Magnesium is moderately leachable in soil and, as with calcium (Ca), greater amounts are often found in the subsoil than in upper parts of the soil profile, especially on older, highly weathered soils. Magnesium deficiency is more prevalent on old, leached hardpan soils, wind-modified sandy soils, and recently reclaimed alkali soils. Fill areas from land leveling are more susceptible due to the further burying of the higher-Mg subsoil. Likewise, young vines are more deficiency-prone until their root systems penetrate subsoils.
Magnesium is mobile within the plant and, under deficient conditions, is readily translocated from older to younger tissue. Thus, older basal leaves show the first signs of chlorosis, usually in mid- to late summer. The chlorosis begins at or near the leaf edge and progresses inward between the primary and secondary veins. Some border of green color remains along the main veins; the chlorotic area may become almost creamy-white. The remaining green tissue surrounding the main veins is best described as a "Christmas tree pattern." The chlorotic margins can become necrotic and turn brown as the deficiency becomes more advanced.
L. Peter Christensen
William L. Peacock
Christensen, L.P. 2000. Mineral Nutrition and Fertilization (PDF). Pages 102-114 in: Raisin Production Manual. University of California, Agricultural and Natural Resources Publication 3393, Oakland, CA. Buy book
Christensen, L.P. 1984. Nutrient level comparisons of leaf petioles and blades in twenty-six grape cultivars on three years, 1979-1981. Am. J. Enol. Vitic. 35:124-33. Abstract
2004. Soil Environment and Grapevine Mineral Nutrition Symposium. Proceedings of the American Society of Enology and Viticulture. To Purchase