Many soils require adding several essential nutrients to alleviate plant deficiencies. Farmers may opt to select a combination of single-nutrient fertilizers or apply a fertilizer that combines several nutrients into each particle. These combination fertilizers (compound or complex) can offer advantages of convenience in the field, economic savings and ease in meeting crop nutritional needs.
Manufacturers make compound fertilizers by using basic fertilizer materials, such as ammonia (NH₃), ammonium phosphate, urea, sulfur (S) and potassium (K) salts. There are many methods used for making these fertilizers, with the specific manufacturing processes determined by the available basic components and the desired nutrient content of the finished product. Here are four brief examples.
Compaction methods (agglomeration) involve binding small fertilizer particles together using compaction, a cementing agent or a chemical bond. Various nutrient ratios can be combined using undersized particles that may not be suitable for other applications.
Accretion-based fertilizers are made by repeatedly adding a thin film of nutrient slurry, which is continually dried, building up multiple layers until the desired granule size is reached.
Pipe-cross reactors are used to chemically melt NH₃, acids containing S or phosphorus (P), and other nutrients—such as K sources and micronutrients—into a solid fertilizer with the desired nutrient content.
The nitrophosphate process involves reacting phosphate rock with nitric acid to form a mixture of compounds containing N and P. If a K source is added during the process, a solid fertilizer with N, P and K will result.
Compound fertilizers contain multiple nutrients in each individual granule. They differ from a blend of fertilizers mixed together to achieve a desired average nutrient composition. This difference allows farmers to spread compound fertilizer so that each granule delivers a mixture of nutrients as it dissolves in the soil and eliminates the potential for segregation of nutrient sources during transport or application. The multiple-nutrient granules of compound fertilizers also allow the farmer to achieve uniform distribution of micronutrients throughout the root zone.
These fertilizers are especially effective for applying an initial nutrient dose in advance of planting. There are certain ratios of nutrients available from a fertilizer dealer for specific soil and crop conditions. This approach offers advantages of simplicity in making complicated fertilizer decisions, but does not allow farmers the flexibility to blend fertilizers to meet specific crop requirements. Turf managers and homeowners often find compound fertilizers especially desirable.
Compound fertilizers are sometimes more expensive than a physical combination or blend of the primary nutrient sources, since they require additional processing. However, when purchasers consider all the factors involved with nutrient handling and use, compound fertilizers may offer considerable advantages.
Among the nutrients, N typically requires the most careful management and reapplication during the growing season. It may not be feasible to supply sufficient N in advance of planting to meet the entire demand (using only compound fertilizer) without over- applying some of the other nutrients. Because of this, growers should consider applying a compound fertilizer early in the growing season and then later add N as needed.
Manufacturers often produce compound fertilizers regionally to meet local crop needs. They typically adjust a wide range of chemical and physical properties to meet those needs. For example, a desire to minimize P in urban storm water runoff has led some communities to restrict the addition of P to compound fertilizers sold for turf and ornamental purposes. In another example, manufacturers customize their products by boosting certain fertilizer elements for regional soils known for deficiency in those nutrients.
Source: Nutrient Source Specifics (No. 4), International Plant Nutrition Institute.