Plants require light, water, air and nutrients to survive and thrive. As a plant grows, it uses up the nutrients in the soil. If the nutrients are depleted at a greater rate than they are replaced, a soil loses its fertility. Fertilizers replace the nutrients in the soil to keep plants healthy.
Plants require nutrients in different amounts, divided into macronutrients and micronutrients. Macronutrients are those needed in large amounts. The three macronutrients plants need to survive are nitrogen (N), phosphorous (PO) and potassium (K). The amount by weight of the macronutrients are placed on the side of a fertilizer's packaging in a ratio such as 14-14-14, nitrogen's weight as the first, phosphorous second and potassium third, according to the University of Florida Extension.
Elements or nutrients needed in smaller amount are called micronutrients. These are needed in very small quantities. Micronutrients are not included in many commercial fertilizers. When soil becomes slightly alkaline, when the pH is 7.0 or above micronutrients are required. Micronutrients are found in animal manures and fish emulsion.
Soil sampling determines how much fertilizer is required in the soil. Samples are taken from several areas of the garden or landscape and combined to determine a soil's pH level and nutrient requirements. Soil probes, according to Michigan State University, are available from most university extension services. Samples are taken from areas that are uniform or similar in composition, and tested using a home pH test or by sending the samples off to a soil lab.
Fertilizer is applied according to the size of the area and the weight of nitrogen in the fertilizer. The weight of the nitrogen in the bag is indicated by the number mentioned earlier. In a 16-4-8 bag of fertilizer, the nitrogen is 16 percent of the bag's weight. A 24-pound bag will contain 3.84 pounds of nitrogen (24 x 0.16 = 3.84). Application of nitrogen into the soil is generally 1 to 2 pounds per 1,000 square feet.
Side dressings of fertilizer are used to promote vegetable, flower or fruit growth from the plant. These are light applications of fertilizer during the fruiting period. Applications are 1/4 to 1/2 pound of nitrate or soda per 100 square feet of area. Potassium may be added as well at a rate of 1/4 pound per 100 square feet during the fruiting period to encourage growth.