This is a guest post by Patrick Déry and Bart Anderson. Patrick Déry is a physicist, energy, agriculture and environment analyst and consultant in Quebec, Canada. Bart Anderson is a former reporter, teacher and technical writer; he currently is co-editor of Energy Bulletin. Peak oil has made us aware that many of the resources on which civilization depends are limited.
But oil production is not the whole story. Nutrients like nitrogen and phosphorus were also required for the “Green Revolution”.
Nitrogen is present in large quantity in the atmosphere (78% of its composition). The Haber-Bosch process for obtaining nitrogen uses one percent of all energy consumed by humans . Nitrogen can also be fixed in the soil using micro-organisms such as rhizobium and azotobacters. If there is sufficient energy, nitrogen will be available.
Phosphorus may be the real bottleneck of agriculture. 
Population growth was only possible because we found phosphorus deposits and cheap energy to extract, transform and transport it to farms. When we plot data of world population versus world phosphate production, we find a significant correlation.
What does this correlation mean? Even if we find a real substitute for fossil fuels, it will be impossible to maintain population growth because phosphate deposits are probably in decline. It will be impossible to maintain an agriculture without recycling nutrients.