Most gardeners apply fertilizer of some kind to their gardens annually. A variety of commercial fertilizers are available to gardeners, and suggestions for homemade fertilizers abound. Some gardeners use only organic fertilizers, while others are committed to chemical fertilizers. In fact, the choices are so many and varied that choosing a specific fertilizer can be quite confusing. The job is simplified by remembering that the purpose of fertilizers is to ensure optimal nutrition for the plants in the garden.
Major Nutrients in Soil
Twenty nutrients have been identified that are essential for plant growth. Carbon, hydrogen and oxygen are supplied by air and water. The other 17 are found in the soil and are absorbed by the plant's root system. Six of these nutrients, called macronutrients, are the most important to plant health. Nitrogen, calcium, phosphorus, sulfur, potassium and magnesium are all needed in relatively large amounts. If the garden soil does not contain enough of these nutrients, fertilizers can supply them.
Other Soil Nutrients
Eleven other nutrients must be present in the soil for plants to remain healthy and grow well. These are the micronutrients, and they are needed only in small amounts. Micronutrients include boron, chlorine, copper, iron, manganese, molybdenum, zinc, iron, selenium, silicon and sodium.
Check the Soil for Nutrients
Before applying fertilizer, it is wise to know what nutrients are already present in the soil. Professional soil testing is the best way to check soil. In the United States, every County Extension Office of the Department of Agriculture can provide this service. Commercial soil testing kits are also widely available.
Other Soil Considerations
Having nutrients present in the soil does not always ensure good plant growth. The plants must also be able to absorb the nutrients. Soil texture and soil pH both influence the availability of nutrients to plants. Both can be altered with soil treatments when needed. Standard soil testing usually includes information about pH levels.
Supplying Missing Nutrients
Fertilizer mixes supply any nutrients that are missing or inadequate in the soil. Fertilizer, however, should also be matched to the kind of plants being grown and the stage of growth. Most plants require more nitrogen early in the growth cycle and sometimes benefit from having a little more applied later. Root vegetables and flower bulbs need higher levels of phosphorus. Potassium is water soluble, so it may be especially needed in light, sandy soils that drain quickly.