We define a fertilizer as a substance that is added/applied to soil or sprayed to crops foliage/leaves to help in the growth of the plant by supplying various nutrients required in the growth stages. A fertilizer may be organic or inorganic/synthetic. Organic fertilizer may be from sea weed, manures and compost. Another way of classifying fertilizer is whether the fertilizer provides a single nutrient, say N,P or K(Straight fertilizer) or multiple nutrients, say N and P or N,P and K(complex fertilizers). It is also important to mention that fertilizers enhance effectiveness of the soil by modifying its water retention and aeration.
Nutrients are broadly categorized into two: Macro nutrients (Nitrogen, Phosphorous and Potassium; NPK) and Micronutrients (Copper, Iron, Manganese, Molybdenum, Zinc, Boron, etc). It is worth noting that there are nutrients we refer to as secondary macro nutrients: Calcium, Magnesium and Sulphur.
On any pack of fertilizer the number show, say 17.17.17, show the percent by weight of the three major nutrients. The first number is always the percent Nitrogen (N). The second is always the percent phosphate (P2O5). The third number is always the percent potash (K2O). The higher the number, the higher the content of that nutrient in the fertilizer nutrient.
Different plants require different nutrients. The best way to select a fertilizer is to have your soil tested. The soil test report will recommend a fertilizer to be used and also comes with a management note that provides guidelines on the rates to be applied for the intended crop.
Never over-fertilize since too much fertilizer hurts more than help. Correct rates should be applied to avoid negative effects to our crops and soil. In the use of organic fertilizers always allow both compost and manure to age and fully break down before applying.