There can be any number of reasons for wanting to use either an organic fertilizer or an inorganic one. Before making that decision, you should understand the purpose of fertilizer in general, distinct benefits and disadvantages of each type, and how to determine exactly what you need. Armed with this information, you can make an educated decision about what is best for your soil, your crop and your budget.
Fertilizer is a soil amendment. It can be anything that is used to enhance the nutritional content of soil to improve plant growth or crop production. Sixteen different nutrients are required to sustain most plant life. Fertilizers can be blended, providing general nutrients most plants need or specialized, providing only specific nutrients. Some fertilizers are organic while others are inorganic.
Organic fertilizers are those which originated from living things. They contain no chemical or synthetic additives. There are many types of components in organic fertilizer. Animal-related products can include manures with or without the animal's bedding material mixed in, chicken feathers, fur, bonemeal and egg shells. Plant matter often includes grass clippings, dried leaves, used straw, peanut shells, pine needles and other discarded plant materials that have not gone to seed.
Inorganic fertilizer is man-made and consists of chemical and synthetic components. Inorganic fertilizers are what are sold to the general public as all-purpose or general fertilizer. It is described by a set of numbers called the fertilizer's "analysis." It designates the percentages of the three most commonly needed nutrients: nitrogen (N), phosphorus (P) and potassium (K). The remaining material is minor nutrients and a filler material that allows the fertilizer to be distributed evenly.
Organic fertilizers create a soil environment that enhances the soil's ability to host living organisms such as earthworms which by their presence also improve the soil. The combination of the organic matter and the presence of the organisms improve the soil's capability to hold moisture and nutrients. Organic materials take longer to break down so nutrients are released more slowly and are less likely to be leached away. Many sources of organic fertilizer are free or available at a low cost.
Inorganic fertilizer is easier to apply and can be distributed evenly by mechanical means. The nutrient content is easily identified and the amount needed is easier to determine. Because commercially prepared blends are labeled with the content, it is easy to match a blended fertilizer with the results of a soil test that identifies your specific needs.
The nutrient content of organic materials tends to be lower than inorganic fertilizers and therefore requires the use of higher quantities to achieve the same goals. Organic fertilizers are more difficult to distribute as most must be applied by hand. The slow-release of nutrients in organic materials can be a negative if nutrients are not available in a timely manner and in sufficient amounts when the plants need them. For this reason they should be applied to the soil well in advance of when the need is anticipated.
Inorganic fertilizers do not contribute to the soil's ability to host living organisms. They can also be considerably more expensive than organic options.
The importance of a soil test cannot be underestimated. The soil's nutrient content, the intended crop and the time frame in which the soil must be improved in order to support that crop all play a role in deciding which fertilizer to use. Your local extension office can guide you in collecting samples and can perform the test. The results will indicate any deficiencies and make recommendations based on soil needs. They can also test the nutrient content of your organic fertilizer so you will have a better idea how much you will need to add.