Fun Fact: Lead gets its chemical symbol Pb from the Latin word for lead: plumbum. Molybdenum gets its chemical symbol Mo from the Ancient Greek word for lead: molybdos (μόλυβδος) as molybdenum ores were confused for lead ores!
Molybdenum (Mo) occurs in soil in even smaller amounts than copper and zinc: 0.6 to 3.5 ppm (parts per million).
Molybdenum is a metal and Mo ions have a positive charge, but in soil it occurs mainly as the complex negative anion MoO42- (molybdate). This makes molybdenum behave more like phosphate (PO43-) or sulfate (SO42-) in soil.
It varies from soil to soil, but most soil molydenum can typically be locked up in various soil minerals in a nonexchangeable form. A smaller fraction is in organic matter, and the breakdown of this releases plant-available molydenum to the soil water.
Molybdate is quite soluble but adsorption to particles (and thus lack of availability) increases with decreasing pH. (This is the opposite for the other micronutrients.) However, even low pH soils with high amounts of organic matter can provide enough molybdenum to plants.
Dry plant matter usually contains less than 1 ppm molybdenum, meaning plants require even smaller amounts of this than they do of copper. It is uptaken as molybdate.
Molybdenum is an essential component of two major enzymes in plants. These convert nitrates to nitrite, then to ammonia, which is used to synthesise the amino acids which form proteins. (Similarly, molybdenum is essential in the nitrogen cycle, where molybdenum-containing enzymes are needed by the nitrogen fixers, the nitrifying bacteria, and the denitrifying bacteria.)
Unlike the other micronutrients, molybdenum is mobile within a plant, and a deficiency will show in older and middle leaves first.
Because of molybdenum’s crucial role in the conversion of nitrates to proteins, a deficiency will strongly resemble a nitrogen one, with stunted growth and chlorosis of the leaves. Flower development may also be affected.
Where a molybdenum deficiency differs from a nitrogen one is at the leaf margins — necrosis appears very quickly here as nitrates accumulate in the leaves and cause ‘nitrogen burn’.
Sulfate ions can compete for uptake and potentially cause a molydenum deficiency.
Lime can overcome a deficiency if the soil is very acidic, but this is slow to take effect. A simple molydenum application (not much is needed) will have the same effect more quickly, and is especially suitable if the soil pH is already within a required range.
Molybdenum toxicity is very rare — some plants can retain levels over one hundred times higher than normal without showing effects. A toxicity, should it appear, is marked by golden yellow leaves with sometimes a blue tinge.
The key nutrients and essential micronutrients certainly amount to a very complex story, especially in terms of trying to work out how well my backyard soil in Canberra meets my plants’ various nutrient requirements. I sometimes wish that us gardeners could simply purchase a truck load of rich volcanic "chocolate" soil, such as that found between Bungaree and Blampied (potato country) in Victoria. But knowing what nutrients plants require, and what level of soil ph is most effective, is very helpful when selecting compost, lime/dolomite, fertilizers and trace elements. To be sure I’ve got everything covered, I like to apply a goodly layer of chicken compost in addition to NPK etc, at least it smells like its got the lot. Thanks Kristi for the post.