A to Z of Crop Nutrients: Iron (Fe) and Molybdenum (Mo)

In a way, Iron and Molybdenum act quite differently in relation to the soil. Molybdenum (Mo) becomes more available as the pH levels in your soil increase, while Iron (Fe) is quite the opposite and becomes more available at lower pH levels. These two micronutrients are a great example of why a balanced pH is very important to help maximize crop production. Soil pH levels in the 6.2 – 6.5 range should be adequate for both nutrients, assuming other imbalances don’t exist. Let’s dig in a little deeper.

Iron (Fe) – Iron is taken up by the plant as the Fe+2 cation and plays an important role in chlorophyll production. Since Iron is not mobile in the plant, the deficiencies will show up on the youngest leaves of the plant or newest growth. Typically the veins are a dark green color while the interveinal tissue is chlorotic.

The most commonly seen Iron deficiency appears in soybeans and is commonly referred to as IDC or Iron Deficiency Chlorosis. Typically these deficiencies are most common in high pH soils because Iron availability declines as pH increases. High levels of other metal cations can also interact with Iron and cause deficiencies. It is very challenging to correct an Iron deficiency with soil applied fertilizer compounds because they are often rendered unavailable by soil reactions. The most common way to correct a deficiency is via foliar applications. Applying sulfur-containing fertilizers in a band can lower the pH enough to improve iron availability locally near the root.

Molybdenum (Mo) – Molybdenum is taken up by the plant as the MoO4(2-) anion. Since soil is negatively charged, it is more mobile in soils. Mo is important in corn because it helps in the enzyme reaction of nitrate reductase. The corn plant takes up the majority of its nitrogen in the nitrate form, but once in the plant the nitrate is converted to ammonium via an enzyme called nitrate reductase. Therefore adequate levels of Mo are important for proper nutrition in the corn plant. In soybeans Mo is important in the symbiotic N fixation process via rhizobia bacteria. It is a component of nitrogenase which is the enzyme needed by rhizobia to fix the N.

In soybeans a Mo deficiency can appear as a nitrogen deficiency because a lack of the nutrient reduces N fixation. Moly is unique because it becomes more available at a higher pH in the soil. Because Mo is mobile in the soil via soil water, dry conditions can increase the likelihood of a Mo deficiency. In sandy or low organic matter soils the retention of the MoO4(2-) ion can also be limited. Higher levels of phosphorus tend to increase Mo availability while high levels of sulfate can create deficiencies because the plant uses the same mechanism to take up both ions so they can compete. If soils are lower in Mo then high sulfate levels can induce a deficiency. Because Mo is required in such small amounts typically a seed treatment is a common method of application, although foliar treatments can correct a deficiency in season.

Note: Much of my information comes from the book Soil Fertility and Fertilizers (Havlin, Tisdale, Nelson and Beaton Eighth Edition) which is a common textbook in colleges. It’s a great resource if you are into reading textbooks.