Answer: Magnesium deficiency is probably the most common deficiency, particularly with tomatoes. (Tomatoes are great for learning about diseases !) The lower, older leaves turn yellow while the veins remain green. If the plants are at the fruiting stage 'green back' occurs. However these symptoms can also be caused by overwatering or by the practice of flooding the tomato beds to reduce watering time. Lazy gardeners will have some experience of this.
The traditional remedy is to apply Epsom salts (magnesium sulphate) as a spray, at the rate of half an ounce to a gallon of water.
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