Bulk fertilizer is available in three main forms: liquid, gaseous, and granular. Fertilizers are manufactured in a wide array of formulas containing varying amounts of the three main plant nutrients: nitrogen, phosphorous, and postassium. Choosing the right bulk fertilizer is a matter of determining which formula and which delivery form best suits your needs.
Gaseous fertilizer, which is a form of ammonia called anhydrous ammonia, is usually only used by large-scale farming operations, as it requires expensive, specialized equipment, which requires special training to run. The ammonia is injected directly into the soil where it bonds with water, making its nitrogen available to the plants. This type of fertilizer is kept under pressure as a liquid but turns to gas as it is dispensed. It is the most nitrogen-rich type of fertilizer available.
Unless you own a large farming operation, anhydrous ammonia is probably not suited to your needs. A liquid or granular bulk fertilizer will be more appropriate, and the first thing to consider when choosing a fertilizer is the needs of the crop to be grown. Different crops have different nutritional needs, so research the crop to find out what its needs are. Timing of nutrient availability can be just as important as the amounts of the nutrients required.
Soil composition is another important factor in choosing a bulk fertilizer. Purchase a soil test kit, and test the soil for nutrient content, or have it tested by your local agricultural extension office. Knowing the level of the nutrients present in the soil will help you choose a fertilizer that will fulfill your crop's needs. Soil test kits are often available at your local garden center or from many on-line vendors.
Compare the results of the soil test to the needs of the crop you plan to grow to determine what kind of formula you need. For example, if you are growing a crop that needs lots of nitrogen and your soil is nitrogen-poor, you need a fertilizer with a high percentage of nitrogen in its composition. Crops grown for their leaves, such as spinach and lettuce, often require greater amounts of nitrogen, while fruit and vegetable crops often require more phosphorous and potassium.
Liquid bulk fertilizer is often purchased in a highly concentrated form, which helps to reduce shipping costs. It is then mixed with water and applied. This type of fertilizer makes nutrients immediately available to the plants, but can be quickly used up or leached from the soil by rain. Both organic and inorganic liquid fertilizers are available. Organic fertilizers are derived from natural sources, while inorganic fertilizers are manufactured from a chemical or mineral source.
Granular fertilizers are available in two types. One is meant to be dissolved in water and applied as a liquid. The other is spread over or tilled into the soil. This type of fertilizer releases its nutrients more slowly, but makes them available to the plants over a longer period. Granular fertilizers are also available in organic and inorganic formulas.