In growing maize for biogas, the crop that does most damage to the soil is being specifically exempted from the rules

In principle it's a brilliant solution. Instead of leaving food waste and sewage and animal manure to decay in the open air, releasing methane which contributes to global warming, you can contain it, use micro-organisms to digest it, and capture the gas.

Biogas from anaerobic digestion could solve several problems at once. As well as a couple of million tonnes of sewage sludge, the UK produces between 16-18m tonnes of food waste, much of which still goes into landfill. Farms in this country generate around 100m tonnes of animal manure and slurry, a major cause of water pollution. It could all be processed in digesters. A tonne of food waste can produce about 300 kilowatt hours of energy: the UK's discarded food, the renewable industry says, could generate enough electricity for 350,000 households. The residue can be used as fertiliser.

It was also a brilliant idea to turn waste chip fat into biodiesel. But the incentives to produce biodiesel, often justified by the claim that they would make use of waste, have created multiple ecological disasters. They have encouraged farmers to feed cars rather than people and financed the conversion of rainforests in Indonesia, Malaysia and West Africa into oil palm plantations, driving orangutans and many other species to the brink of extinction. In most cases, biodiesel, as a result of the changes in land use, has much higher greenhouse gas emissions than the fossil fuel it replaces.

A tractor loads maize silage into hoppers. Photograph: David Levenson/Getty Images

Biogas is now going the same way. Provide the money to do the right thing and if you're not careful it will be used to do the wrong thing.

Part of the problem is that there's not enough money on offer to make the conversion of waste alone economically viable. That's because the yields of gas are often quite low. For example, slurry from cattle and pigs produces only 15 to 25 cubic metres (m3) of biogas per tonne of material. Purpose-grown crops are much more productive. Grass silage produces 160-200m3 per tonne, while silage made from maize (which in the US is called corn) generates 200-220m3, and potatoes 280-400m3.

Economic modelling commissioned by the government tested eight different mixes with which farmers could feed an anaerobic digester, to try to work out which were profitable. All of them included grass, wheat, maize or potatoes, and in some cases the models specified a higher tonnage of these specially grown crops than the waste the digesters are supposed to process. As maize has both a high yield per hectare and a high yield of biogas per tonne, it has become what the farming press calls the biogas "core crop". There could scarcely be a better formula for subverting everything biogas is supposed to achieve.

John Jackson, a farm manager, inspects the gas compressor area. Photograph: David Levenson/Getty Images

The first and most obvious problem is that it means taking land out of food production. According to Farmers' Guardian, a biogas plant with a capacity of one megawatt, "requires 20,000-25,000 tonnes [of maize] a year, accounting for 450-500 hectares of land".

Consider, when you read that, that the average capacity of an offshore wind turbine is four megawatts. Four hundred and fifty hectares of land or one concrete pillar in the seabed – can there be any doubt about which is the better option?

And this isn't any old land, but prime arable fields. Another article in the same magazine reveals:

"Maize-growing in the UK is being concentrated into more intense and specialised regions while some growers in marginal areas have opted out of the crop. This trend is being compounded by the rapid growth of maize for biogas production, represented by the high dry matter, later-maturing varieties which are unsuited to marginal sites."

It reports that the area of maize being grown for biogas in the UK has trebled to 15,000 hectares in the past two years alone, and is likely to rise to 25,000 hectares next year. This is an astonishing rate of growth. If, as the National Farmers Union (NFU) advocates, 1,000 medium-sized biogas plants are built by 2020, and maize supplements the slurry and manure they process, that will mean the use of between 100,000 and 125,000 hectares.

Biffa's anaerobic digestion plant in Cannock, England. Photograph: Diensen Pamben/Getty Images

So when you hear the NFU insisting that we cannot remove even the most barren land (such as hills which can support only one sheep on every two hectares) from farming for the purpose of rewilding and flood prevention because that might have an impact on our food supply, remember that the same organisation wants 100,000 hectares of the best land in Britain to be taken out of food growing and used instead for gas.

But it gets worse, because maize farming could scarcely be better designed to cause soil erosion, compaction and run-off, which threaten the fertility of the land, the health of our freshwater ecosystems and the homes at risk from flooding.

As the government department responsible for regulating farming points out:



"Soils are most susceptible to erosion when they are left exposed with little or no vegetation cover … maize is susceptible to soil erosion since ground cover is slow to develop after sowing, and the soil surface can be poorly protected until mid-summer."

After harvesting, maize fields tend to be left bare until the following spring, without the dense stubble that helps to protect the soil after other cereal crops have been cut.

A paper in the journal Soil Use and Management reports that 75% of the maize fields it sampled in south-western England "were found to have degraded structure generating enhanced surface-water run-off".

That means more flooding, more soil loss, more siltation, more trouble all round.

Maize, which comes from the sub-tropics, requires a great deal of fertiliser and pesticide to grow well in this country, and much of this washes off with the soil, into the rivers and the sea, helping to wipe out many of the animals they harbour.

So you'd expect the government to apply stringent rules to the way maize is grown. But only if you had no idea how the Department for the Environment, Food and Rural Affairs (Defra) really operates. What it has done – astonishingly - is to remove maize from the rules which govern other crops.

I have stated this before, and the NFU claims it isn't true.

So let me direct the NFU to a document which its officers should know like the backs of their hands: the Guide to Cross Compliance in England, 2014. This contains all the rules that farmers must abide by if they are to receive their subsidies. The guide makes just one mention of maize, and it goes as follows:

"Post-harvest management of land. If your land has carried a crop of oil-seeds, grain legumes or cereals (other than maize) which has been harvested by either combine harvester or mower, then: You must meet one of the following conditions on that land …"

In other words the crop that does most damage to the soil is specifically exempted from the rules designed to protect the soil. I have asked Defra six times for an explanation, and it has failed on all of these occasions to provide one. My conclusion, which holds until it deigns to provide an answer, is that maize could not be grown in this country if it were subject the rules that apply to other crops.

If you want to know where we might be heading, take a look at Germany. Two years ago Der Spiegel reported:

"Subsidies for the biogas industry have led to entire regions of the country being covered by the crop … Plans called for transforming Germany into a bio-wonderland by peppering it with numerous small eco-power plants. What resulted was a revolution in the fields, a subsidised gold rush – and an ecological disaster. Corn [maize] is now being grown on 810,000 hectares in Germany."

As a result, "for the first time in 25 years, Germany couldn't produce enough grain to meet its own needs."

On some soils, the German Nature and Biodiversity Conservation Union estimates: "growing corn releases 700 grams of carbon dioxide into the atmosphere for every kilowatt hour of energy it produces. And this happens for years on end. This is comparable to the carbon-released-to-power-produced ratio of some coal-fired power plants."

In other words, it's the biodiesel story, all over again.

Because the anaerobic digestion of waste food and slurry makes sense, I don't want to see a biogas moratorium imposed. But I would like to see a ban on the use of all purpose-grown feedstocks. To make biogas viable, this ban would have to be accompanied by an increase in the subsidies available for converting waste. Yes, that means extra expense, but it's got to be a better deal than trashing the food supply, the soil, the rivers and our living rooms – all in the name of protecting the planet.

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