Fertilizer: what do those numbers mean?
By Tim Daly
Gwinnett County Extension Agent
The soil is the most important component of plant growth. It supplies the plant with water and nutrients in addition to anchoring it in place.
Sometimes, fertilizer has to be applied to garden plants and lawns to provide supplemental nutrients for optimal growth when the soil lacks them. Many types of fertilizers exist, but not all are created equal. The correct choice depends upon the particular plant and its soil.
Before applying fertilizer, consider having your soil tested through Gwinnett County Extension. The results will show the nutrient status and the relative acidity of the soil (pH) through soil testing. Applying fertilizer without a soil test can lead to the application of too much or too little lime and fertilizer required for optimum growth.
Plants require several nutrients for sustenance. Nitrogen, phosphorus, and potassium are the ones they need in the highest quantity whereas sulfur, magnesium and calcium are needed in lesser amounts. Some nutrients, such as zinc, iron, and manganese, are only needed in small levels.
Three numbers on fertilizer bags indicate the guaranteed analysis or the fertilizer grade. They give the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K), commonly referred to as the N-P-K ratio. For example, if a 100-pound bag of fertilizer is labeled 20-10-15, it has 20 pounds of nitrogen, 10 pounds of phosphorus and 15 pounds of potassium for a total of 45 pounds of nutrients and 55 pounds of filler material. Other nutrients are sometimes listed on the label. The law requires the fertilizer manufacturer to guarantee that the N-P-K ratio of fertilizer on the label is correct meaning that the nutrients listed on the label are contained in the fertilizer. If you have your soil tested, the report will recommend the type of fertilizers and amounts that should be applied.
Various formulations of fertilizers are used. Complete fertilizers contain all three of the major nutrients. Incomplete fertilizers, such as 0-20-20, do not include all three of these nutrients. Slow release fertilizers include a coating of materials that allow the nutrients to be slowly made available to the plants over a period of time. Water, heat and microbes break the material down. Some fertilizers are combined with pesticides. However, timing for a fertilizer application and treatment for the pest may not coincide. For example, some fertilizers are formulated with a pre-emergent herbicide. In the spring, the best time for use of a pre-emergent is in March. The fertilizer should not put down on warm season grasses at that point since it could cause them to come out of dormancy prematurely and increase the risk of cold damage from a late season freeze.
Many gardeners question whether the use of organic fertilizers is advantageous over synthetic fertilizers. Bone meal, cottonseed meal, blood meal, manure, compost and other sources are considered organic fertilizers. The advantage to using them is increasing the organic matter content of the soil and improving its physical structure. They also are less likely to burn the plant material. However, the nutrients in organic fertilizer are not readily plant available. They have to be broken down by soil microbes to be released into the soil. With synthetic fertilizer, the nutrients are immediately released. Also, the nutrient levels from organic sources are relatively small. Even if you use a synthetic fertilizer, consider incorporating organic matter into the soil to improve it.
Choosing the correct type of fertilizer and applying the appropriate amounts of it will help ensure healthy vigorous plant growth. Having some basic knowledge of fertilizers will contribute to improving the quality of your home landscape and productivity of your vegetable garden.
Timothy Daly, is an Agricultural and Natural Resource Extension Agent with Gwinnett County. He can be contacted at 678-377-4010 or firstname.lastname@example.org