Learn the science behind:
How Lettuce is Grown Without Soil
We humans have been growing food for thousands of years, using the soil around us. Soil supplies plants with water and all the nutrients they need to grow and thrive. Combined with the power of the sun and rain, the soil is what makes plants.
But our earth is being stretched and challenged to its limits. We’re with so many people that growing enough food becomes more challenging. We’re exerting enormous pressure on our available natural resources.
Luckily, there are plenty of initiatives to lower this impact, ranging from big to small. We should try to grow our food as efficiently as is possible, limiting the water, land, and energy that we need to do so.
Lettuce and salad leaves are a great refreshing add to a meal. However, the leaves are often prone to spoilage since they’re so delicate, can be full of dirt and often need to be transported long distances before they reach their final consumers. How nice would it be if that growing can be done closer to the consumer?
Nowadays it’s possible to grow salad greens without any soil, in such an efficient manner, that even in areas where the climate isn’t suitable to the leaves, they can be grown! And, more interestingly, without using any soil.
The complexity of soil & Greenhouses
Most salad greens are still grown on large farms, in the soil. The soil captures nutrients and water and ‘feeds’ the plant. Soils are very complex systems. For instance, a lot of different microorganisms call them their home. A good, healthy soil is essential for producing food sustainably, not just today, but in the years to come.
Maintaining a high quality soil can be challenging. On top of that, not all climates are suitable for growing a lot of crops. It’s why greenhouses came along. From small ones in people’s backyard, to huge complexes. It’s what allows a country like the Netherlands to grow tomatoes, strawberries and bell peppers year-round!
In greenhouses farmers can control the humidity, the amount of water and, the soil and nutrients that a plant receives. Good greenhouse management is complex but can result in significant yield improvements.
Soils in greenhouses
Maintaining good soil quality in a greenhouse isn’t easy either. Taking soils from the ground and placing them in a greenhouse proved to be challenging, the soils didn’t remain healthy. As such, as far back as the 1930s, people were already interested in replacing soil with something else.
Replacing soil isn’t easy though. You need to fully understand what it is that soil does to make a plant thrive. You need to know which nutrients your plants need and this may vary between varieties!
Whereas soil still isn’t fully understood, we now know enough to be able to imitate soil for a wide variety of different crops. And, even better, growers can optimize it such that we need no soil and fewer resources overall to grow these crops!
For example, tomatoes and cucumbers can be grown in rock wool systems, no soil required. Growers carefully drip the exact right amount of moisture and nutrients into the system for the roots of the plants to take up. Other crops such as lettuce can be grown submerged in water. The water, filled with the exact right amount of nutrients, continuously flows, giving the plants great access to all the nutrients they need.
Of course, in order to control the feed so very precisely, the plants shouldn’t get their resources in another way. It’s why they’re grown in greenhouses where all they get from the outside is sunlight. Nowadays though, even vertically stacked systems are used or plants are growing in basements, using artificial light as their energy source.
Replacing soils this way and feeding all nutrients through the water is called hydroponics and it’s growing in volume in a lot of areas around the world.
Growing lettuces without any soil requires a lot of expertise. You have to ensure that the water you feed your plants contains the exact right mix of nutrients for whatever it is that you’re growing. But that’s not all. Since you can control every aspect of growing the plants, the temperature, the humidity, in some instances the amount of light, you can optimize it all.
What is your plant’s favorite temperature to grow in? What about their favorite water temperature to drink? Or humidity. Since you can control them all, you can optimize them one by one. What’s more, you can even control the possible pests within the greenhouse!
Growers claim that thanks to all this optimizing, they can grow ten times (in some cases 100x!) as much salads on the same amount of land compared to regular soil farming. Also, since the plants are fed just enough, the amount of water required to grow them can be reduced by a factor 10! Those numbers are huge and can be just one of the many solutions to help us create a sustainable food supply in the long run. Unfortunately, this growing system is a lot more energy intensive, especially due to the need for climate control in the growing areas. The amount of energy depends on the climate, a bigger deviation from outdoor temperatures will result in more energy consumption.
Why soil-less lettuce?
Whether or not a farmer/grower decides to grow produce using hydroponics depends on a lot of factors. Of course, the methodology should be viable, that is, profitable for the grower. Using hydroponics requires a decent amount of investment as well as expertise. For now, it is mostly profitable for more expensive vegetables and lettuce is one of those. Since lettuce leaves don’t store and hold well (unlike wheat kernels for instance) it is worth the effort to spend more growing and caring for them especially in areas where lettuce can’t naturally be grown year round.
Also, lettuces grow quickly and can be harvested regularly. As a result, if you plan it well, you can seed, grow, and harvest continuously. This is important to efficiently use your space. Trees on the other hand or yearly crops likely wouldn’t be worth the investment to be grown within a greenhouse.
But it’s not just because of commercial reasons. There are biological reasons as well. These plants don’t have very extensive root systems, or tall complex structures (such as trees!) that need some soil to help support them. Those crops wouldn’t be as easy to feed and maintain this way.
Is it any better?
But what about the flavour and nutritional value of this value? And does this lettuce stay good long enough to prevent spoilage? Well, there is still a lot of on-going research within this area.
Some claim the nutritional value of hydroponically grown lettuce is better than ‘conventional’ ones, since growers can specifically focus on improving nutrient content. Others claim it is less so due to the complex processes that occur within the soil that ‘enrich’ the plants.
Some initial evidence from a few years back showed that hydroponically grown lettuce had a shorter shelf life. The leaves would wilt a few days sooner. However, work is on-going to improve the shelf life by optimizing growing conditions and manufacturers claim the opposite. Of course, if you don’t have to transport your produce as far anymore, that reduces the need for a longer shelf life, solving part of the problem itself.
Personally, I’d happily eat my locally grown hydroponic lettuce leaves. The amount of water and land that we can save sounds very promising and based on my experience, they taste delicious and are somehow more delicate and crisp than most other lettuce leaves. Of course, that may well be a bias, by being excited about a new technology, but nevertheless, the salad tasted great!
Barbosa, G.L., et al., Comparison of Land, Water, and Energy Requirements of Lettuce Grown Using Hydroponic vs. Conventional Agricultural Methods, Int. J. Environ. Res. Public Health 2015, 12, 6879-6891; doi:10.3390/ijerph120606879
John W. Doran, Michael R. Zeiss, Soil health and sustainability: managing the biotic component of soil quality, Applied Soil Ecology, Volume 15, Issue 1, 2000, Pages 3-11,
Elejalde-Ruiz, A., ‘We see a lot of room for growth’: Gotham Greens is making money farming lettuce on the South Side. Now it’s adding a 2nd greenhouse in Pullman, Nov-13, 2019, link
Gotham Greens, link
Green and Vibrant, What is the Nutrient Film Technique – NFT? How does it work?, Dec-13, 2018, link
Grodan, link; rock wool substrate suppliers for greenhouses
Kotsiras, A., et al, Innovative harvest practices of Butterhead, Lollo rosso and Batavia green lettuce (LactucasativaL.) types grown in floating hydroponic system to maintain the quality and improve storability, ScientiaHorticulturae201(2016)1–9, link
Nxawe, S., Laubscher, C.P., Ndakidemi, P.A., Effect of regulated irrigation water temperature on hydroponics production of Spinach (Spinacia oleracea L), African Journal of Agricultural Research Vol. 4 (12), pp. 1442 – 1446, December, 2009, link
Rorabaugh, P.A. Jensen, M.H., Giacomelli, G., Introduction to Controlled Environment Agriculture and Hydroponics, 2002, link
The Lanka Salad Company, link; hydroponics salad grower in Sri Lanka