Fertilizers
Inorganic Commercial Fertilizers
Inorganic fertilizers, also referred to as commercial or synthetic fertilizers, are chemically manufactured from petroleum products or from naturally occurring minerals containing one or more plant nutrients. For example, a common phosphorus fertilizer is monoammonium phosphate that contains 11 percent nitrogen in an ammonium (NH4⁺) form, and 22 percent phosphorus in a phosphate (PO₄³¯ form. This compound is the result of treating mined and finely ground rock phosphate with sulfuric acid, to first produce phosphoric acid that is afterwards reacted with ammonia. Characteristics of inorganic synthetic fertilizers are that they dissolve in water and are immediately available to the plant for uptake. When used according to recommendations, these types of fertilizers are safe for the environment and can supply the required nutrients for plant growth. However, excessive rates of these fertilizers can injure the roots of plants causing death and potentially lead to environmental degradation.
Fertilizer Labeling
By law, all products sold as fertilizer require uniform labeling guaranteeing the minimum percentage of nutrients. The information required on the label and the way in which the information is given are specified by the fertilizer law. The information on the label (Figure 12.1) is the guaranteed amount of the primary nutrients given in a series of three numbers, such as 10-6-4, and is referred to as the “fertilizer grade.”
Fertilizer Ratio
The fertilizer ratio is the relative proportions of primary nutrients in a fertilizer grade divided by the highest denominator for that grade. A 27-3-9 fertilizer has a ratio of 9-1-3. This fertilizer is high in nitrogen, low in phosphorus, and moderate in potassium, relatively speaking.
Other Nutrients
In addition to primary macronutrients, fertilizers may contain other nutrients, such as sulfur (S), iron (Fe), boron (B), zinc (Zn), and molybdenum (Mo). These nutrients may be added as additional nutrients or may be constituents (impurities) remaining in the fertilizer material following mining and manufacturing processes. If present as additional nutrients (Figure 12.1), nutrients will be listed on the fertilizer label on an elemental basis, similar to nitrogen.ssing the activity of PPO, sulfite ions limit the oxidation of phenolic compounds that serve as the primary substrates. The oxygen reacts with phenols and produces a compound known as quinone and in doing so causes browning reactions in juice.
Regulatory Standards
The law establishes minimum allowable levels of nutrients and provides specific labeling requirements. The law only requires that the manufacturer guarantee what is claimed on the label. So, in some cases, a fertilizer will contain secondary nutrients or micronutrients not listed on the label, because the manufacturer does not want to guarantee their exact amounts. For this reason, some fertilizers (especially organic fertilizers) may have a higher total nutrient content than what is listed on the label.
Single- or Multiple-Nutrient Fertilizers
Based on their primary nutrient content (N-P-K), fertilizers are referred to as being single-nutrient or multiple-nutrient. Single-nutrient fertilizers, such as urea (46-0-0), contain only one primary macronutrient and are also called “simple” or “straight” fertilizers. In contrast, multiple nutrient fertilizers contain more than one nutrient, such as diammonium phosphate (DAP, 18-46-0) and may be referred to as “mixed” or “complex” fertilizers. A fertilizer or blend containing all three primary nutrients (e.g., 15-30-15) is a “complete” fertilizer.
Fertilizer Forms
Fertilizer materials are available in either solid (dry), liquid, and gaseous forms. Each form has its own uses and limitations, which provide the basis for selecting the best material for the job.
Solid Fertilizers
Solid fertilizer forms are classified by size and shape and include granules, prills, pellets, and powder. Granular fertilizers are the most common form of solid fertilizer used. A desirable granular fertilizer contains hard granules of uniform size and composition and has good storage and handling qualities. Such materials are required by today's fertilizer applicators. Granular fertilizers are sold in bags or as bulk materials for direct spreading onto fields, and are also used to make bulk blended fertilizers.
Blended Fertilizers
Blended fertilizers are mixtures of dry fertilizer materials when two or more chemical compounds may be mixed together in a suitable proportion to supply nutrients at the same time from the same material. In blended fertilizers, the individual particles remain separate in the mixture, and there is a potential for segregation of the nutrients. The result is nonuniform distribution of the fertilizer nutrients in the field.
Liquid Fertilizers
Liquid fertilizers can be diluted or concentrated for precise, even application. Liquid fertilizers are commonly applied through irrigation water (fertigation), sprayed, knifed-in, broadcast, or banded. Unlike solid fertilizers, liquid fertilizers are applied on a volume rather than a weight basis and the liquid density (pounds nutrient per unit volume) is needed to calculate application rates. Liquid fertilizers generally have a lower nutrient analysis than solid fertilizers due to the solubility limitations of nutrients in a solution. Critical factors to consider when using liquid fertilizers are climate and storage. Most liquid fertilizers will “salt-out” (precipitate out of the solution into solid form) when temperatures become too cold.
Solution Fertilizers
In solution fertilizers, nutrients are completely dissolved in water. The major advantage is ease of handling. The disadvantages are that they are generally low-nutrient analysis, especially when the material contains potassium, and the cost per unit of nutrients is generally higher than suspension fertilizers.
Suspension Fertilizers
Suspension fertilizers are manufactured from liquid fertilizers similar to those that are used to make solution fertilizers. Suspensions, however, have more material added to the water than can dissolve, so it becomes a thin slurry. Clay is added to keep the very fine, undissolved fertilizer particles from settling out.
Gaseous Fertilizers
Gaseous fertilizer, such as anhydrous ammonia, requires special handling. Anhydrous ammonia is stored as a compressed liquid that expands during application, becoming a gas that must be injected into the soil to prevent release to the atmosphere.
Fertilizer Salt Index
Most fertilizer materials are readily soluble because they are salts. Once they are dissolved in the soil, they increase the salt concentration of the soil solution, which in turn increases the solution’s osmotic potential. The greater the osmotic potential, the more difficult it is for the plants to extract the soil water they need for growth. The salt index of a fertilizer is expressed as the ratio of the increase in osmotic pressure of the salt solution produced by a specific fertilizer to the osmotic pressure of the same weight of sodium nitrate, which is based on a relative value of 100.
Fertilizer Acidity/Basicity
The potential acidity or basicity indicates how the fertilizer will affect soil pH. The potential acidity refers to the fertilizer’s tendency to cause the media pH to decrease, while the potential basicity refers to the fertilizer’s tendency to cause a media pH increase. Many water-soluble fertilizer labels state the potential acidity or basicity of the fertilizer in units of equivalent pounds of calcium carbonate (CaCO3 or agricultural lime) per ton of fertilizer, referred to calcium carbonate equivalent (CCE).
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