The last time we were in Wessex, I showed that its denizens circa 2039 could probably feed themselves quite comfortably using organic farming methods with 20% of the population concentrating largely on neo-peasant subsistence farming using 40% of existing lowland farmland, and the remaining 80% of the population fed by larger-scale, more cereal staple oriented farming from the remaining 60% of the farmland, plus a bit of upland grazing.

However, as it stands that scenario does depend on a fossil energy-intensive ‘business as usual’ approach on the large-scale farms. It seems worth pondering an alternative, zero fossil energy scenario. Here we begin to exceed even my own generous comfort zone for idle speculation about the future – if there’s no fossil fuel use in Wessex farming in 2039 (or beyond), what might be the social and economic correlates? Probably not one with 80% of the population still happily residing in towns and working as video game programmers, conservatory salesmen or whatever.

Still, I don’t propose to worry about that too much in this post. For now, let’s just consider the farming side of it, and see if we can find another way to power the food production for 80% of Wessex’s population.

That immediately plunges us into a speculative debate about the shape of the future energy mix which could go on until…well, 2039. So here I’m going to curtail it brutally by making the following doubtless highly debatable assumptions. I’m going to assume that there won’t be enough renewably generated electricity to power electric, fuel cell or electro-synthesised hydrocarbon tractors. I’m going to assume that none of the magic, much-touted next-generation or generation-after sources of limitless clean power such as thorium or nuclear fusion have come through. And I’m going to assume that wood methanol isn’t a viable source of agricultural energy, as a couple of people have suggested to me that it might be. The way I read the runes on that one is as follows:

You get about 27 litres of methanol from a tonne of wood, and you get about 3 tonnes of wood from a hectare of managed woodland, so you get about 80l of methanol from a hectare of woodland. Methanol has about half the energy density of diesel. You need about 100l of diesel (so 200l of methanol) to farm a hectare of arable land each year. I’ll assume you need about a quarter of that to farm a hectare of permanent grass, minimally, about as much again to manage the rest of the production and transport economy around food. That works out at about 1.2 million hectares of managed woodland to service 1.8 million hectares of farmland, which would exceed the land area of Wessex by nearly a third (while also neglecting the energy needs of the woodland management). Methanol can be made from other carbon-rich waste, but it seems to me a stretch to think it could be a major agricultural energy source unless anyone can provide some radically more promising figures.

Another suggestion I received was to put aside my West Country obsession with cows and make methane instead of milk from the grass via anaerobic digestion. Now, I’ve always regarded these straight-to-methane schemes as a dastardly vegan plot to deny me the froth I so badly need on my morning cappuccino, but after crunching a few numbers I’ve got to admit that the plan has something to commend it. In fact, the numbers seem to stack up so spectacularly well that I feel I must have made a terrible error somewhere, so let me run through my arithmetic in some detail with the hope that someone can either corroborate it or else point out the error of my ways.

Let’s start by calculating how much energy we need to run our Wessex food system. I’m going to assume that we need 100 litres of diesel per hectare on the farm for arable operations, and 25 litres for grassland management. Then to fuel the entire food economy from farm to fork, I’m going to assume we need another 200 litres of diesel equivalent per hectare (for both arable and grassland) – an assumption loosely based on the emissions scenarios in Tara Garnett’s Cooking Up A Storm. Diesel has an energy content of 38.6 MJl-1. So if we take our 166,000 ha of cropped arable at 300 l/ha diesel and our 795,000 ha of permanent grassland and arable ley at 225 l/ha and multiply that sum by 38.6 MJ we get a total energy requirement of about 8.8 billion MJ (or 8.8 GJ if you prefer).

On the supply side I’m assuming 20 tonnes of fresh silage per hectare1 (or 5.5 tonnes dry matter), grown organically (average conventional yields are more than double that), and 160m3 of biogas per tonne of silage2, with an energy content of about 22 MJ/m3 – so that works out at about 68,000 MJ/ha. If we take a quarter of our permanent pasture – some 223,000 ha – and set it aside for silage as biogas feedstock, that’ll give us 15.1 GJ of energy, which is nearly double our energy requirement. As I understand it, methane-powered tractors are already a reality at engine efficiencies similar or above those of conventional diesel, and though the biogas coming out of the digester needs a bit of refining, the process efficiency is quite high. Embodied energy of plant construction seems to turn out at around 10% of total energy output3, so the overall energy costs seem manageable.

Obviously we need to re-run our food productivity figures in the light of taking out a quarter of the permanent pasture (hopefully rotating cows over it and returning some or all of the digestate to it will keep the silage production sustainable). But since this part of the farm system otherwise produces relatively low-output grass-fed cows, the overall loss of productivity may not be too severe. And so it proves – removing 25% of the permanent pasture for biogas drops the supply/demand ratio for food energy from 1.07 to 0.99, with all the other nutritional ratios remaining >1. An energy ratio of 0.99 is doubtless a bit too close for comfort, but it shouldn’t be too difficult to find an extra bit of productivity. The lazy way would be to plough some more permanent pasture for wheat – about 22,000 ha or 3% of the total permanent pasture diverted to wheat would restore food energy productivity to the 7% surfeit we were experiencing with fossil diesel (call it 6% to make provision for a ley). But there would be other more elegant, if more labour intensive, ways of doing it. And remember that I’m making a lot of conservative assumptions about yields.

Originally I’d been thinking in terms of biodiesel from oilseed rape as the way we’d have to go in a fossil-fuel free Wessex. That method produces almost, but not quite, as much fuel energy per hectare as biogas from organic silage, but only by devoting a big chunk of precious cropland to the oil crop. And the rape would have to be grown conventionally, using synthetic fertiliser and pesticides, with additional energetic and environmental implications. An advantage of rape is that the meal or press cake from the oil extraction process yields a high energy livestock feed, which partially compensates for the loss of cropland. But rape just doesn’t seem to me to stack up as well as biogas – particularly since it looks like I can keep enough cows to get my cappuccino in the morning and still have fuel to start up the tractor. Another advantage of anaerobic digestion and biodiesel over the photovoltaics we were discussing in my last post is that the basic engineering technologies in both cases seem simpler, which perhaps gives them a better chance of making it through the climacteric as per the previous discussion.

Well, there you have it. As I’ve said many times before, I’m not trying to suggest in this exercise that it would a simple or even a likely thing for a future Wessex to feed itself, especially if it were as energy-constrained as the one I’ve been discussing here. I don’t want to come over all ecomodernist (not that ecomodernists have much time for such down home energy technologies as anaerobic digestion). But my proposition for discussion is that it may be a possible thing.


  1. See, for example, the Organic Farm Management Handbook, or this.

Photo credit: December from the Shephearde’s Calendar.