Plants make their own food using energy from the sun, but need a variety of chemical compounds to build stems, leaves, roots and flowers. Some of these--principally, nitrogen, potassium and phosphorus--are often in short supply in the soil and application of these nutrients as fertilizer can result in larger leaves, more flowers or other improvements in growth.
Liquid fertilizers have the advantage of quick absorption and almost immediate effect. These are often used in starting young plants or whenever a quick greening from extra nitrogen is needed. Unfortunately, any nutrients not absorbed will wash out of the root zone within a short time.
Granular fertilizers, such as lawn fertilizer, can be spread easily by hand or with a mechanical spreader and often release nutrients over a few weeks, allowing for more absorption of the formula.
Pelleted, time-release fertilizers can be bought that need only one application per season. While fairly expensive, they are convenient and dependable, especially if you're inclined to forget to fertilizer regularly.
Organic materials such as manure and compost contain nutrients but are primarily used to increase the water and nutrient holding capacity of the soil. Others, such as blood meal, fish meal and cottonseed meal, are used mainly for their nutrients.
On the front of a box or bag of fertilizer, you'll find three numbers, often called NPK, for nitrogen, phosphorous, potassium. These might be 10-10-10, or 10-5-5. They have been mixed from a variety of chemical compounds to this precise formulation. The types of plants a gardener is growing and the nutrient level of his soil will determine the best type of fertilizer for his purpose.
For instance, a fertilizer for lawn grass is heavy on the nitrogen, perhaps being a 30-10-10. A fertilizer for a peach tree, on the other hand, will be higher in the last two numbers because phosphorus and potassium are needed for flower and fruit production.
Organic vs Inorganic
Roots take up nutrients as simple water-soluble chemical, regardless of its origin in ammonium sulfate or in fish meal. Organic fertilizers have the advantage of increasing the amount of humus in a soil, helping to hold moisture and nutrients, but also have the disadvantages of being uneven in levels of nutrients and being slowly made available to the plants.
Inorganic fertilizers are exact in formula, but, if applied too liberally, can burn plant roots and may build up to toxic levels of salts in the soil.
Single Element Fertilizers
Instead of buying a pre-mixed formulation, you can often find materials with only one element and either mix them yourself or use one for a specific purpose. Ammonium nitrate, for example, is 34-0-0, and is often used on citrus crops and pastures. Super phosphate, 0-46-0 is a very concentrated form of phosphorous. Blood meal, 13-1-0.6 is mainly nitrogen and can be used by itself to promote leafy growth.
Though nitrogen, phosphorous and potassium are the primary elements needed by most plants, there are also a variety of nutrients needed in small quantities. Usually these are present in the soil, but occasionally a lack may need to be remedied by applications of fertilizer with boron, zinc, copper, sulfur, iron and other micronutrients.
Many organic fertilizers have a variety of other nutrients included simply because they come from living material, but you can also buy inorganic fertilizers with micronutrients added. If you have a serious deficiency in one micronutrient, you can also buy compounds specifically design to correct that deficiency.