There are three main nutrients found in fertilizer, each with its own special benefit. Consequently, there are three main numbers you'll find on a fertilizer bag -- in order: nitrogen, phosphorus and potassium; they're listed in alphabetical order.
Nitrogen is the first number on the fertilizer bag and it just may be the most important. Nitrogen is one of the most common elements and it's critical to all forms of life, especially plants. Nitrogen is what makes plants have healthy green foliage. Too little nitrogen and plants look anemic and lack full leaves. Too much nitrogen in the garden and your plants will be all leaves and stems and no fruit.
The second number on a fertilizer bag stands for phosphorus. Phosphorus is critical in the development of blooms. Since each bloom becomes a fruit or vegetable, it's very important in the garden. Tomato plants need phosphorus to have larger and more uniform tomatoes. Annual or bedding plants bloom only when there is enough phosphorus in the soil. Most gardeners add rock phosphate to their garden since phosphorus is often lacking in most soil types.
The third number on the fertilizer bag represents potassium. Plants that are low in potassium may look weak or thin and may have fruit that doesn't fill out completely. Potassium helps the main systems of the plant function together, helps with photosynthesis and controls the exchange of carbon dioxide. Green sand and wood ash are full of potassium that can be easily absorbed by plants. These can burn plants if added too closely so follow the instructions on the package for best results.
Almost all fertilizers have micronutrients. These are elements that are important to plant growth, but are needed in much smaller quantities than nitrogen, phosphorus and potassium. The main five are magnesium, copper, iron, manganese and zinc. You'll see on the label that they come in fractions of a percent. These nutrients add to the fertility of the plant, help with fruit production, and make plants more resistant to pests and diseases.
Slow-release fertilizers are a safe and easy way to feed your plants. The slow release coating means the nutrients are broken down over time so the plant won't get burned. This type of fertilizer works because the granules only break down with water and heat. As you water and as your garden soil warms up, your garden plants grow faster, just in time to absorb the fertilizer as it's released into the soil.
Liquid fertilizers are a great supplement to granular fertilizers. They give plants a quick boost because the nutrients are absorbed right into the leaves. They're helpful with plants that are suffering from lack of nutrients and they make healthy plants much more productive. Excess fertilizer drips from the leaves down the ground below. Because liquid fertilizers are absorbed by the roots as well as the leaves, none of the nutrients are wasted. Be sure to follow the directions carefully to get the proper water to fertilizer ratio.
Organic fertilizers are another useful tool for improving soil nutrition in the garden. Fish emulsion is used as a foliar or leaf spray to add nitrogen to a plant. Bone meal is what the name implies and is a powder that is added to the soil. Cow manure is an organic fertilizer than can be purchased in a bag form your garden center. Be sure to understand the proper use for any fertilizer you add to your garden. Ask a garden expert for advice if you're unsure which product is best for you.