Magnesium plays an important role in photosynthesis as the center of the chlorophyll molecule and acts in enzyme reactions forming proteins, and aiding the plant in protein synthesis.
Deficiencies of magnesium in grapevines are most common on sandy, acidic soils. Additionally, high rates of K fertilizer without an Mg source can induce a deficiency. A grapevine petiole K:Mg ratio that exceeds 5:1 can increase the likelihood of a Mg deficiency. Soil levels of Mg are likely to result in a deficiency when magnesium base saturation is less than 5% of CEC or total exchangeable Mg is less than 25 mg/kg of soil. Applying a balanced fertilizer like Intrepid Trio® with both K, Mg, and S in each granule will keep your grapevines supplied with the nutrients it needs to continue to produce high quality fruit.