Fertilizers are what we use to distribute nutrients into the root zone of plants. Technically, they are NOT PLANT FOOD since plants make their own food through a process called photosynthesis. Fertilizers merely provide the nutrients that are used as the building blocks of carbohydrates and sugars in the plant. Unless they are present along with the proper temperature, water, light and chlorophyll levels, growth will not occur.

Generally, the home gardener is concerned with three major factors when it comes to fertilizers:

1. Nutrient Levels - By law, all fertilizers must have a listing of the amounts of various nutrients to found in them. At a minimum, the label must contain the percentages of nitrogen (N) - phosphorus (P in the form of phosphate) - potassium (K in the form of phosphate). Based on the needs of the particular plants or on the results of a soil test, the amount of each nutrient needed may determine the fertilizer selected.

In addition to the three major nutrients, fertilizer containers will show the percentage of minor nutrients that might be available. For instance, a container of ammonium sulfate fertilizer will have an analysis of 21-0-0 but the label will also give the percentage of sulfur included.

2. Fertilizer Form - There are many different formulations used to help us deliver the nutrients to the plants. These fall into one of three general categories including solid, liquid and gas.

  • Solid - The most common solid form of fertilizers is the type called granular fertilizer. This is the classic type found in bags at every garden center, plant nursery and hardware store. Generally, the nutrient elements are combined with a type of clay to form the granules. That is why the bag gives an analysis of 15-10-10 which only adds up to 35%. The remaining 65% is made up of "inert matter", primarily clay.

  • Liquid - This includes all those fertilizers that we mix with water in order to apply them to containers.

  • Gas - Farmers often apply a form of nitrogen called anhydrous ammonia to their corn in the summer. However, most home gardeners will never have use for this formulation of fertilizers.

3. Nutrient Availability - Nitrogen is nitrogen but the rate at which it becomes available to the plants can vary depending on the form of nitrogen used. This can also be a factor in other nutrients. Some forms are highly water soluble and are available to the plant immediately. Other sources are coated with materials that slowly deteriorate and release their nutrients of time. Organic fertilizers provide the same nutrient elements but usually have to be first broken down by microorganisms before they are in a form useful to the plants.

4. Fertilizer Calculations - How do you figure out how much fertilizer to apply? Here is a simple formula.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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