Magnesium is the most commonly deficient secondary nutrient. In many cases deficient plants show no obvious symptoms, except reduced yields. The most common visual symptom is the yellowing of older leaves, especially in the areas between the veins (leaf margins and veins stay green), giving the leaves a mottled effect. These yellow patches may eventually turn into necrotic lesions. The leaves (or their margins) often take on a reddish hue and their tips may curl upward. Affected leaves commonly drop and it can eventually defoliate most of the plant. Magnesium is quite mobile in the plant, so symptoms first appear in older leaves. Cauliflower and Potato are good indicators of magnesium deficiency.
Reasons for deficiency:
Leaching: Magnesium cations are easily leached, so a deficiency sometimes occurs in light sandy soil in humid areas.
Low pH: Magnesium is most available at a pH of 6.0 to 8.5, so may be unavailable in acid soils. Simply raising the pH may solve a deficiency problem.
Nutrient imbalance: An excess of potassium or calcium can cause a deficiency of magnesium.
Sources of magnesium:
Organic matter in the form of compost or manure is the best source of magnesium for a mature organic garden.
The most common source of magnesium for the garden is dolomitic lime (calcium magnesium carbonate). If your soil is deficient in magnesium always lime with this in preference to calcitic limestone. Basic slag is another liming agent that contains magnesium.
In alkaline soils you can’t use dolomitic lime, because it will raise the pH even further and can lead to other problems. In such a situation you can dissolve 10 ounces of Epsom salts (magnesium sulphate) in 10 gallons of water for every 100 square feet of soil. If a magnesium deficiency shows up in the early stages of growth, you can use Epsom salts as a foliar feed (3 ounces in a gallon of water). You may also try a liquid seaweed fertilizer.