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The fertilizer bag label contains much useful information. Here are some tips on how to read the label.
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At the top of the label will be the description of the brand name of the plant food or fertilizer. Directly under that will be three numbers. Each number will be separated by a dash such as 5-10-5. These three numbers stand for the percentages of nitrogen(N), phosphorus(P), and potassium(K) found in the fertilizer. Let's look at each component in more detail.
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The first number, nitrogen (N), will usually be further subdivided into the different forms of nitrogen found in the particular fertilizer. Nitrate nitrogen is a very soluble and rapidly available form of nitrogen, ammoniacal nitrogen is less soluble and slower to be available, and insoluble nitrogen is very slow to release. Nitrogen is the most frequently needed element in most garden soils. A plant without enough nitrogen shows pale yellow leaves and lacks vigor.
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The next number on the fertilizer bag stands for phosphorus (P). This element is critical for most aspects of plant growth but is particularly important for producing healthy roots and maturing crops. Deficiencies are difficult to detect but they sometimes show up as red stems.
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The third number is potassium (K). It is also called potash. It is used in large quantities and is necessary for the general health of all plants. It's very important for the structural strength of plants.
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After the three major fertilizer components are given, many times elements in smaller quantities, called trace or minor elements will be listed. These elements are generally required in lesser quantities. They include Iron(Fe), Magnesium(Mg), Copper(Cu), Zinc(Zn), Boron(B), Sulfur(S), and Calcium(Ca).
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Lastly, most fertilizer bags will list the materials that the different elements are derived from. Before amending outdoor soil with fertilizers, you should first test the soil to determine what it's lacking. Additionally, different plants have different needs so what type and how often you fertilize can vary greatly.