The not so Mellow Yellow story of the simple leaf
Iron (Fe) is required by all plants and without it plants cannot function as they should. Iron plays a part in many of the vital functions of the plant. Iron deficiency, known as Iron chlorosis, can affect many plants, and yellowing of the leaves, whilst the veins remain green, is a good indicator of iron deficiency. Typically yellowing starts at the tips of new growth and works its way to older leaves as the deficiency gets worse.
Iron chlorosis in plants is typically caused by one, or more, environmental factors. A soil with a pH that is too high, making it more alkaline.
Magnesium (Mg) is another nutrient deficiency that can cause yellowing of leaves. Plants lacking magnesium will usually turn a pale green or yellow colour, again between the leaf veins and often with red, purple or brown hues and early leaf fall. First signs of a magnesium deficiency are the yellowing of older leaves near the bottom of the plant. Common in apples, cherries, camellias and roses, amongst others.
Similar to iron deficiency there is not usually a shortage of magnesium in the soil, unless you have a light sandy soil, but over application of potassium rich fertilisers can cause a problem as the plant will take up the potassium in preference to the magnesium. Reduce application of potassium fertilisers and apply a foliar feed during the summer, of Epsom salts. Or for a longer term solution apply to the soil around the roots.