The average citizen of a developed country today effectively consumes 66 barrels of oil a year, such is the dependency of our modern food systems on fossil fuels.
Chemical fertilisers and pesticides are derivatives of fossil fuels, including oil and natural gas. The use of fossil fuels is the reason the earth is experiencing dangerous climate change.  Aside from transport- cars, trucks and aeroplanes – agriculture is the most fossil fuel intensive industry. We use, in the industrial world, about 10 calories of energy for every calorie of food we produce, process, and transport. … We’re going to have to transform our entire agricultural system very quickly if we’re going to avert a global food calamity. 
The use of oil is essential to modern agricultural practices. Oil is used in the pumping of water for irrigation,the machinery used on the farm and the long road of transport from the farm all the way along the supply chain to our plate. Oil is so interlinked with our food system that the price of food is closely correlated to the price of oil. Oil prices are a key contributing factor to the increase in the price of food. We saw a spike in food prices in the 1970’s, which correlated with the oil crisis at that time and again during the food riots of 2008.
Farmers in developing countries are the hardest hit by rises in oil and subsequent fertiliser costs because they have just begun the switch to Western, oil intensive systems of food production. This is illustrated by the fact that China has increased its fertiliser usage by 44% in recent years, India 33%, Pakistan 61% and Brazil 137%. In contrast, Europe has cut its fertiliser use by half15 as farmers return to organic and agro-ecological farming methods. 
Over 95% of all the food grown in (Britian) is totally reliant on synthetic fertilizer. [4 ] We’ve used fossil fuels, essentially, to grow plants in soil that is otherwise dead. That works as long as we have the cheap fossil fuels. …
When we don’t have the cheap fossil fuels, we’re going to need living soils once againa and that living soil is something that requires time and care to build. 
This reliance on oil is also clear in the United States where most food crops eaten … are produced on large land tracts planted in monocultures (single crops). … Chemical fertilizers, irrigation, pesticides, herbicides, and new seed varieties were the immediate stimulants of the 20th-century yield increases, but petroleum was, and continues to be, their essential energy source. Petroleum contributes most ingredients for manufacturing the pesticides and herbicides essential for controlling pests and weeds that thrive in monoculture production and supplies the energy to mine, process, and deliver phosphate and potash to farms. (Natural gas is the primary ingredient in most nitrogen fertilizers.) Petroleum also supplies energy to manufacture and operate the farm equipment that prepares the soil, sows and harvests crops, and irrigates fields. Finally, petroleum transports agricultural inputs such as pesticides and feed, agricultural products, and food
According to the International Energy Agency, peak oil and gas are due in the coming decade. These spell scarcity and soaring prices in the primary nutrients – nitrogen (N), phosphorus (P) and potassium (K) – that sustain all advanced farming systems worldwide. In food production there is no substitute for these three nutrients: they are as essential to plant growth as water and light.
The energy to power the world’s tractors, trucks, trains, planes and ships that move the food cannot come from the farm in the form of biofuel: to do that would reduce world food output by 10-30%, at the same time as we need to double it due to population growth and ever growing expectation of improvmentsin lifestyles.
Furthermore, oil facilitates globalization of the food supply. In 2005, the United States imported 44% of fruits and 16% of vegetables, including a significant portion of those eaten when local produce is available. Even within the United States, food travels long distances. One Iowa study found that conventional produce traveled on average 1494 miles to institutional markets. As oil prices rise, long-distance transportation will increasingly become a luxury, leading to substantial changes in food distribution networks to supply healthy diets. 
References: 1. http://www.sustainabletable.org.au/Hungryforinfo/Conventionalfarmingdamagetheenvironment/tabid/117/Default.aspx (c. 2011)
2. Richard Heinberg in ‘A Farm For the Future’ – the full length film on peak oil, farming & permaculture. Rebecca Hosking & Tim Green (February 20, 2009)
3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3154242/ (September, 2011)