What is Fertilizer?

Wikipedia defines fertilizer as any organic or inorganic material of natural or synthetic origin (other than liming material) that is added to a soil to supply one or more plant nutrients essential to the growth of plants. From a gardeners perspective, fertilizer may contain NPK and minor nutrients (sulfur, magnesium, iron etc). The NPK number is a short form for nitrogen, phosphorous and potassium.

Synthetic Fertilizer

Synthetic fertilizer is also called inorganic fertilizer or commercial fertilizer to differentiate it from organic fertilizer. This fertilizer is either mined from the ground or synthesized by man. It consists of granular material and comes in bags.

Organic Fertilizer
Any fertilizer that originates from an organic source is considered to be organic. Some examples include fish extracts, manure, and compost. It can be purchased in bags or bottles and can be ordered in bulk as trailer loads.

What Does the Plant Need?
To better understand the differences between fertilizers it is important to understand things from the plants point of view. What nutrients can a plant use? Can it distinguish between a synthetic and an organic source?

Garden Myths book by Robert Pavlis
Plants get almost all nutrients through their roots. You can think of the outside wall of the root as having small holes or pores. These holes are used to let certain molecules into the root. The process is more complex than this, but it is not a bad analogy.

These holes are quite small and so only small molecules can get through them. Water molecules consisting of H2O are small enough and get into the root. Nitrate is also a small molecule, NO3, and it is also absorbed into the root. Other nutrients such as phosphorous, potassium, iron, magnesium etc are all small enough to enter the roots.

Large molecules like proteins, DNA, carbohydrates etc are huge in size compared to the nutrients. They just don’t fit through the holes. In fact most organic molecules fall into this category.Plants can’t use most organic molecules found in the soil! Almost all of the organic material in compost, manure etc is of little use to plants – they simply can’t get the molecules into the roots.