Three Reasons Your Crop Might Be Low on Iron
Why Plants Need Iron
Iron (Fe) is an essential element required for plant growth and reproduction that is needed by plants in only relatively small amounts, thus making it a micronutrient. Interestingly, soils often have abundant iron, but most of it exists in insoluble forms plants cannot use (e.g., iron oxide). Iron is involved in the manufacturing process of chlorophyll and is critical for a few specific enzyme functions such as those involved in nitrogen fixation in legumes. Iron is also found in the cytochromes, which are part of the electron transfer systems in chloroplasts and mitochondria. Therefore, proper iron nutrition is critical for optimizing the overall plant energy and nutrient metabolism, which generally contribute to crop yield and quality goals.
Iron’s involvement in chlorophyll synthesis is the reason for the chlorosis (yellowing) commonly associated with Fe deficiency. Iron is relatively immobile once incorporated into the tissues in the upper parts of the plants, and as a result, the translocation of Fe from one plant part to another is minor. Therefore, Fe deficiency symptoms show up first in the youngest leaves. Poor iron nutrition can also result in poor nodulation of legume crops, leading to reduced size and yields.
Why Iron is Unavailable to Your Plants
Managing the iron nutrition of your crops can be difficult due to the following three mechanisms:
- Alkaline pH soils - Soils with an alkaline pH often exhibit crops with an Fe deficiency because Fe is largely in the oxide and unavailable form. Furthermore, high levels of calcium, carbonate and bicarbonate in the soil can lead to Fe deficiencies due to the formation of insoluble compounds and competition of Ca with Fe uptake by the root.
- Acidic pH soils - Acidic soils also can lead to Fe deficiency due to manganese competition with Fe uptake. Furthermore, in acidic soils, phosphate and iron readily form insoluble precipitates, thus rendering both nutrients unavailable for plant uptake.
- Saturated soil - Crops that are irrigated with the flood irrigation technique can show symptoms of iron chlorosis. This is also true for crops that are overwatered or have poorly draining soils. Water-logged soils result in a chemical environment where insoluble iron compounds are allowed to persist, and ionic constituents, such as bicarbonates, form insoluble iron compounds in the irrigation water. Well aerated, freely draining soil can help promote a chemical environment that favors the formation of a higher concentration of more soluble iron compounds that can be used by plants.
Iron chlorosis is a common nutrient deficiency in crops in the Western US as explained by the three mechanisms above. Also, certain crop varieties, such as citrus and related cultivars, are relatively susceptible to iron deficiency. Soil and tissue testing can help identify potential issues with crop iron nutrition in order to take corrective action (Figure1).
Figure 1 – A crop of lemons had severe iron chlorosis due to acidic soils and heavy applications of phosphoric acid, leading to a localized reduction of available Fe for crop growth. As a result, the plants were severely chlorotic and stunted (far left). A soil-applied and foliar Fe program was implemented, based on soil and tissue test results, which resulted in a permanent improvement in crop appearance in growth over the course of 2 months (far right).
- Iron Nutrition of Plants - http://edis.ifas.ufl.edu/ss555
- Control of Iron Chlorosis in Ornamental and Crop Plants - http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1884&context=extension_curall
- Iron Chlorosis - http://www.ext.colostate.edu/mg/gardennotes/223.pdf
- Dr. Karl Wyant, Western Division Lead Agronomist