When unsure about what fertilizer to use, it is best to begin by testing the soil's nutrient and pH levels using a soil tester. Once the soil's pH and nutrient levels are determined, it is easier to select a fertilizer with the necessary ratios of nitrogen, phosphorus and potassium that can best replenish the soil. The type of fertilizer is also dependent on the type and age of the plants.
Nitrogen in fertilizer helps promote healthy foliage in trees, shrubs and flowers, as well as vibrant green blades of grass. Plants that have a nitrogen deficiency exhibit symptoms such as yellowing of the foliage and little to no growth.
Phosphorus is mainly responsible for helping the plant establish a healthy, strong root system. Fertilizer that contains high levels of phosphorus is generally used on young plants. Phosphorus is also important for promoting flowering and fruiting in plants. However, too much phosphorus can cause nutrient burn, which manifests itself in yellow, drooping leaves. This is why it is important to perform a soil test before applying to fertilizer so as not to introduce the wrong amount of nutrients and potentially harm the plant.
Potassium is generally always included in fertilizer due to its ability to boost the immune system of plants, prevent infections and promote overall hardiness.
The percentages of nitrogen, phosphorus and potassium in a fertilizer are generally indicated by three numbers that are prominently displayed on the fertilizer container or bag. The first number is nitrogen, then phosphorus and lastly potassium.