The fertilizers which contains three kinds of macronutrients essential for plants including nitrogen, phosphorus and potassium are collectively called Compound fertilizer. Accordig to the manufacturing technique it can be divided into synthetic compound fertilizers, formulation fertilizers and bulk blending fertilizers(BBF).
Synthetic compound fertilizer
Synthetic compound fertilizer is a kind of fertilizer with certain nutrient content and ratio manufactured by chemical synthesis or extraction separation. 
the nutrient content and ratio is determined by chemical reaction during processing and chemical constitution of compound formula. Normally synthetic compound fertilizer doesn’t contain organic principle but sometimes there are some impurities or moisture in it, such as diammonium phosphate with 18% nitrogen and 46% phosphorus, diammonium phosphate with 11% nitrogen and 44% phosphorus, potassium potash with 13% nitrogen and 44% potassium, potassium metaphosphate with 58% phosphrous and 37% potassium and so on.
Formulation fertilizer
Formulation fertilizer is a kind of compound fertilizer with multielements which can be manufactured by processing two or more kinds of sole fertilizers again with fertilier granulator. The ratio of the nutrient elements of compound is much steady and fixed according to the requirements of agriculture formula. At present, there are many fertilizer plants producing formulation fertilizer while plants with different size adopt different raw materials, such as some manufacturers adopt phosphoric acid, liquid ammonia as the raw materials for nirtogen and phosphorus and muriate of potash( or potassium sulphate) for potassium to 
produce compound fertilizer with high concentration about 15:15:15, and most plants adopts urea, thiamine and muriate of ammonia as the raw materials for nitrogen, muriate of potash for potassium, potassium sulphate for special fertilizer of watermelon and fruit tree. Fertilizer plants in north usually adopts superphosphate and mono ammonium as the materials for phosphorus source while those in south adopts calcium magnesium phosphate for phosphorus source. On the other hand, the ratio of NPK of compound fertilizer can be adjusted according to the requirements of crops to manufacture different special fertilizers for different crops such as wheat and corn.
Bulk blending fertilizer
Bulk blending fertilizer is manufactured by mixing and blending two or more kinds of one-nutrient fertilizer (usually in granule) according to the requirements of agriculture and farmers. In the mixing process the granules are not damaged and there is no chemical reaction. In the fertilizer warehouses of some countries like America and New Zealand, there are many kinds of one-nutrient fertilizer such as urea, diammonium phosphate, muriate of potash and so on. Pick up different fertilizers according to the requirements of customers and mix them into bulk blending fertilizer using fertilizer mixing machine, then transport the fertilizer to the field after simply packing.
If you are planning to start compound fertilizer plant, Whirlston Fertilizer Machinery recommend Mini Compound Fertilizer Production Line–specially designed for new investors.
