Chapter 17

Fertilizer Management for Grapevines

Inorganic Fertilizers

Inorganic fertilizers, also referred to as commercial and 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 form, and 22 percent phosphorus in a phosphate 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.

Fertilizer Grade

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 (See Figure 17.1) 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.”

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.

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.

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.

Fertilizer Forms

Fertilizer materials are available in either solid (dry) or liquid forms and are often blended to meet individual nutrient needs. Each physical form has its own uses and limitations, which provide the basis for selecting the best material for the job.

Solid Fertilizers

Depending on the nutrient content and manufacturing processes, solid fertilizers may differ in size, shape, color, and bulk density. Solid fertilizer forms are classified by size and shape and include granules, prills, pellets, and powder.

Liquid Forms

Liquid fertilizers are categorized into two groups: clear solutions and suspensions. 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.

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). Potential acidity or basicity indicates the type of reaction produced, while calcium carbonate equivalency indicates the strength of that reaction.

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 vines to extract the soil water they need for growth.

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