The high-tech and high yield agricultural systems of today rely heavily on fossil fuel inputs in the form of pesticides, fertilizers, machinery and vehicle fuel, irrigation and crop drying energy, among others. It is estimated that food grains produced by modern agricultural methods contain between four and ten calories of fossil fuel for every calorie of solar energy. Such modern agricultural dependence on fossil fuels closely links the world's fossil fuel resources to the Earth's food supplies and food prices. Energy intensive agricultural inputs already cost the farmer 10 to 15% of the value of the crop produced, and this figure will likely rise significantly as the world's fossil fuel deposits, particularly oil and gas, continue to deplete rapidly. This pattern is being exacerbated by rapid global population growth that necessitates further energy intensive agricultural inputs in order to raise yields and expand arable land. Since fossil fuel reliance is not a sustainable option for agriculture to pursue, agricultural systems will necessarily need to reduce their dependence on fossil fuels in favour of other energy sources.