When searching for a fertilizer for your garden, you will be confronted with many very different types of chemical and organic substances, all promoted as soil-enrichers that will help your plants reach their full potential. Navigating this maze of information can be difficult, but by understanding the basic types of fertilizer you can choose the best ones for your plants. (Note: "Organic" and "inorganic" as used here simply refers to the source of the fertilizer; it does not indicate whether or not it is suitable for use with organic crops.)
Organic Nitrogenous Fertilizers
Nitrogenous fertilizer is simply fertilizer that is rich in nitrogen, a substance needed by all plants for healthy growth and reproduction. Natural sources of this element come from byproducts of plants and animals. Animal manure is one prime example, but oil-cakes (used along with chemical fertilizer), blood meal, cottonseed meal, corn gluten meal, crab meal, feather meal, leather meal, and fish meal are also used for increasing nitrogen in the soil. Urea is another organic fertilizer, noted for its ability to produce quick results. Compost is another organic material that can be used to improve the soil, although the nutrients it provides are not just limited to nitrogen.
Inorganic Nitrogenous Fertilizers
Inorganic (or chemical) fertilizers come in many varieties, each with their own specific use. Sodium nitrates (also called Chilean nitrate or Chilates) and ammonium sulphate nitrate are used to make nitrogen quickly available to plants. Ammonium sulphate, ammonium nitrate and ammonium chloride are all used as general nitrogen fertilizers and are specialized for different methods of fertilization. Ammonia itself can also be used in a liquid form put directly into the water supply for the plants. All of these chemicals can cause damage to the soil, plants or water if used improperly, especially in rendering the soil acidic.
Organic Phosphate Fertilizers
Phosphorous is a major nutrient chemical necessary for growth and reproduction in plants. Organic sources of this nutrient include bone meal and bat guano.
Inorganic Phosphate Fertilizers
Rock phosphate (natural deposits in rocks), superphosphate (manufactured from rock phosphate), and slag (a by-product of steel mills) are all used in soil that is overly acidic or deficient in phosphorous.
Organic Potassium Fertilizers
Potassium is not often needed in soil, and fertilizers containing it should only be applied if a deficit is certain. If it is, kelp meal is an organic fertilizer used to increase the potassium content in the soil.
Inorganic Potassium Fertilizers
Muriate of potash (potassium chloride) and sulphate of potash (potassium sulphate) are used to enrich the soil with potassium. Both are very soluble in water and are available quickly to plants. Granite meal and greensand are also used for potassium enrichment.