Plants fall into "ruderal," "circumventor" or stress-tolerant categories. Ruderals, including annual and perennial plants, create many seeds that reproduce quickly. Circumventors, including both plants and trees, are less prolific in seed production, more tolerant of their surroundings than ruderals and have some protection against disease and pests. Stress tolerant plants and trees thrive with little water and no fertilizer. They also protect themselves from predators with bark and resins that kill certain insects and inhibit diseases.
Synthetic or chemical fertilizer is created from liquid ammonia. It is inexpensive to produce and has revolutionized American agriculture. Between 1950 and 1975, total farm product output increased by more than 50 percent and farm labor hours decreased by 60 percent. Mechanization, genetic improvement and increased use of chemical pesticides and fertilizers created this change. Environmental science now identifies atmospheric nitrogen overload from synthetic fertilizer as a primary cause of global pollution, according to a report by the World Resources Institute.