Nitrogen-phosphorus-potassium (N-P-K) fertilizers are manufactured using ammonium phosphate to which potassium (usually potassium chloride) and water have been added. Additional nitrogen and other nutrients also can be added.
Special N-P-K formulations (9-18-9 or other grades) are made by reacting furnace-grade phosphoric acid with potassium hydroxide. This gives a neutral pH P-K product. Nitrogen is then added, usually as urea. Small amounts of ammonia may sometimes be used for part of the nitrogen. The resultant product is high quality; however, because of the ingredients used in manufacturing, the cost is much greater than with regular fertilizers.
Caution must be indicated when these fertilizers are applied with the seed. The salts of the nitrogen and potassium fertilizers can damage seed by dehydration. The potential damage per unit of “saltbased nutrient” is similar for all fertilizer materials.