Ever notice the 3 numbers on a fertilizer bag? These represent the total percentage of weight as packaged for N - P - K.
N = Nitrogen
Nitrogen is the first major element responsible for the vegetative growth of plants above ground. With a good supply, plants grow sturdily and mature rapidly, with rich, dark green foliage.
P = Phosphorus
The second major element in plant nutrition, phosphorus is essential for healthy growth, strong roots, fruit and flower development, and greater resistance to disease.
K = Potassium (Potash)
The third major plant nutrient, potassium oxide, is essential for the development of strong plants. It helps plants resist diseases, protects them from the cold and during dry weather by preventing excessive water loss.
Organic nutrients have lower N-P-K values than chemical nutrients, but don't let this fool you. Organic fertilizers can still easily burn plants if over fertilized. The difference is that the organic fertilizer is released into the soil much slower and for a longer period of time compared to the chemical-type fertilizers.
Powdered nutrients also tend to have higher N-P-K values than liquid nutrients because they are not diluted with water.What really matters is how strong these nutrients are when diluted to recommended levels.
Nutrients needed by plants are composed of mineral ions, which are absorbed in solution from the soil through the roots and used with carbon dioxide and water to make more complex compounds. These nutrients include N-P-K. The macro's also include, but to a lesser degree, Magnesium (Mg), Calcium (Ca), and Sulfur (S). All of the macros are required in relatively large amounts by plants.
Besides the macro-nutrients, plants also require micro-nutrients. The micro-nutrients include a wide arrange of elements. These elements are called trace elements. And while trace elements aren't required in such large quantities as the macro-nutrients, they play an equally important role in healthy plant growth.
The main difference between fertilizer types are either organic and inorganic. There is a degree of confusion concerning these differences. Both types are made up of natural ingredients. The difference is in the degree that they are processed, and the way they are absorbed by the plant.
The processing differences are only in degrees of processing. Going from bulky organic fertilizers, the least processed form of fertilization, to concentrated organics, to highly processed, inorganic soluble fertilizers. The term organic relates to the fact that the supplemental elements added to the soil, must first be processed by microbes in the soil before the fertilizers can be used by the plants. Inorganic fertilizers are in such a form that this intermediate microbe step is not required. As soon as they become wet, or soluble, they are ready for use by the plant.
Weight for weight, bulky organic fertilizers supply fewer nutrients than inorganic fertilizers. They do contain high levels of the micro-nutrients, but the real benefit is in their long term supply of Nitrogen (N). The addition of bulk organic fertilizers, besides being long-term sources of Nitrogen and micro-nutrients, also improve the structure and water content of most soils. This in turn encourages root growth and the ability of the plants to absorb more nutrients of the soil. Some examples of Bulky Organic Fertilizers are: compost and rotted-manures.
Concentrated organic fertilizers are prepared commercially, and thus are more consistent in content. Through the reduction process, much of the bulk and water has been removed to provide a small, easily handled substance. They are also more expensive. These organic fertilizers don't release their elements into the soil without some interaction between the fertilizers and microbes living in the soil. It is in this interaction that the nutrient are released into a form plants can absorb. This is a highly beneficial process, however, the process slows and at times stops completely during cold weather.
Soluble fertilizers, also called inorganic fertilizers, contain high percentages of given nutrients. Through the processing of these chemicals, they have become easy to handle and apply. They are usually cheaper and readily available. These products are highly soluble which means that when they come in contact with water, they quickly break down into free-flowing ions. Once applied, the nutrients are almost immediately available for plant absorption without the intermediary activity of soil microbes.
Advanced technology has introduced slow-release fertilizers. This is a complex blend of inorganic fertilizers, in which some of the fertilizer particles have been coated with a polymer shell that has tiny holes. This perforated membrane allows water to pass through the shell, absorb some of the nutrients inside the shell, and then pass back through the membrane and into the soil. To regulate the disbursement of the nutrients over a period of time, different thickness of the polymer shell are used. While more expensive, this time-release property overcomes many of the drawbacks of soluble fertilizers where it is either feast or famine for the plants.