First we will need some sulfur, referenced in the Bible as brimstone. Elemental sulfur can be found in nearly pure form in the ground, and is an essential element for all life on earth. It's the oldest known pesticide in use. Homer described its "pest averting" qualities 3,000 years ago. It's pretty cool stuff; it can be used as an insecticide and fungicide, is responsible for the smell of skunk and garlic, is part of gunpowder, and burns with a blue flame and melts into a blood-red pool of fluid. If we can't find any pure sulfur, we can always find some fools gold (pyrite), abundant in Iowa, and allow it to rust. The rust will be sulfate, a form of sulfur.
Now we need some saltpeter (potassium nitrate), which was described by an Arab chemist in the year 1270. To make our saltpeter, we'll create a compost pile using campfire ashes and straw, which we will cover from the rain and keep moist with urine. We'll mix it up often and after a few months we'll wash the resulting chemical salts off of the straw, filter the water through more ashes, and air dry in the sun. Congratulations! You've just manufactured a wonderful fertilizer that is not permitted for use in organic farming! If you want to use bat poop or scrape the walls of a saltpeter cave in Chile, then you can use it. But if you made it like this in your backyard and fertilize your garden, your garden is no longer "organic."
Now we will make some sulfuric acid, which has been studied for over two thousand years and used to be called vitriol. To make vitriol, we need to burn our sulfur and our saltpeter, collect the smoke and combine it with steam. This was the method used in 1736 London to begin the first large-scale production of the acid. Alternatively, you could chop up a bunch of onions and collect their fumes (propanethiol S-oxide) and mix that with steam. Congratulations! You've just manufactured another chemical that is not permissible in organic farming!
Now that we have sulfuric acid, let's make some pure iodine. First, we'll collect a pile of seaweed and burn it. We'll repeat this a few times until our fire pit is coated with a residue. To remove the residue, we'll dump a bunch of our sulfuric acid on it, and watch a giant purple cloud of iodine vapor appear, and begin to crystalize on everything around us. We'll collect these crystals, which are pure iodine, discovered in the same way in 1811.
Next we'll need to make up a quick batch of methanol (wood alcohol), used by ancient Egyptians in the embalming process. Pure methanol was first produced in 1611 using the wood of the boxwood tree. To make the stuff, we need to bake some wood and collect the vapors. A clay and bamboo distillation setup should work very well. If you spray plants with methanol, it will protect them on hot days and increase yields. But be careful, methanol is poisonous. Drink a little (10 ml) and you'll go blind. Drink a bit more (30 ml) and you'll die. It is found in small amounts in fruit juices, beer and wine. It is toxic via ingestion, skin contact, or inhalation. Congratulations! You've just made a toxic poison that is allowed for organic farming! Spray away to your heart's content, just don't breathe the vapors!
Next we'll need red phosphorus, discovered in 1669. First, to make white phosphorus, we'll need to evaporate a bunch our urine and collect the residue, which will be a white, flammable, waxy substance that glows in the dark. At this point it's pretty volatile, nasty stuff. If burned and inhaled over time, it will make your jaw fall off. To stabilize it into red phosphorus, which is mostly harmless, we need to either bake the stuff at over 482 °F, or simply expose it to sunlight until it turns red. We'll choose the sunlight method. This is another element that is essential to all known forms of life; no animal or plant can live without it, and a normal person excretes about 1-3 grams of phosphorus daily. Red phosphorus is used to make matches and methamphetamine, and is not permitted in organic farming, even though it is another lovely fertilizer.
To make our final product, we need to mix three of our basic ingredients: iodine, methanol, and red phosphorus, which will produce a chemical reaction yielding methyl iodide. To extract the it from the mixture, we'll throw it back into our clay distillation apparatus and heat gently, collecting the steam and allowing it to condense into a colorless liquid. Methyl iodide is also naturally produced and emitted by rice plantations in small amounts, so if wanted to get really creative we could try to collect it from that source. Obviously, this stuff is not permitted in organic farming either. By now I hope it is becoming somewhat obvious that "organic" farming is a somewhat arbitrary term, and the "chemicals" that it bans have been around for centuries and don't seem so scary when you know how to make them.
Now when we want to grow strawberries or other fruits that are easily attacked by diseases and bugs and pests, we first mix some of our colorless fluid into the soil. It won't hurt the strawberries, but it will stop those pests from eating all the fruit and starving the village. Just don't drink the stuff, about six grams of it will kill you, but as you can see, it's a natural product that can be produced in my backyard with simple tools from rocks, trees and urine.