25% runoff⁸. At 20% to 25% drainage, every 25 L transpired needs 31 to 33 L applied, by our arithmetic. Capturing and reusing that drainage (Managing a recirculating solution) saves both water and fertiliser, and semi-closed systems normally reduce effluent by about 90% compared with no recirculation⁹.

The fertiliser itself has a footprint. Industrial nitrogen comes from the Haber–Bosch process, whose hydrogen and heat come from fossil fuels¹². Ammonia production averages about 41 GJ per tonne, and about 70% of ammonia goes into fertilisers¹⁰. Since ammonia is 82% nitrogen by mass, that is about 50 MJ per kg of nitrogen, our arithmetic from¹⁰ and the atomic masses. FAO estimated that nitrogen fertiliser production alone accounts for about half of the fossil fuel used in primary production¹¹. Phosphorus comes from phosphate rock, a non-renewable resource under threat of depletion¹³.

For an indoor farm this energy is small. Casey and colleagues list about 2 g of nitrogen per kilogram of lettuce⁶; at 50 MJ per kilogram that is 0.1 MJ, under 0.2% of the farm’s 15 kWh of electricity, by our arithmetic from^6,10. Recovering nutrients from wastes avoids part of this fertiliser demand; PonicLabs describes two approaches on its anthroponics and bioponics pages.

Energy: heating and cooling in greenhouses, lighting indoors

A greenhouse uses free sunlight but must hold its air near the crop’s set point against the outside climate. In Barbosa’s Yuma model, heating and cooling dominated, and lighting took 15,000 kJ per kilogram, about 4.2 kWh by our conversion¹. The authors note that a greenhouse in a climate closer to its set point needs much less energy¹.

An indoor farm buys every photon as electricity. Graamans and colleagues modelled lettuce in plant factories and in greenhouses in the Netherlands, the United Arab Emirates and Sweden⁵. In plant factories the lighting load exceeded all other loads⁵. Producing 1 kg of lettuce dry weight took 247 kWh of purchased energy in a plant factory, all of it electricity, against 70, 111, 182 and 211 kWh in the greenhouses of the Netherlands, the Emirates and Sweden with and without supplementary lamps, where the gas burned for heating is counted one-for-one as electricity⁵. At the 7% dry matter they assumed⁵, that is 17.3 kWh per kilogram of fresh lettuce in the plant factory and 4.9 kWh in the Dutch greenhouse, our conversion.

Other estimates cover a wide range. Kozai’s 2013 review found that only 0.7% of the electricity in closed plant production rooms ended up as chemical energy in the crop, which he converted to 794 kWh per kilogram of dry matter⁴. Casey’s container farm used about 15 kWh per kilogram of fresh lettuce for lighting, cooling, ventilation and pumping⁶. The spread reflects lamp generation, climate and how much of the building each study counted.

Worked example 2: electricity to light a kilogram of lettuce

Inputs. Light use efficiency (LUE), the grams of dry matter produced per mole of photons reaching the canopy (Light: PPFD, DLI, spectrum and photoperiod): 0.55 g mol⁻¹ on average in vertical farms¹⁴, and 0.87 g mol⁻¹ in a controlled trial at 400 µmol m⁻² s⁻¹¹⁵. LED fixture efficacy (Choosing and sizing grow lights): 2.5 to 2.8 µmol J⁻¹ for good white and red fixtures¹⁶. Dry matter 5%³.

1. Dry matter in 1 kg of fresh lettuce: 50 g, as before.
2. Photons needed: 50 g ÷ 0.87 g mol⁻¹ = 57.5 mol.
3. Electrical energy: 57.5 mol = 57.5 × 10⁶ µmol; 57.5 × 10⁶ ÷ 2.5 µmol J⁻¹ = 23.0 MJ.
4. In kilowatt-hours: 23.0 MJ ÷ 3.6 MJ kWh⁻¹ = 6.4 kWh.

With the vertical-farm average of 0.55 g mol⁻¹ the result is 10.1 kWh; with the best trial value of 1.01 g mol⁻¹¹⁵ and 2.8 µmol J⁻¹ it is 4.9 kWh. These are our estimates, for lamps only, and they assume every photon reaches the canopy. They sit below Graamans’s 17.3 kWh and Casey’s 15 kWh, which also include cooling, dehumidification and fans^5,6. Graamans assumed 7% dry matter; at 7% our lamp estimate is 6.9 to 14.1 kWh, still below his figure. Lamps alone take as much energy as Graamans’s whole Dutch greenhouse bought per kilogram, gas heating included, or more, by our comparison with⁵.

