History

• Until the mid-20th century, when a method for manufacturing nitrogen was discovered, most fertilizer was organic. The switch to lower-cost chemicals enabled farmers to grow more food, but the downside was a decline in soil health, as well as excess fertilizer being washed into waterways.
  
  

Formulation

• The nutrients in both types of fertilizer are much the same. The differences lie in their source, quantity, availability to plants and long-term effects on the microlife of the soil. Organic fertilizers get their N-P-K (nitrogen, phosphorus, potassium) from plants, animal waste and natural minerals. They also provide micronutrients, such as boron, copper, iron and manganese. Synthetic fertilizers contain nutrients from many sources, including fossil fuels, processed to remove any substance other than the pure nutrient.
  
  

Amendments

• Although they provide nutrients, substances such as composted manures, peat moss and compost made from yard debris and vegetable waste are considered amendments, not fertilizers. They are invaluable in improving the structure of the soil -- loosening heavy clay or thickening sandy soils -- but their N-P-K ratios are variable.