“With the prospect of a peat ban coming ever closer on the horizon, lettuce growers need to start looking for another solution,” says Lise Maibom, business development manager at Grodan and project lead for the lettuce trial. “We believe that stone wool offers the answer, but it requires a different irrigation approach – not least because it is a non-organic substrate.”
This means that, unlike when using peat, growers have to give their plants not only water, but also nutrients in the shape of fertilisers right from the start. “If growers are not familiar with using a non-organic substrate, it can seem more complex at first. That’s why we provide extensive support to our customers in the shape of detailed guidelines and tailored advice. This helps growers achieve optimum plant health, quality, yields, efficiency and therefore profitability when using our stone wool growing media,” she continues.
“Moreover, at Grodan, we believe in continuous improvement. Therefore, we are conducting ongoing research in conjunction with various objective, industry-leading partners to advance our knowledge. We constantly integrate our new learnings into our advice to growers for their specific crops and markets,” adds Lise.
Pressure to move away from peat
The recent lettuce trial at the Proefstation voor de Groenteteelt research centre in Sint-Katelijne-Waver (SKW), which has extensive knowledge of standard peat-based substrates, is a prime example of Grodan’s commitment to research. “Peat has been the main growing medium for lettuce for many years, and it is appreciated for its buffering capacity. However, growers are under mounting pressure to move away from peat – not only from governments in the form of legislation, but also from consumers who are creating bottom-up demand with their increased focus on health, food safety and sustainability,” comments Ben Vanvoorden, researcher at SKW.
“As a cleaner substrate, stone wool addresses many of these issues, such as by eliminating soil contamination on the lower leaves of a ‘living lettuce’, and by supporting highly automated systems that eliminate the need for human intervention,” he says. “Therefore, when Grodan approached us for this research trial, we saw it as an excellent opportunity to explore how to help growers to achieve the same quality, production and speed on stone wool as on peat.”
The cleanliness of stone wool offers other practical benefits, according to Lise: “Unlike with peat, which can bring unwanted pests and diseases into the greenhouse, the high temperatures involved in the production process of stone wool blocks make this almost impossible. Additionally, whereas peat detritus can get into the gutters and cause abrasion in the irrigation system, the stone wool blocks remain fully intact – even when used in automated systems.” These are all extra reasons why lettuce growers should consider ‘switching to clean’, she says.
Experimenting in a risk-free environment
Ben acknowledges that many growers are unaware that stone wool offers an alternative to peat, or are concerned about the perceived risks associated with switching to a new system. “That’s why, together with Grodan, we’re conducting experiments in a risk-free environment on the growers’ behalf. The great thing about being a trial station is that we’re able to take things to the extreme in order to find the optimal levels. Then by applying our insights, we can develop the perfect ‘game plan’ for hydroponic lettuce cultivation on stone wool. This trial marks a major step in our commitment to fine-tuning and further optimising lettuce cultivation on stone wool – just as we’ve been doing at SKW with peat for the past 60 years.”
Supporting precision growing
The fact that stone wool is a non-organic substrate opens up opportunities for precision growing, according to Lise: “Because there are no nutrients in the stone wool substrate itself, growers know for sure that they are starting from a baseline of zero. Whatever growers give in the form of fertilisers will not be influenced by anything that might already be in the substrate.”
Ben agrees, commenting: “Whereas the buffering capacity of peat holds some of the nutrients back, all the nutrients you give in stone wool are readily available to the roots. So unlike with peat, growers know that the plants and roots will receive all of what they are given.” Through the nutrient recipe, growers have full control over the electrical conductivity (EC) and can use it to precisely steer plant health and growth throughout the crop cycle.
The optimum EC strategy
“In stone wool blocks, EC is crucial in the early stages of lettuce cultivation, both during the propagation phase and the initial wetting, to promote plant health and growth. In every research project we conduct, we notice that if the right plant quality isn’t achieved in the propagation phase, you will end up with a lower-quality lettuce at harvesttime,” states Ben.
“In our research trial, we knew that for stone wool – unlike peat – we wanted to explore the right EC to use in the propagation phase to compensate for the lack of naturally occurring nutrients at the beginning. And we assumed that the plants on stone wool would require a higher EC after that too, but what is the optimal level?” he continues.
To answer that question, the team started out using an EC of 1.5 for the initial wetting, as is the norm, and used the same value in propagation and also in the gutter phase. “However, we soon realised that the EC in propagation and perhaps even the initial wetting needed to be higher. Therefore, we tried various EC levels of between 3 and 5.5 to test the limits, using sensors to constantly monitor the EC level and other parameters in the root zone,” explains Ben.
Combining quality with efficiency
With various end-product requirements in mind, the team evaluated the two growing systems by analysing harvests of three different lettuce sizes: 150g, 250g and 400g. “We found that a higher EC in the first phase produced the best results for all three harvests on stone wool,” reveals Lise. “We also discovered that stone wool didn’t actually require a higher EC in the gutter phase; we could work with similar values as with peat.”
“All the plants had a good germination rate and good plant uniformity. Additionally, the overall quality was better compared to peat: fewer yellow leaves and significantly less rot, resulting in a longer shelf life,” states Ben. “We also evaluated the root growth and root health, such as browning, and saw that the root parameters were good.”
Higher-quality lettuces mean that growers can command a better price for their end products. “Meanwhile, the plant uniformity, combined with fewer yellow leaves and less rot, makes harvesting easier and faster for growers. It also reduces waste, improving the operational efficiency,” comments Lise. “Additionally, as a non-organic substrate, stone wool supports clean and food-safe products that are popular with retail customers and consumers. Moreover, since there are no organic fibres that can get into the gutters, stone wool reduces wear and tear on the grower’s irrigation system.”
Faster growth rate on stone wool
“Based on the results from this trial, we can already conclude that a higher EC level during the early phase of lettuce propagation and cultivation on stone wool supports better nutrient availability, leading to clearly improved growth and health of the roots and plants,” says Ben. “We expect a higher EC to have an even bigger positive impact on quality and efficiency in winter/spring crop cycles, when the propagation period can be as long as two months, compared to less than two weeks in the summertime. Therefore, the next step is to validate these findings in another trial in early 2025.”
Lise is looking forward to further fine-tuning the EC levels in the next trial. “In the first two sowings of this trial, the stone wool lettuces were smaller than the peat ones when they were transferred to the gutter system. However, they showed a strong growth rate in the gutter phase, enabling them to catch up. And they had reached the same size and quality as the peat-based lettuces by harvest time of 400 grams,” she says. “This implies that if we can find the optimal EC during propagation to ensure that the plants are the right size when entering the gutter phase, stone wool could potentially enable growers to produce high-weight lettuces even faster than on peat.”
Tailored advice
“These findings already demonstrate that stone wool growing media can support better growth, quality, shelf life and efficiency for growers. By validating these findings, we will be able to further tailor our advice and guidance to growers who are keen to ‘switch to clean’ for their hydroponic lettuce production,” she concludes.
