The Amish don't buy bagged fertilizer. Everything needed is already on the farm.
Manure
Cow: rich in nitrogen and potassium. Compost 6-12 months (fresh burns roots). Horse: drier, warmer — good for heavy clay soils. Chicken: most concentrated — dilute 1:10 or compost.
Wood Ash
Potassium + calcium. Raises pH of acidic soils. 1-2 kg per 10 m2. Not for potatoes or blueberries (they like acid). Only from wood, not coal.
Bone Meal
Phosphorus + calcium. From butchering bones: burn, grind to powder. Apply at planting — stimulates root development. Especially for bulbs and root vegetables.
Green Manure (Cover Crops)
Clover, alfalfa, mustard — sow, grow, plow under. Nitrogen + organic matter. Amish seed fields with clover after harvest — by spring, soil is enriched.