Mixed complete fertilizers are compounded by manufacturers for home use. They are made of incomplete chemical fertilizers—usually some forms of ammonia, nitrate, phosphate, and potash—treated to give the final product the qualities the manufacturer desires. Some of these qualities are described below.
Fertilizers are made in one of several forms:
Granulated fertilizers are manufactured for easy spreading and mixing into soil. They can be broadcast by hand or with a spreader, or incorporated into soil when potting or tilling planting beds. The highest-quality granules are even in particle size, free of dust, and deliver a mix of quick-release and slow-release nutrients.
Liquid fertilizers are usually diluted in water and applied with the water. They are usually quick-acting, without sustained release or persistence in the soil. They can be applied with a watering can, through a garden hose, or injected into a drip irrigation system.
Powdered fertilizers are soluble powders that, like liquid fertilizers, are delivered in water. They can be used in any way liquid fertilizers can be used. Before using them in a drip irrigation system, be sure they dissolve completely, without leaving a residue that will plug up the fine pores.
Pelleted fertilizer looks like granules, but the pellets are very round and uniform in size and shape. They are composed of pieces of soluble fertilizer coated with sulfur and/or polymers to slow the rate of release. This is one type of slow-release fertilizer, made to be mixed into the soil within the root zone.
Tablets are another slow-release fertilizer. They are buried in the soil within the root zone of plants. Most last for several months to a couple of years, releasing nutrients slowly.
Spikes are similar to tablets. These slow-release forms are made to be pressed into planted soil or driven in with a hammer.
Rate of Release
Fertilizers can be formulated to release their nutrients quickly, or over a long period. Many brands are composed of mixes of different release rates, to release measured amounts of nutrients over a given period.
Most fertilizers have a slight alkalizing effect on the soil. Some are made especially to have an acid reaction. These are usually marketed as fertilizers for some group of plants that grow better in acid soils, such as azaleas, camellias, and rhododendrons. In addition to making the soil more acid, these fertilizers frequently contain chelated iron and other micronutrients, which helps this class of plants.
Besides being useful for plants that prefer acid soil, acid-reaction fertilizer is good for most plants in areas that have alkaline irrigation water, which makes the soil too alkaline over years of irrigation. The acid reaction fertilizer counters the effect of the alkaline water. This is especially useful in houseplants, which are not exposed to seasonal rainfall that washes alkalinity out of the soil.