Of all the micronutrients recognized today, molybdenum is needed in the smallest amount. It was identified as a micronutrient in 1939 and is required by all plants for proper development and nitrogen assimilation.

Alfalfa, soybeans and other legumes are especially sensitive to low levels of molybdenum.

How It Behaves.

In plant development, molybdenum aids with the formation of essential amino acids.

“It’s a critical nutrient in the nitrogen conversion process, converting nitrates to ammonium in the plant,” says Ron Olson, senior agronomist for Mosaic.

In legume plants, molybdenum is required for the nitrogen fixation process, and in non-legumes molybdenum is required for proper nitrogen uptake through the roots. Consequently, molybdenum deficiency closely resembles — and is sometimes misdiagnosed as — nitrogen deficiency in plants.

Initial symptoms appear as yellowing and stunted growth, with marginal scorching and rolling, or cupping of the edges of the leaves occurring if the deficiency is more severe.

Plants take up molybdenum as the molybdate ion MoO42-, which is an anion in the soil. While uncommon, deficiencies appear more often in sandier, heavier-weathered soils. Molybdenum tends to be more available in young volcanic soils, as well as soils with more organic matter or those derived from shale or granite.

Olson notes that sulfate sulfur and molybdenum can compete in the soil.

“High levels of sulfate sulfur in the soil can suppress molybdate because it may act as an ion antagonist. However, this is very rare, usually occurring in situations with a lot of manure or ammonium…