Carbon: it depends on the grid

Electricity becomes a climate footprint through the grid that supplies it. Casey and colleagues modelled the same container farm on six electricity sources and delivered lettuce to Britain from four field supply chains⁶. Their functional unit was 1 kg of fresh lettuce at a regional distribution centre, from cradle to gate⁶. The container farm emitted 8.9 kg CO₂-equivalent per kilogram on the British grid and 17.8 kg on the coal-dominated South African grid⁶. On wind power it emitted 0.48 to 0.56 kg⁶. When the carbon cost of occupying farmland was added to field lettuce, wind-powered container lettuce had among the lowest footprints of all ten chains⁶. Field lettuce ranged from 0.15 kg (grown in Britain) to 10 kg (air-freighted from the United States)⁶.

Martin and Molin assessed a basil vertical farm in a Stockholm basement, using annual production of 60,000 plants as the functional unit¹⁷. They found 0.27 to 0.74 kg CO₂-equivalent per kilogram of edible plant on the Swedish grid, depending on the pot and growing medium, which is mostly hydropower and nuclear, and large increases in emissions on the Nordic grid mix, which has more fossil fuel¹⁷. Pots and growing medium also mattered¹⁷.

Land and yield

Hydroponic yields per square metre are high because the crop grows all year at close spacing¹. Barbosa’s modelled greenhouse gave 41 kg m⁻² per year against 3.9 kg in the field¹. Graamans’s Dutch greenhouse model gave about 31 kg m⁻² of fresh weight⁵. The Cornell handbook needs 660 m² of pond to supply 1,245 heads of 150 g per day¹⁸, which works out at about 103 kg m⁻² per year by our arithmetic. Casey’s container towers produced 154 kg per m² of floor per year, up to 38 times more than the field systems⁶.

Stacking layers multiplies yield per square metre of floor in proportion to the number of layers⁵. It does not multiply yield per mole of light. Each layer needs its own lamps, and the dry matter per mole stays near the values in Worked example 2, our inference from^14,15. Land saved indoors is paid for in electricity.

What the evidence supports

Hydroponics saves water, because a recirculating system loses little beyond transpiration, and a sealed room can recover even that^1,4,5. It saves land per kilogram^1,6. It costs energy: a modest amount in a greenhouse in a mild climate, much more in a hot or cold climate, and most of all indoors under lamps^1,5. Its climate footprint follows that energy and the grid behind it⁶.

Key numbers

Common mistakes

1. Quoting “90% less water” as a fixed property of hydroponics¹. It is one modelled greenhouse against furrow-irrigated desert fields, and the saving depends on the field baseline¹.
2. Expecting a recirculating system to use less water than its plants transpire. Transpiration is set by the crop and the air around it (light, temperature, humidity, wind), not by the growing system, so only vapour recovery in a closed room goes below it^2,4.
3. Forgetting the drainage in a drain-to-waste system. With 20% to 25% runoff, a fifth to a quarter of the water and fertiliser applied leaves the system⁸.
4. Comparing farms in kilowatt-hours without saying what is counted. Lamps alone, all electricity, or all energy including heating give different numbers for the same farm^1,5,6.
5. Treating local food as low-carbon by default. On a coal-dominated grid, indoor lettuce emitted more than lettuce air-freighted between continents⁶.

Summary

The “90% less water” claim comes from a 2015 Arizona study that compared a modelled hydroponic greenhouse with furrow-irrigated desert fields¹. The same study found the greenhouse needed about 82 times more energy, mostly for heating and cooling¹. Water use in a recirculating system falls to roughly what the crop transpires, about 20 to 30 litres per kilogram of lettuce by our estimate, and a sealed room can recover much of that^2,3,4. Drain-to-waste systems add the drainage. Indoors, lighting dominates, and our estimate for lamps alone is about 5 to 10 kWh per kilogram of fresh lettuce^5,14,15,16. Whether that is a large or small climate footprint depends on the electricity source. Fertiliser energy matters less than lighting indoors, but it rests on fossil fuels and mined phosphate.

