Molybdenum Fertility in Crop Production
Function in Plants
- Molybdenum is a micronutrient required in very small amounts for plant growth.
- Molybdenum is a component of the enzyme nitrogen reductase which regulates the nitrogen reduction process in plants. This process involves the conversion of nitrate (NO3) to the amino form (-NH2) to build proteins.
- In legumes, such as alfalfa and soybean, molybdenum is also a component of nitrogenase, an enzyme needed for nitrogen fixation.
- Most crops require less than 1.0 ppm of molybdenum. Of the 17 essential nutrients, molybdenum and nickel are needed in the smallest quantities.
- Leguminous crops such as alfalfa and soybean require more molybdenum than grasses and other non-legumes.
- Molybdenum deficiency is very rare in corn.
- Molybdenum deficiency can occur in soybean in acidic and highly-weathered soils, and can result in significant yield reductions.
Availability in Soil
- Molybdenum is taken up by plants in the anion form molybdate (MoO42-).
- Molybdate is released from the weathering of soil minerals.
- Soils typically contain between 0.25 and 5.0 ppm total molybdenum.
- Molybdenum is the only plant micronutrient that becomes more available as pH increases (Figure 1). Solubility increases 100x for every point increase in pH.
- Deficiencies rarely occur in soils with a pH greater than 6.2.
- High concentrations of sulfate in the soil can limit molybdenum availability, as sulfate (SO4) and molybdate (MoO42-) compete for root uptake sites.
- Addition of phosphate can promote plant uptake of molybdenum by causing molybdate adsorbed to soil solids to be released.
Figure 1. Relative availability of molybdenum by soil pH.
- Since molybdenum is essential for nitrogen metabolism, a deficiency of molybdenum will manifest in plants as nitrogen deficiency, with leaves that are light green or yellow.
- Leaves may yellow, cup or roll, have scorching in leaf margins, and older leaves can become chlorotic.
- Molybdenum is mobile in plants so deficiency symptoms can appear over the entire plant, often appearing first on the oldest leaves.
- In most soils, liming to increase the soil pH can increase the concentration of available molybdate and eliminate deficiencies, making liming the best molybdenum fertility strategy in most cases.
- In soils where liming is not practical and molybdenum concentrations are low, molybdenum fertilizers can be applied.
- Sodium molybdateis the most common form of molybdenum fertilizer. It can be banded or broadcast on the soil, applied with a foliar treatment, or incorporated in a seed treatment (Table 1).
- Soluble molybdenum sources, ammonium molybdate and sodium molybdate, are suitable for foliar application and are typically applied at a rate of 2-3 oz/acre.
- Seed treatments that include molybdenum fertilizer are frequently used in areas with molybdenum deficiencies. A rate of 0.5 oz/acre is usually adequate.
Table 1. Fertilizer sources of molybdenum.
- Soybean yield responses to molybdenum fertilizer have been documented in soils with pH between 5.6 and 6.0 (Rasnake, 1982).
- At soil pH levels below 5.5, molybdenum fertilizers may not be effective.
Authors: Samantha Reicks and Mark Jeschke
IPNI. 2014. Molybdenum. Nutrifacts No. 13. International Plant Nutrition Institute.
Rasnake, M. 1982. Use of molybdenum for soybean production. Soil Science News and Views. Vol. 3 No. 4. Univ. of Kentucky Extension.
Sawyer, J. 2012. Nutrient deficiencies and application injuries in field crops. IPM42. Iowa State Univ. Extension.
Schulte, E.E. 1992. Understanding plant nutrients: Soil and applied molybdenum. A3555. Univ. of Wisconsin Extension.