Seedling to salad: Sustainability in UK tomato production
The UK currently grows 95,000 tonnes of tomatoes per year, almost all of this is in heated glasshouses. The number of growers has decreased to <40 as the UK moves towards more industrial scale methods, and I was fortunate enough to visit the Teesside site of the UK’s largest supplier of British tomatoes, APS Group.
The lifecycle of Teesside tomatoes
APS Teesside produces 3,000 tonnes of tomatoes annually in 9 ha of glasshouse. I’m going to give a brief overview of the whole process which has been carefully optimised to minimise waste and inputs. However, a particular point of interest for me is the CO2 consumption, during daylight hours the glasshouses are fed CO2 up to concentrations of 1600 ppm (atmospheric is currently 403), plenty of which is supplied by the neighbouring CF Fertiliser factory with a back-up supply from burning natural gas. A potential application of the biological scrubbing system which I am working on could be to scrub the flue gas from burning agricultural waste from tomato growth which could then be fed back to the glasshouses, creating a more circular economy within the company. A drawback of this is that there wouldn’t be enough fuel for this from the tomato plants alone, but there may be some from wood waste from companies in the local industrial hub of Middlesbrough.
Glasshouses are cleaned and prepared for cultivation. Thermal screens control heat retention in the latter hours of the day reducing heating requirements overnight. 6 week old plants are bought from a nursery in the Netherlands, these are planted in rock wool beds and fed hydroponically. Pictured here is one of the glasshouses being prepared for the winter crop.
The glasshouse is supplied with NPK fertilisers, carefully buffered and mixed. APS go to great extents to collect and recycle rainwater for this process, the system is well optimised for minimising water loss.
Combined heat and power are generated from burning natural gas. Before the CO2 enters the glasshouse the gases must be monitored and scrubbed of compounds which may be damaging to the plants, environment and people working in the glasshouse.
The flue gas is scrubbed using urea to remove nitrogen oxides (NOx), cooled and filtered to remove particulates. NOx cause environmental problems such as the greenhouse effect, acid rain and photochemical smog, therefore there are stringent industrial regulations on release. These gases also reduce growth and photosynthesis in plants and in humans can cause constriction of airways and fluid build-up in the lungs. Particulates are filtered as they can contain heavy metals and toxic hydrocarbon compounds which can pass through the respiratory tract when < 10 µm, particles which are < 2.5 µm can be deposited within the alveoli and particles < 0.1 µm can migrate through membranes and into the blood. Pictured in the foreground, is the NOx analyser post scrubbing and the pipes in which the gas is cooled before filtration.
A gas analyser monitors the CO, CO2 and ethylene content before it enters the glasshouse. CO is dangerous to humans as it competitively binds to haemoglobin and therefore stops O2 uptake. Ethylene is controlled as it causes senescence and early ripening in the plants, which is not ideal if you’re trying to grow large fruit.
The gas permeates through tubular bags along each row, beneath the beds, the concentrations in the glasshouse reach as high as 1600 ppm (atmospheric concentrations are about 405 ppm). The plants are heated by pipes which run along each row above the beds at approximately 40 °C. All inputs optimised are very carefully monitored and semi-automated. Pictured here is a row of tomatoes, the water pipe running above the beds is visible.
The majority of the pest control is done biologically using carefully selected species of insects such as Macrolophus pygmaeus for biological control of whitefly, and Amblyseius sp. for spider mites; some fungicides are also used as there currently aren’t efficient ways of biologically controlling fungal pests. Moths are controlled using sticky traps. Pictured left is a trap.
The glasshouse is stocked with 90 bumble bee hives, supplied by a Belgian company. They are viable for about 5 weeks, during which they bimble around in beetopia, collecting nectar for the hive, completely free of predators and pests. Pictured right is a hive.
The plants grow vertically and are maintained daily by hand. Each worker is in charge of about 25 rows which they access using a cherry picker (pictured left). The view from the top of this is quite impressive.
The leaf litter, fallen fruit and prunings are swept to the bottom to decompose a bit but can’t be left for too long as they become a breeding ground for a variety of insects, some of which may harm the yields. The company with whom I am doing my project (Freeland Horticulture) are planning to collect this waste for composting into topsoil. This is otherwise used for compost for their organic tomatoes and at one of the sites they are converted into by-products by anaerobic digestion, such as bioplastics and leaf fibre cellulose used for making punnets and packaging film.
