What constitutes a fertilizer is variously defined—one source has it being “a substance (as manure or chemical mixture) used to make a soil fertile.” Another definition reads, “Fertilizer is any organic or inorganic material of natural origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants.”

More specifically, a fertilizer generally contains one, two or all three of the designated fertilizer elements nitrogen (N), phosphorus (P) and potassium (K). Chemical fertilizers consist of both generated and naturally-occurring substances. Chemical fertilizer nitrogen is derived from a fossil fuel (usually natural gas), phosphorus from acidified rock phosphate and potassium as potassium chloride, a naturally occurring mineral.

The percentages of the elements N, P and K in a fertilizer are expressed for N in its elemental form, P as its pentaoxide (P2O5), and K as its oxide (K2O)—which is why on a fertilizer label you will see three numbers, expressing the percentage of N to P2O5 to K2O in the product. Sometimes there is also a fourth number, which notes the percentage content of sulfur (S).

Frequently the term complete is used to identify a fertilizer that contains something other than just the three basic fertilizer elements—usually one, several or even all of the micronutrients boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn).

Inclusion of these micronutrients actually limits the use of such fertilizers to those situations where there is a probable deficiency; however, if there is no deficiency, then using a mixture that includes these extra elements could lead to excess or even toxicity.

In some instances the amount of the included micronutrient in the complete fertilizer mix might also be insufficient to meet the plant’s requirements. That’s why it is recommended that the inclusion of any micronutrients in a fertilizer be based only on specific determined need.

Since many of the commonly used rooting media, such as soilless organic mixes containing peat moss or pine bark, composted milled pine bark, composted wood byproducts, perlite, rockwool or coir, are naturally occurring products, they will also contain various levels of these micronutrients, sometimes in sufficient quantities to meet the plant’s needs. This is another reason why including micronutrients in an applied fertilizer mix could lead to excesses and possible toxicity.

The form of the fertilizer you use—whether it’s a solid, a liquid (either completely soluble or in slurry form) or a gas, such as anhydrous ammonia (NH3)—will determine its use as well as its ease of handling as a fertilizer material. Solubility of ingredients can be a significant factor in a fertilizer’s interaction with the rooting environment (particularly in soil), affecting the availability of the essential plant nutrient elements that it contains.

This is particularly important for the fertilizer element P, since the orthophosphate anion is a highly reactive ion, forming complexes that can significantly reduce the availability of P in the ionic form necessary for root absorption.

An organic fertilizer is a substance that consists of combined carbon (C), hydrogen (H) and oxygen (O). Unfortunately, what designates a substance as an organic fertilizer has not been well defined and there might be some question about the use of the term fertilizer for identifying substances that do not contain substantial quantities of any of the fertilizer elements (NPK) sufficient to meet the requirements of a growing plant.

Most organic substances that have been identified as being fertilizers have low essential nutrient element levels and are therefore not suitable for use when growing plants with high nutrient element requirements.

Being low in elemental content, a large quantity of organic fertilizer would be required to meet the demands of even a low nutrient element requirement plant. In addition, being an organic substance, the fertilizer must undergo decomposition in order for any plant-essential elements to be released into the rooting medium solution in the ionic form necessary for root absorption.

The use of most common organic fertilizers can result in excesses or insufficiencies when used for meeting the nutrient element requirement for just one element—for example, animal manures are a good source for N, but they contain other elements that can become present in excess with continued manure applications.

Composted chicken litter is another organic material that can result in elemental imbalances between the elements K and Ca and Mg, leading to either an induced Mg or Ca deficiency.

A determination of the ‘fertilizer value’ of an organic material can be reached by performing a total elemental content determination or a water equilibrium analysis. A total elemental determination can be performed by a plant analysis laboratory, while the water equilibrium test is available from some soil testing laboratories.

A water equilibrium test will determine the level of ‘plant available’ elements and will not only include the fertilizer elements but the other essential elements as well—such as Ca, Mg, S and some of the micronutrients. A total element analysis will identify those elements potentially available upon decomposition.

Organic compost that is the end product of microbial decomposition is a stable substance, resistant to further decomposition. Some of these organic composts are suitable for use as rooting media due to their stable physical structure—worm castings, for example, depending on the source material, can be an excellent potting medium.

In general, though, organic composts are not suitable sources for the fertilizer elements—or the other elements plants sometimes require—so they should not really properly be considered as fertilizers at all.

An inorganic substance can be classified as being organic, based on the fact that it is a naturally occurring substance, not on the basis of being a compound of combined carbon, hydrogen and oxygen. This designation applies to those substances that can be used in the production of food plants or products in order to meet state or federal regulations for designation as being ‘organically grown.’

The challenge for the grower, whether using a chemical, organic or organic-inorganic fertilizer, is to provide the nutrient elements necessary to meet the plant’s requirements without insufficiency or excess. To this end you should try to understand the plant’s nutrient element requirements and balance them with the nutrients being supplied by the fertilizer and those which already exist in the rooting medium.