At one time during history, there were no chemicals in farming. Ladybugs are natural pest controllers. Manure can fertilize quite well. Are there any tractors and combines nowadays that don't burn those dirty fossil fuels?
Man's sheer numbers: major environmental threat. Man has overpopulated well beyond ecological soundness.
more people = more pollution, more waste, more litter, more consumption of resources, some rare and/or non-renewable, more erosion of land, greater loss of habitat for wildlife, endangerment and extinction of vital species, greater dependency on destructive mass agriculture which erodes the land, pollutes the air with dust and relies on unhealthful chemicals and genetic engineering
Humans are too numerous for their own damn good.