There are 16 essential elements that plants need to grow. Oxygen, carbon, and hydrogen are used in the largest quantities, and the plant derives them from air and water. The remaining 13 nutrients come from the soil, and can be added through fertilizing.
Fertilizers are labeled with 3 numbers, separated by dashes, for example: 24-8-16, the analysis on Miracle Gro All Purpose Plant Food. This is called the analysis, and it refers to the percentage of the 3 macronutrients by weight. The macronutrients are: nitrogen, phosphorus, and potassium. So the example above would include 24% nitrogen, 8% phosphorus, and 16% potassium. A fertilizer does not have to have all 3 macronutrients. For example, a label reading 20-0-0 would include 20% nitrogen, and no phosphorus or potassium. These 3 elements are the ones that are usually deficient in soil.
As you’ve probably already noted, the 24-8-16 fertilizer in the example only adds up to 48%. So what else is in the fertilizer?
There are 3 additional macronutrients which are not included in the analysis. They are calcium, sulfur, and magnesium. These are present in adequate quantities in most soils, and only need to be added if a soil test indicates. If included, the percentage will be shown on the full list of ingredients on the fertilizer container.
We’ve now accounted for 9 of the 16 essential elements. The remaining 7 are referred to as micro nutrients, or trace elements. They are boron, chlorine, copper, iron, manganese, molybdenum and zinc. These micronutrients are just as vital to plant health as the macro nutrients, but they are needed in very small quantities. They are present in sufficient quantities in most soils, but may leach out of very sandy soils. A more common problem is that the elements are present in the soil, but factors like incorrect pH inhibit uptake by the plant. A soil test is recommended before adding these elements, as overdose can be toxic to plants.
The last ingredient in your fertilizer is filler. This is inert matter that makes it easier to apply the fertilizer.