There are two main concerns. Farms now use vast amounts of chemicals, as fertilisers and pesticides, plus increasing quantities of energy, particularly fossil fuels. For every calorie of energy in food that is derived from the sun, between four and ten calories are added from petrochemicals. Increasingly, these artificial components replace the natural systems, including people's skills, for producing food. One of the consequences is that fewer crops are grown in ever larger units of production in order to make 'economies of scale'. The costs in terms of long term fertility of the soil and contamination of the environment are not counted. It is clear that this sort of farming is not sustainable as the resources it uses are finite and will run out in the next 100 years. The concern is that by the time we recognise the limitations, will the earth be capable of providing what we want?