Dr Catherine Keeling, Warwick Crop Centre

Published June 2013

“It smells a little, don’t you think?” I venture cautiously.

“Hold the mesh tighter. Ready?”

“I think so…”

A thick torrent of warm liquid gushes out of the pipe. For a moment it looks like our DIY filtration mechanism will hold, but, alas, overwhelmed by the weight of this charmingly fragranced mixture, the mesh caves in. Splash back. I’m covered. It’s in my wellies. It’s running down the cuffs of my gloves.

I’d been warned not to screw the container lids on fully in order to allow the ammonia to escape from the cooling digestate. Car windows down on the way home then.

We’d been collecting the liquid by-product of the process of anaerobic digestion (AD). AD is the breakdown of biodegradable material in the absence of oxygen by micro-organisms. It produces biogas, a renewable source of energy that can be used to generate electricity and heat for on-site operations, in this case at an organic dairy farm, with excess electricity being exported to the National Grid. The liquid by-product (digestate) is rich in plant nutrients and generally used as a fertiliser for field crops.

Feedstocks for AD can include anything that is derived from plant material: manure, crop residues, compost, food waste, paper and waste water. In short, it turns waste into a resource.

A need has been identified to explore appropriate markets for digestates, since the potential for this technology to expand in the UK is significant. The project I’ve been working on sought to explore the potential of one such market: table top strawberry production, where substituting conventional feed for digestate would reduce the quantity of synthetic fertilisers used. Fertilisers are made from fossil fuels and cost money. Potential wins, therefore, in terms of improving both the environmental and economic sustainability of strawberry growing.

Sounds good, right? What could go wrong?

Well, strawberries have a reputation for being difficult to grow. They can be sensitive and fussy eaters; made more so when grown in bags rather than soil. Soil-grown plants have larger areas available for their roots to occupy, and the complex nature of the soil helps to buffer the root-zone against deleterious changes.

Secondly, table top strawberries are fed via drip irrigation systems. Grower folklore warns us gravely against using non-synthetic feeds in such systems as the lumpy organic bits can cause blockages.

Thirdly, and perhaps most alarmingly, analysis of the digestates confirmed our fears that 100 per cent of the nitrogen that they contained was in the form of ammonium; which would be potentially toxic to the plants and lead to a very acid root-zone.

Fortunately I like a challenge. Using the digestate analysis as our starting point we figured out a 'recipe' that might turn the digestates into something more appealing to a strawberry plant.

Firstly, we had to dilute the digestates. All six of the digestates that we looked at contained far too much nitrogen to feed directly to the plants without burning them.

Secondly, we attempted to balance out the ammonium-nitrogen with a little nitrate-nitrogen. In conventional strawberry production, growers supply their plants with additional nitrogen and potassium during flowering and fruiting. We took advantage of this fact and supplemented the digestates with potassium nitrate. We also added in some phosphorus and an extra sprinkling of some of the micronutrients.

Finally, we plumbed in a coarse filter to take out the worst of the lumpy bits, whilst leaving the majority of the suspended organic matter untouched.

Success? Well, yes. Three of the six digestates that we looked at did as well as the synthetic feed, in terms of both fruit quality and quantity. Plants grown with the other three digestates had a bit of a struggle in taking up enough calcium, not because these digestates contained less calcium, but because the way in which root-zone pH responded to these digestates reduced the availability of the calcium for plant uptake. This is a situation that could easily be remedied by tailoring the starting pH of the system to the individual digestate. We also conducted regular taste testing which actually revealed a preference for the flavour of the digestate-grown strawberries (yes, really!).

Alright, so two of the digestates blocked the drippers (meaning that four of them didn’t), and consumers might take some convincing but this was only a preliminary look at the feasibility of growing strawberries with digestates. The yield data and happy taste testers confirm that there is real potential. Most striking for me, however, is the remarkable alchemy that we could start with something that you wouldn’t want to share a car with, and turn it into something so delicious.

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Dr Catherine Keeling studied as an undergraduate at the University of Sheffield, before completing an MSc in Environmental Sustainability at the University of Edinburgh. For the next few years she worked as an organic grower in mid-Wales, where she ran a market garden, supplying salad vegetables to local businesses. Her PhD looked at the use of saline irrigation water in UK horticultural production, investigating the saline tolerance of some novel or underutilised crops. She joined the Warwick Crop Centre for two years in 2011.

Image: Yummy by José Eduardo Deboni (via Flickr).