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Major Fertilizer Components
Although commercial inorganic fertilizers can come in a variety of forms liquid or solid, the chemistry of them is most generally described in terms of three numbers, called NPK numbers. The "N" refers to nitrogen, the "P" to phosphorous and the "K" to potassium (Latin, kalium). NPK value is given in number terms. Thus 20-20-15, not unusual for lawn fertilizer, means the fertilizer contains twenty percent nitrogen, twenty percent phosphorous (as pentoxide) and fifteen percent potassium (as oxide). These categorizations were established by the Association of American Plant Food Control Officials to provide a uniform standard. Of course, trace nutrients are important to plants as well. The need for these may be ascertained through testing by an extension laboratory.
Nitrogen
Nitrogen gas is nearly chemically inert and so must be modified if it is to be successfully used in fertilizer. Although ammonia gas (a compound of nitrogen and hydrogen) can be dissolved in irrigation water to produce acceptable fertilizer, this approach is not useful for the average planter and gardener. Instead, water-soluble ammonium salts and nitrates are used. The former contain nitrogen in the positive portion of the molecule---the latter in the negative portion. Another common source of available nitrogen is sodium nitrate. As an organic alternative, urea may be used. There are times when one form of nitrogen fertilizer serves better than another does. For instance, fish emulsion is one of the best nitrogen fertilizers, but its odor is a turn-off.
Phosphorous
Phosphorous is most generally supplied (in its inorganic form) as rock phosphate or superphosphate. It is always located in the negative portion of the compound used. A less common inorganic phosphorous fertilizer is pulverized slag from steel mills. Superphosphate, unlike rock phosphate, is completely water soluble. Reduced solubility---slow release---is at times the superior choice. Among the slow release varieties is the highly desired, organic bone meal.
Potassium
Potassium is one of those rare substances that is soluble in nearly every form. An alkali metal element, potassium forms positive ions that combine with negative ions to form salts. Potassium sulfate or potassium muriate (potassium chloride) are commonly used in commercial fertilizers. To consolidate, some of the potassium may be supplied as the nitrate, thus providing both potassium and nitrogen. However, generally more of one is desired than the other, so part of the more greatly desired element needs to be provided in another form. Wood ashes supply potassium derived from organic sources (though its active ingredient is---technically---an inorganic compound).