Next in the series: Meters and measurement: pH, EC, DO, temperature

References

1. Barbosa, G. L., Gadelha, F. D. A., Kublik, N., Proctor, A., Reichelm, L., Weissinger, E., Wohlleb, G. M. and Halden, R. U. (2015). Comparison of land, water, and energy requirements of lettuce grown using hydroponic vs. conventional agricultural methods. International Journal of Environmental Research and Public Health, 12(6), 6879–6891. https://doi.org/10.3390/ijerph120606879
2. Castilla, N. (2013). Greenhouse Technology and Management, 2nd ed. CABI, Wallingford. Section 11.6.7.
3. Dediu, L., Cristea, V. and Zhang, X. (2012). Waste production and valorization in an integrated aquaponic system with bester and lettuce. African Journal of Biotechnology, 11(9), 2349–2358. https://doi.org/10.5897/AJB11.2829
4. Kozai, T. (2013). Resource use efficiency of closed plant production system with artificial light: concept, estimation and application to plant factory. Proceedings of the Japan Academy, Series B, 89(10), 447–461. https://doi.org/10.2183/pjab.89.447
5. Graamans, L., Baeza, E., van den Dobbelsteen, A., Tsafaras, I. and Stanghellini, C. (2018). Plant factories versus greenhouses: comparison of resource use efficiency. Agricultural Systems, 160, 31–43. https://doi.org/10.1016/j.agsy.2017.11.003 (full text read in the open-access reprint: Graamans, L. (2021). STACKED. PhD thesis, TU Delft, chapter 3. https://doi.org/10.7480/abe.2021.05
6. Casey, L., Freeman, B., Francis, K., Brychkova, G., McKeown, P., Spillane, C., Bezrukov, A., Zaworotko, M. and Styles, D. (2022). Comparative environmental footprints of lettuce supplied by hydroponic controlled-environment agriculture and field-based supply chains. Journal of Cleaner Production, 369, 133214. https://doi.org/10.1016/j.jclepro.2022.133214
7. Lieth, J. H. and Oki, L. R. (2008). Irrigation in soilless production. In: Raviv, M. and Lieth, J. H. (eds), Soilless Culture: Theory and Practice. Elsevier, Amsterdam, chapter 4. https://doi.org/10.1016/B978-044452975-6.50006-X
8. Resh, H. M. (2013). Hydroponic Food Production, 7th ed. CRC Press, Boca Raton.
9. Raviv, M., Lieth, J. H., Bar-Tal, A. and Silber, A. (2008). Growing plants in soilless culture: operational conclusions. In: Raviv, M. and Lieth, J. H. (eds), Soilless Culture: Theory and Practice. Elsevier, Amsterdam, chapter 13, 545–571. https://doi.org/10.1016/B978-044452975-6.50015-0
10. International Energy Agency (2021). Ammonia Technology Roadmap: Executive summary. IEA, Paris. https://www.iea.org/reports/ammonia-technology-roadmap/executive-summary
11. FAO (2011). "Energy-smart" food for people and climate. Issue paper. Food and Agriculture Organization of the United Nations, Rome. https://www.fao.org/4/i2454e/i2454e00.pdf
12. Cooper, J. E. and Scherer, H. W. (2012). Nitrogen fixation. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London, chapter 16. https://doi.org/10.1016/B978-0-12-384905-2.00016-9
13. O'Neal, J. A. and Boyer, T. H. (2013). Phosphate recovery using hybrid anion exchange: applications to source-separated urine and combined wastewater streams. Water Research, 47(14), 5003–5017. https://doi.org/10.1016/j.watres.2013.05.037
14. Jin, W., Formiga Lopez, D., Heuvelink, E. and Marcelis, L. F. M. (2022). Light use efficiency of lettuce cultivation in vertical farms compared with greenhouse and field. Food and Energy Security, 12(1), e391. https://doi.org/10.1002/fes3.391
15. Carotti, L., Graamans, L., Puksic, F., Butturini, M., Meinen, E., Heuvelink, E. and Stanghellini, C. (2021). Plant factories are heating up: hunting for the best combination of light intensity, air temperature and root-zone temperature in lettuce production. Frontiers in Plant Science, 11, 592171. https://doi.org/10.3389/fpls.2020.592171
16. Kusuma, P., Pattison, P. M. and Bugbee, B. (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research, 7, 56. https://doi.org/10.1038/s41438-020-0283-7
17. Martin, M. and Molin, E. (2019). Environmental assessment of an urban vertical hydroponic farming system in Sweden. Sustainability, 11(15), 4124. https://doi.org/10.3390/su11154124
18. Brechner, M., Both, A. J. and CEA Staff (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture Program, Ithaca, NY. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.*