Vineyard Nutrient Management
Essential Plant Nutrients
Magnesium
Role and Deficiency Symptoms
Magnesium has several functions in the plant. It is the central component of the chlorophyll molecule - the green pigment responsible for photosynthesis in green plants. Magnesium also serves as an enzyme activator of a number of carbohydrate metabolism reactions. In addition, the element has both structural and regulatory roles in protein synthesis.
Magnesium symptoms appear in mid- to late season and include marginal leaf yellowing or reddening of basal leaves, which extends to the interveinal area, while the mid-vein region remains green (See Figure 16.5). Magnesium is mobile within the vine and, under deficient conditions, is readily translocated from older to younger tissue.
Assessing the Need for Magnesium Fertilizer
As with most other macronutrients, tissue sampling can be used to determine the vines’ magnesium status. Tissue analysis results (Table 16.3) coupled with visual observations should indicate whether to apply magnesium.
Time of Application
Magnesium can be applied almost any time of the year because it is not rapidly leached through the soil profile.
Application Methods
Direct Soil Surface Application: Dry magnesium fertilizers are applied directly to the soil surface either as broadcast or banding. Broadcasting potassium fertilizers over the entire vineyard is less efficient and less economical. If banded, the magnesium fertilizer should be applied in a 3 to 5 feet (0.9 to 1.5 m) wide band, parallel to the row. Take care to keep the band 18 to 24 inches (46 to 61 cm) away from the base of the vines.
Fertigation: Magnesium applied by drip irrigation offers better control of application.
Foliar Application: The use of foliar spray with macronutrients has limited benefits when compared to micronutrients.
Soil Factors Affecting Availability
Magnesium deficiency is often observed where vines are grown in soils of low pH (less than 5.5) as well as high pH soils and where potassium is abundantly available.
Magnesium Toxicity
Problems associated with high magnesium soils may include the following; reduced vine growth and crop load, potassium deficiency, poor soil structure, and slow water infiltration.
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