A fertilizer is any soil treatment that contains N, P or K, the chemical symbols for Nitrogen (N), Phosphorous (P) and Potassium (K). These are designated by the three N-P-K numbers on a fertilizer bag that represent the % of each element. Examples of simple chemical fertilizers include (21-0-0) for ammonium sulfate, (33-0-0) for ammonium nitrate, (13-0-45) for potassium nitrate and (0-0-55) for potassium phosphate. Organic fertilizers include steer manure, blood meal and corn gluten. A soil treatment that lacks all of the N-P-K elements but still contains vital plant nutrients is classed as a soil amendment and can’t be called a fertilizer. Soil amendments include lime, Epsom salts and micronutrients. Unlike simple fertilizers complete fertilizers have all three N-P-K elements. Examples include (20-20-20), (15-10-30) and (5-10-30).

Nitrogen is the most important element of fertilizers because it is the most rapidly depleted by plants. That is why farmers apply so much fertilizer to plants so frequently. Nitrogen is also the most expensive element to place into fertilizers. Why? Because it has to be converted, or “fixed” into a form where plants can use it. This fixation process requires a huge amount of energy. There are four basic forms of fixed nitrogen that plants can use. They are nitrate-nitrogen (NO₃^-), ammonium-nitrogen (NH₄⁺), urea-nitrogen (CONH₂) and complex nitrogen (amino acids and proteins). All of these forms are recycled and converted in nature by plants, animals and microorganisms in a complex process call the Nitrogen Cycle. When you purchase a bag of chemically-produced, nitrogen-fertilizer it contains nitrate, ammonium or urea, alone or in mixtures. Look at a label and it will tell you if the nitrogen is “nitrate-N” or “ammonical-N” or “urea-N”. This is important, because the different forms will give different results depending on the plant and the time of year it is applied. Another noteworthy aspect is whether the nitrogen is (WS) water “soluble-N” (i.e. quick release) or WIN water insoluble (long lasting). The best form for plants depends on the situation. The following table lists some simple nitrogen fertilizers and with the N% of each.

Simple fertilizers (Table below) are usually salts made up of a positively (+) charged cations and negatively charged (-) anions. They are the cheapest fertilizers available. They are NOT “organic” or “natural”, but they are very effective at making plants grow if correctly used. Avoid spilling them on soil or applying too much as they can overdose plants and cause salt burn. See below for those that are best for warm vs. cool season, quick vs. slow release and if they can be applied foliarly. The higher the acidity number, the more acid forming they will be when applied to a soil.