Chapter 17

Fertilizer Management for Grapevines

Inorganic Fertilizers

Inorganic fertilizers, or synthetic fertilizers, are chemically manufactured from petroleum products or from naturally occurring minerals containing one or more vine nutrients. Nitrogen fertilizers are normally made from petroleum or natural gas. Phosphorus, potassium and other trace element fertilizers are often mined from the earth. Vines need sufficient nutrients for proper growth. If the vineyard soil does not contain adequate nutrient levels than commercial fertilizers may be added.

Fertilizer Grade

By law, all products sold as fertilizer require uniform labeling guaranteeing the minimum percentage of nutrients. The three-number combination (fertilizer grade or analysis) on the product identifies percentages of nitrogen, phosphate, and potash, respectively (See Figure 17.1).

Other Nutrients

In addition to primary macronutrients, fertilizers may contain other nutrients, such as sulfur, iron, boron, zinc, and molybdenum. 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 (as in Figure 17.1), nutrients will be listed on the fertilizer label on an elemental basis, similar to N.

Single- or Multiple-Nutrient Fertilizer

Based on their primary nutrient content, 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 fluids in which nutrients are dissolved in a water medium and include both solutions and suspensions. Solutions contain completely dissolved nutrients without solids. Suspensions contain some dissolved nutrients and some undissolved solid nutrients.

Other Fertilizer Considerations

Effects on Soil pH

Soil pH can be affected by certain fertilizer applications with some having the potential to lower the soil pH while others have the potential to raise the soil pH. In general, nitrogen and phosphorus fertilizers have the greatest effect on soil pH. Of these two nutrients, nitrogen fertilizers are commonly of greatest concern because they are applied in larger quantities and may have longer term effects on the soil following years of application.

Solubility

Fertilizer solubility is a measure of how much fertilizer material will dissolve in water and is a property that strongly influences the availability of nutrients to a crop and type of application methods?fertigation and direct soil application. Highly soluble fertilizers will dissolve easily in water and the soil solution, making nutrients available for vine uptake.

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.

Nutrient Mobility in Soils

Nutrients differ in their mobility in the soil, which affects their ability to be lost through leaching. As a good general rule, ammonium, potassium, calcium, and magnesium are more mobile than the micronutrients.

Nitrogen Fertilizers

Many different chemical and physical forms of nitrogen (N) fertilizers exist. Some of the more common fertilizer nitrogen sources are given in Table 17.3. Vines can use nitrogen in one of two forms: ammonium nitrogen or nitrate nitrogen. Ammonium nitrogen (NH4⁺) is changed to the nitrate nitrogen form by bacteria, a process known as nitrification. The conversion of ammonium to nitrate nitrogen generally occur very rapidly providing aerobic soil conditions and soil temperatures are above freezing.

Phosphorous Fertilizers

Many different chemical and physical forms of phosphorous (P) fertilizers exist. Some of the more common fertilizer phosphorous sources are given in Table 17.4. Vines most often absorb phosphorous in the form of orthophosphate.

Potassium Fertilizers

Many different chemical and physical forms of potassium (K) fertilizers exist. Some of the more common fertilizer potassium sources are given in Table 17.5. Vines most often absorb potassium in the form of the ion which dissolves readily in water.

Sulfur, Calcium, and Magnesium Fertilizers

There are numerous sources of calcium, magnesium, and sulfur for fertilizing grapevines. Appendix C, Sulfur, Calcium, and Magnesium Fertilizers lists the most common fertilizer sources. In addition, materials such as bone meal, wood ash, manures and sludge can contain adequate significant amounts of these elements.

Micronutrient Fertilizers

There are many different fertilizers that are marketed as micronutrients. Usually, micronutrients are mixed with fertilizers containing nitrogen phosphorous, and/or potassium. Because micronutrients are needed in such small amounts, the best method to correct a micronutrient deficiency is usually by application is by foliar fertilization. Appendix, D, Micronutrient Fertilizers lists some of the more common micronutrient fertilizers used in fertilizing grapevines.

Chelates

Some metal micronutrients are “chelated,” meaning synthetic organic compounds wrap around a metal ion to neutralize it electrically in order to increase its availability. Chelates of zinc, manganese, iron, and copper have become standard products for foliar application in many vineyards.

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