Yellowing of Crop Leaves? It Could Be Iron Chlorosis.

Iron chlorosis is a yellowing of plant leaves caused by iron deficiency. However it is not always a true iron deficiency but rather an iron tie-up in plants and soil. Yellow leaves indicate a lack of chlorophyll, the green pigment responsible for photosynthesis (sugar production) in plants.

The causes of iron chlorosis are complex and not completely understood. It could be a deficiency in the soil or the plant or an iron tie-up in the soil or plant. Many reactions govern iron availability and make iron chemistry in the soil complex. Iron chlorosis generally occurs in calcareous soils with a high pH. Even though these soils have plenty of iron, the high pH causes chemical reactions that make the iron unavailable to plant roots

Plants that are native to high pH soils don’t illustrate symptoms of iron chlorosis, because they have evolved to use iron efficiently and can obtain iron from the soil. Recent research has discovered that bicarbonates (HCO3) play a major role since they are readily produced in high pH soils, especially when moist conditions limit iron availability. Even in alkaline soils, bicarbonate is much higher in the calcareous soil than in the non-calcareous soil.The causes of iron chlorosis are complex and not completely understood. It could be a deficiency in the soil or the plant or an iron tie-up in the soil or plant. Many reactions govern iron availability and make iron chemistry in the soil complex. Iron chlorosis generally occurs in calcareous soils with a high pH. Even though these soils have plenty of iron, the high pH causes chemical reactions that make the iron unavailable to plant roots

In calcareous soils, bicarbonate inhibits mobilization of accumulated irons from roots to foliage and affects availability of iron in the soil by buffering soil pH. When high bicarbonate irrigation water is applied, iron deficiency is enhanced because bicarbonate is supplied to the soil. The adverse effects of high bicarbonate levels are exacerbated in saturated soils, very dry soils, or compacted soils, where bicarbonate levels increase, leading to diminished root growth and nutrient uptake.

Gypsum can help remediate iron chlorosis. This is because gypsum reacts with bicarbonate to form calcium carbonate (CaCO, or lime) decreasing the amount of bicarbonate in the soil that affects iron uptake and iron availability.

Daniel Davidson, Ph.D. Agronomist