Understanding Crop Iron Nutrition & Deficiency Issues
Iron (Fe) is abundant in many rocks and minerals. However, it’s availability for plant uptake is low due to conditions in the soil that promote poor solubility (Elkins and Fichtner 2012). Despite the large amounts of iron in the Earth’s crust, plants can only take up the nutrient as Fe2+, or sometimes as Fe3+. It is the poor solubility of these forms of iron, and their interactions with factors as detailed in this article, that cause the iron deficiency symptoms seen in many Western US crops.
Crops require iron for the following reasons (IPNI Iron Factsheet #12):
- Iron is essential for chlorophyll development & function – this is what keeps a plant green
- Acts as an oxygen carrier & is a constituent of certain enzymes & proteins
- Involved in reactions involving cell division & growth
- For legume crops, iron is an important component of the enzyme nitrogenase, which helps bacteria turn atmospheric nitrogen into a more plant available form
Iron is plant immobile, and therefore, deficiency issues will show up on the youngest or newly formed leaves. Fe is a main component of chloroplasts, so Fe-deficient plants exhibit reduced photosynthetic activity and tend to have a yellow coloration or “Christmas tree” type pattern on their younger leaves (Elkins and Fichtner 2012).
Iron deficiency, also called lime induced chlorosis, is generally caused by alkaline soil pH and the presence of bicarbonates in the irrigation water and soil. There are some other factors, (e.g., oxygen availability, organic matter %, etc.) but pH and bicarbonates are the big two. Effect of pH – Despite the large amounts of iron in the Earth’s crust, plants can only take up the nutrient as Fe2+, or sometimes as Fe3+. Iron becomes oxidized in high pH soils (>7), rendering it unavailable to plants.
- Bicarbonates buffer pH changes in the root zone. This prevents the plant from acidifying the soil environment to solubilize adequate Fe supplies needed for growth.
- Bicarbonates also inhibit mobilization of accumulated Fe from roots to foliage, which limits the transfer of the nutrient to the plant parts that need it for proper photosynthesis (e.g., new growth). Furthermore, roots can become crusted with bicarbonates as water evaporates, preventing normal root growth and function needed to solubilize and take up Fe.
Fe Deficiency Impacts Yields:
Citrus – Iron deficient citrus trees produced 20% less yield than trees that had been treated. Furthermore, fruit set was decreased by 38%, and fruit drop increased by 30% on iron deficient trees, relative to treated trees (http://www.yara.us/agriculture/crops/citrus/key-facts/role-of-iron/)
Treatment and Response Time:
Pre-plant or long-term treatment - Soil-based pre-plant treatments to reduce pH and bicarbonate levels include sulfur (S) and acid applications.In-season treatment - Foliar application of inorganic Fe salts (e.g., ferrous sulfate) OR soil or foliar application of synthetic Fe chelates (e.g., metal ion surrounded by organic molecule)
Table 1 – Advantages and disadvantages of iron fertilizer products (Liu et al. 2012).
inorganic Fe salts (e.g., ferrous sulfate)
Inexpensive and commonly found
Fe ion is not protected from adverse soil conditions (pH oxidation and bicarbonate) and can become unavailable to plant; Charged ion can be washed off waxy plant surface
Fe chelates (e.g., Ferrilene)
Chelate protects Fe ion from adverse soil conditions (pH oxidation and bicarbonate); Chelate structure allows for better penetration through waxy leaf cuticle
Chelation type strongly influences the stability of the fertilizer and protection of the Fe ion
Crops that are Sensitive to Fe Deficiency:
- All citrus
- All Cane berries
Understanding and improving the iron nutrition of your crop requires the integration of several avenues of thought across your farm including soil type, soil test results, crop specific Fe needs, and available nutrient sources. Contact your local Helena advisor for more information on meeting crop Fe demand.
Causes and Control of Lime-induced Fe Deficiency in California Fruit and Nut Crops, Rachel Elkins, University of California Cooperative Extension, Lake and Mendocino Counties; Elizabeth Fichtner, University of California Cooperative Extension, Tulare County, https://tinyurl.com/yca3yo2v
IPNI Nutrient Factsheet – Iron, https://tinyurl.com/y8dhakvg
Effects of Fe Deficiency Chlorosis on Yield and Fruit Quality in Peach, Ana Alvarex-Fernandez, Pilar Paniagua, Javier Abadia and Anunciancion Abadia, https://www.sciencedirect.com/science/article/pii/S0098847210002868