Correcting Iron Deficiency

Yellow leaves on a green plant can indicate a number of problems, including lime-induced chlorosis. The symptoms of this condition are yellow leaves with darker green veins. It occurs on the new growth and when it's severe it can cause the whole leaf to become pale yellow or almost white. Lime-induced chlorosis affects many types of plants and happens when iron in the soil is 'locked up' and not available to the plant.

Alkaline soil is the major source of this problem and this is caused by the application of alkaline water, mulch or compost (such as spent mushroom compost). Sometimes it can be caused by lime leeching out of the render in walls and buildings.

To correct this problem, apply iron chelates. This should be mixed with water and can be applied as a foliar spray or around the root zone. If the weather is still warm, the leaves should start to green up within a week, however it will probably take a couple more applications at the rate of one application every two to four weeks until the leaves are as green as they should be. 

Treating your plants with iron chelates is only a short-term solution to the symptoms of chlorosis and it doesn't actually treat the soil. For a long-term solution, apply agricultural sulphur.