(Written by request and stemming from a discussion in Jerome's latest peak oil diary.)
One aspect of peak oil that is generally ignored by those who believe biodiesel will save the world is that of food. Our current food supply is threatened by many factors, including genetic manipulation and monocultures, but here I will deal only with our food supply as it relates to peak oil, and how the unworkable "magic elixir" of biodiesel will only make the issue worse.
Part the First: Biodiesel and why it can't solve the problem
Loss ratio -- According to sources like Cornell University and the peak oil documentary "The End of Suburbia", biodiesel is subject to an energy loss ratio, meaning that it takes more joules of energy than you get from the finished fuel. True, some of this energy can be in the form of wind or solar or other alternative sources of energy, but even if it's only a 2:1 loss ratio of fuel only, you're still losing. It's like spending $2.00 to buy $1.00... it just doesn't make any sense and is clearly not sustainable.
A problem of scale -- Someone in a Yahoo discussion group I'm in actually got mad and said I had "a defeatist attitude" when I tried to explain why his idea of growing rapeseed organically with horses and grinding it into oil with windmills couldn't solve the world oil crisis. He gave me the example of a 1500 acre farm he knows of that is doing just that with half its growing land, meaning that 750 acres is being used to produce enough biodiesel for that farm. So let's think about that for a moment... 750 acres to power a few farm vehicles. How many acres does the average person live on? Now multiply that by 6 BILLION people. As you can see, there is simply not enough farm land in the world to produce the amount of oil used yearly by the entire world... even if you cut the amount used in HALF. As it is now, we use four centuries worth of plants and animals in the form of fossil fuel in one yearat our current rate, showing that it's impossible to create the sheer volume of oil needed from one year of plant crops alone.
How does this related to food? Ignore the loss ratio for a moment and imagine if every possible bit of acreage in the entire world was used to grow crops for conversion to biodiesel. Where would our food come from? Mars? We are already using 40% of the earth's surface for agriculture and there is little room for expansion. But this has problems related to oil as well...
Part the Second: Soil depletion and why you are eating oil (and gas)
I'm sure you've probably seen images of the Dustbowl, that time during the 1930s when millions of tons of agricultural topsoil were literally "gone with the wind". It blew away and was washed into the sea when the rains finally did come. What used to be part of the Great Plains were subjected to depleting agricultural practices like deep plowing, lack of crop rotation and aggressive weed suppression, leaving nothing to hold the soil in place when the drought came. This happens on a yearly basis on a conventional farm, but in slow motion: 1.8 BILLION metric tons of soil is lost every year from US farms" (1.94 meg .pdf file).
How to combat the annual reduction in soil and soil fertility? Why, add more chemicals of course. Artificial fertilizers and pesticides are used by the ton on conventional farms just to get something to grow. Where do all these chemicals come from? Primarily from petroleum and natural gas.
Some fertilizers are mineral salts, like the familiar "Miracle-Gro". True, they may be more "natural", but salt buildup in the soil makes it harder and harder to grow anything. In general, plants don't like salty soil, the exceptions being native things that grow by the ocean or by brackish deltas naturally. Crops cannot grow where there is salt buildup, and of course more chemicals are thrown at the problem to help "solve" it.
12 billion pounds of synthetic fertilizers and one billion pounds of pesticides are used on US crops annually. Most fertilizers and pesticides are derived from cracking petroleum or methane (natural gas) Basically, we've turned oil into food.
What has decades of chemical use done to the soil? It's become a sterile moonscape, unable to support much more than a few tough weeds if left to its own devices. It's a sterile powder, likely to blow away if it gets too dry, unable to nourish crops on its own, devoid of animal and microorganism life that helps to pollinate, protect or feed the plants.
The vast majority of our agricultural land that feeds us is as dead as the moon, kept alive with the feeding tube called agricultural chemicals, which are derived from oil and gas. These are both at or past peak production.
Yes, the land can be brought back. Even volcanic ash and grit can allow plants and animals to exist, even thrive, given enough time. But it's time we don't have. Growing organically (as opposed to conventional farming, which uses chemicals) means that the soil must be productive enough that artificial chemicals aren't needed. This takes time. Green manure crops that are grown and then tilled in to enrich the soil must somehow be coaxed to exist on the desolate powdery landscape. Microorganisms, worms, bees, spiders, and all the other creatures in the web must somehow be coaxed back to the toxic soil. Natural fertilizers like manure must be collected, trucked in and spread. All this takes time. In that time, no crops can be grown while the soil is being repaired. Some things could be grown, perhaps beans or alfalfa or other legumes that gather nitrogen from the sky and collect it on their roots, but even these might be hard pressed to produce a crop under such stressed conditions. Organic farms can be created where there is only sterile cropland now, but it takes years to even begin to approach that level of production while also trying to rebuild the ecosystem of the soil. We're talking 3 to 6 years to achieve it, and what would the farmer do for money in the mean time? No crops means no money for the farmer during that 3 to 6 years... only a huge megafarm that could afford to take hundreds of acres at a time out of production, and it's pretty unlikely that most of them would do that, especially considering how many are owned by the very chemical companies that have killed the soil (such as Monsanto).
We've painted ourselves into a corner, and I'm honestly not sure how US agriculture can ever get out of it once the oil is gone. The picture isn't pretty, and I think the new Victory Garden will be one of desperation, planted in backyards and public spaces so that people don't starve. Because right now we're eating oil... and the well is drying up quickly.