The result of this highly optimised process is sweet and juicy tomatoes which we can buy in the shops less than 48 hours after being picked. I was lucky enough to take some home with me, they did not last long.
The future of tomato production in the UK
Brits have a voracious appetite for tomatoes consuming about 500,000 tonnes of fresh tomatoes per year, the majority of the 400,000 tonnes imported is from the Netherlands. Domestic production is increasing and will probably continue to in a post-Brexit Britain; with a similar climate to the Netherlands it may be worth considering how we can make improvements on their existing system. Despite fresh, local tomatoes having better flavour, they have a higher carbon footprint in the UK compared to varieties grown in Southern Europe, due to the heating and lighting requirements (see table) (DEFRA, 2009). It’s clear that APS growing methods are well optimised for high yields but my thoughts always draw back to “how can this be made into a circular system?”. The use of waste CO2 from neighbouring industries, recycling water and composting leaf litter are all good steps towards reducing waste, but unfortunately there must be a back-up system during factory closure or low CO2 production to maintain growth rates to meet our plates.
Table 1: Carbon footprint of tomatoes consumed in the UK.
Could biomass burning work?
The green waste from the tomato plants is excellent for compost and topsoil, but there are some neighbouring companies which could provide feedstock for biomass burning. The problem with biomass burning is that it produces much dirtier flue gas than natural gas, so the scrubbing system required would be more complex. In addition to large quantities of particulates, biomass is likely to produce a higher content of volatile organic compounds, some of which are harmful to humans and plant growth. The concentrations of these are still largely unknown and an important part of my current research (Dion et al., 2011).
High yield = high carbon
Unfortunately, optimising the yield of fruit is a highly energy and CO2 intensive process. I would argue that considering the current climate trajectories, it is necessary to factor climate change into economic modelling (Stoerk et al., 2018), and therefore understanding the longevity and sustainability of industrial practices is reliant on accurate modelling of CO2 emissions. Divesting from fossil fuel generation of CO2, heat and electricity would initially mean more expensive tomatoes, but in the long run it would be more sustainable. So how can we cut down the carbon footprint of the tomatoes we eat?
Decouple heat and CO2, this reduces the need for venting the glasshouse in the summer and therefore losing CO2.
For combined heat and power, use responsibly sourced biomass or biogas generated from anaerobic digestion.
Divest from fossil fuels. CO2 generated from biogenic sources such as fermentation is a good alternative.
Seasonal cropping in summer months only would ultimately drive down the environmental cost of tomato production, however this would have negative effects on labour and cost
Eat seasonally! The carbon footprint of tomatoes in the winter can be more than ten times that of the summer crop.
Consume less. The greatest danger to our planet is overconsumption not overpopulation (speaking generally, not just tomatoes).
Don’t worry so much about this. Be aware of the scale of your carbon footprint, eating tomatoes is not a particularly high impact activity, but flying to the other side of the planet is.
DEFRA (2009) Scenario Building to test and inform the development of a BSI method for assessing greenhouse gas emissions from food. Technical annex to final the report.
Dion, L.M., Lefsrud, M., and Orsat, V. (2011) Review of CO2 recovery methods from the exhaust gas of biomass heating systems for safe enrichment in greenhouses. Biomass and Bioenergy 35: 3422–3432.
Stoerk, T., Wagner, G., and Ward, R.E.T. (2018) Policy Brief—Recommendations for Improving the Treatment of Risk and Uncertainty in Economic Estimates of Climate Impacts in the Sixth Intergovernmental Panel on Climate Change Assessment Report. Rev. Environ. Econ. Policy 12: 371–376.
Williams, A.G., Audsley, E., and Sandars, D.L. (2006) Determining the environmental burdens and resource use in the production of agricultural and horticultural commoditites. Main report. Main Report. Defra Res. Proj. IS0205. Bedford Cranf. Univ. Defra. Available www.silsoe.cranfield.ac.uk, www.defra.gov.uk 97 pp.