There are 16 elements that plants need to grow. These elements are broken down into two groups: macronutrients and micronutrients. The nine macronutrients are nitrogen, potassium, phosphorus, magnesium, calcium, carbon, sulfur, oxygen and hydrogen. The three nutrients that are commonly added to fertilizer are potassium, phosphorus and nitrogen. The micronutrients, which are needed in smaller amounts, are chlorine, copper, molybdenum, zinc, manganese, boron and iron.
All Around Fertilizers
All around fertilizers contain an even amount of potassium, phosphorus and nitrogen. These fertilizers are used for ongoing maintenance, and can be water soluble or time released. You often see these fertilizers marked as 10-10-10 (10 percent nitrogen, 10 percent phosphorus and 10 percent potassium). These fertilizers are used for plants planted in healthy soil to keep the nutrients replenished.
Slow-release fertilizers may be natural or synthetic. Natural fertilizers are made from plants or animals. There are two types of synthetic fertilizers: organic (such as fertilizers made from urea) and inorganic (such as fertilizers made from superphosphate). The nutrients in the fertilizer are released over time, and should be applied as directed on the package. Most slow-release fertilizers need only to be applied once each year, in the spring. The benefit of using a slow-release fertilizer is that you can use more at each application without fear of burning the roots. Because slow-release fertilizers are used efficiently, less nitrogen is released into subsurface water and streams (the plant uses more of the nitrogen).
Water Soluble Fertilizers
Water-soluble fertilizer is convenient, as it can be applied to the foliage or the soil. It also comes in many different forms to meet the gardener's needs, such as plant starter with a higher concentrate of nitrogen. The water-soluble fertilizer also comes formulated with chelated iron, which is used for the control and prevention of iron deficiency in plants such as rhododendron and azalea.