On-shelf availability of lettuce tackled with precision

Gs Growers_red and green lettuce

G's Growers and PDM Produce were involved in the trials

Jim Monaghan fresh produce lecturer at Harper Adams University
Dr Jim Monaghan is the principal fresh produce lecturer at Harper Adams University

Using precision irrigation on lettuce crops can lead to “more crop per drop”, a four-year lettuce study reveals

Precision irrigation will help improve the quality of the UK lettuce crop and reduce growers’ water usage, as well as safeguarding future supplies to customers around the country, according to Dr Jim Monaghan, the principal fresh produce lecturer at Harper Adams University in Shropshire.

With the United Nations declaring 2015 as the International Year of Soils (IYS), Dr Monaghan’s findings are very timely. The aim of IYS is to raise awareness of the importance of soils for food security and essential ecosystem functions, and Dr Monaghan’s research indicates that sufficiently watered soils make for a happier crop.

The four-year HortLINK project looked at how precision irrigation – or, as Monaghan puts it, “applying water where it’s needed and when it’s needed” – can benefit lettuce crops. The initial impetus behind HortLINK sprang from the need to conserve water, particularly as water is over-abstracted in many areas of the UK.

“Lettuce crops require irrigation,” Dr Monaghan says. “Irrigation can be crucial during the drier months to maintain crop yield. However, dwindling water supplies and uncertainties associated with a changing climate mean Europe’s irrigated agriculture sector needs to improve water efficiency and produce more ‘crop per drop’. We are not growing leafy salads where we have water available, so we are genuinely under pressure for water,” he adds.

To carry out the work and examine soils in fields used for lettuce growing, Dr Monaghan’s team of researchers teamed up with two of the UK’s biggest lettuce producers – Cambridgeshire-based G’s Growers and Shropshire’s PDM Produce.

The research team used electrical devices, including an electromagnetic induction (EMI) scanner, to map the lettuce fields into ‘zones’ – for example, areas that had plenty of moisture and areas that were a bit parched.

The EMI, says Dr Monaghan, “measured” the saltiness of soils as – just like eating too much salt can make humans dehydrated – so a high salt content correlates with low soil moisture content. Then his team took plant samples out of these different soils and examined them.

Results – variable soil moisture affects yields

A graph from the study reveals that the marketable head weight of lettuce was about 0.65 kilogrammes (kg) in drier zones of the field compared to 0.72kg in zones with more moisture. These results will no doubt be of interest throughout the supply chain – from commercial growers, who are constantly striving to improve their yields and grow food in a more sustainable way, to buyers of fresh produce, who are increasingly keen to source high-quality food responsibly.

The results of the trial also indicated that this kind of effective irrigation affects the quality of lettuce after it’s been harvested. This is because it reduces pinking – that is, a pink colouring that some lettuce forms in the butt and ribs of its outer leaves. Again, this is another positive outcome for the fresh produce sector at large, because reducing food waste represents another key goal of the food industry.

What happens next?

So, could this system work in a commercial environment? According to Dr Monaghan, the ideal system would be one where growers could adjust their irrigation according to the ‘zones’ that have been identified in their field.

However, whilst conducting the trial researchers had difficulty applying variable water pressure with the machinery they were using. The difficulty was in the booms – the part of the machine that actually sprays out the liquid – because the water pressure within the booms varied.

“We are nearly there - The answer is in the engineering,” Monaghan states, adding that the “elephant in the room” is Britain’s weather. “The challenge is that it rains,” he points out. “You cannot guarantee that you will get return from this kind of irrigation at all points in the season. Drier summers are predicted for the UK, but we could have much wetter summers as well. We are in an unpredictable scenario.

“Our Spanish colleagues say it’s harder to do precision irrigation in the UK because you cannot predict the weather here.”

Hopefully, improved weather forecasting services for the horticulture sector will help growers. Meanwhile, Dr Monaghan says a new three-year study, part of the Horticultural Development Company (HDC) studentship programme, is continuing the good work.

He explains: “We have a PhD student (Yara Boubou) who is asking: ‘Is it just moisture – or is it moisture and nutrition?’ So we are now looking at other aspects as well as water.”

Dr Monaghan tells Produce Business UK that the majority of lettuce growers are using some form of variable input – such as applying a bit more fertiliser in certain parts of their fields. He says: “They have improved the uniformity of [their] irrigation but they have yet to move to the next stage.” Hopefully, a useable system will be available to growers in the not too distance future – and the benefits of such a system will no doubt be felt throughout the whole supply chain.”

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