Molybdenum (Mo) is an essential trace element for the production and function of nitrate reductase, an enzyme found in soil.
Although required in minute amounts, Molybdenum (Mo) is instrumental for normal plant development. Most likely deficient in poinsettias, Mo helps regulate various functions that plants rely on and should never go overlooked when improving crop growth and quality. With the ideal balance between 0.3-1.5 parts per million vegetation tissue or .01-.20 ppm growing medium — it's safe to say everything will come up roses.
Molybdenum's Function in Plants
● The symbiotic nitrogen-fixation mechanism involving Rhizobia bacteria in legume root modules requires molybdenum.
● Molybdenum is essential because it converts nitrogen into amino acids and because nitrogen fixation occurs in legumes, which boosts crop yields.
● Plants can only absorb molybdenum in its molybdate form.
● Nitrogenase and nitrate reductase, two enzymes crucial to nitrogen-fixing and nitrogen reduction, need molybdenum to carry out their respective activities.
● Molybdenum is required as a co-factor for nitrogenase enzymes in symbiotic N fixation, where it catalyzes the redox process that transforms elemental N into ammonium (NH4+) ions.
Molybdenum in Soil
Plant-available Molybdate (MoO4), an anion of Mo, is present in the soil. Normal weathering processes liberate it from solid minerals, where it subsequently undergoes several reactions. Clays, metal oxides (Fe, Al, and Mn), organic molecules, and carbonates are all susceptible to adsorption processes after MoO4 anions have been dissolved. Similarly to the chemically related nutrient phosphate, soil pH has a significant impact on MoO4 solubility. The plant availability of molybdenum is the only micronutrient that increases with increasing soil pH. For each increment in soil pH, molybdate solubility rises by a factor of around 100. Therefore, liming acid soils is a crucial management strategy for increasing Mo availability.
Deficiencies in Mo are infrequent in soils with a pH of 6 or above. Sulfate fertilizer reduces MoO4 absorption because it competes with MoO4 for root uptake sites.
Fertilizing peanuts with single superphosphate (SSP) was proven to reduce the plant Mo concentration by a remarkable 70%, whereas applying triple superphosphate (TSP), which contains no sulfates, buoyed these levels back up by 20%. Phophorus-rich fertilizers can actually incentivize soil solids to release more molecules of Mo that are subsequently absorbed and stored in plants.
Molybdenum Deficiency
Molybdenum has a unique property that stands out from other micronutrients - its mobility. However, this comes with a downside, as any deficiency can cause symptoms in older and middle leaves before spreading their influence throughout the stem of the plant and impacting new leaves too.
In poinsettias, this manifests as chlorotic margins on the outer edges of the leaves, which later become necrotic.
Paleness spreads throughout the leaf; in certain crops, it might cause necrosis at the leaf margins. Cauliflower 'whiptail,' a leaf deformity caused by the plant's rapid growth, has been linked to this issue. What we call "whiptail" occurs when the leaf's midrib expands, but the leaf blade's breadth is severely limited.
Plant development and flowering will get stifled later on.
Crucifers (cauliflower, broccoli, cabbage), legumes (peas, beans, clovers), poinsettias, and primroses are the most vulnerable crops to molybdenum deficiency.
Primarily feeding nitrate fertilizer will produce a molybdenum shortage before treating it with ammoniacal fertilizer because molybdenum is required to convert nitrate to ammonia inside the plant.
When it comes to plants, molybdenum is the only micronutrient that can't stick around when pH levels drop—but research shows that high sulfates, in particular, might speed up this process. So if your garden's not providing enough of this essential trace mineral, check out its pH first and foremost. And for an extra boost? Consider throwing a fertilizer supplement into the mix.
Fertilizing Crops with Molybdenum
Using a liming agent to raise the pH of many soils can free Mo trapped in insoluble forms. Soybean yields, for instance, were found to be the same whether Mo fertilizer was applied to unlimed soil or lime was added beforehand. However, the chemical release of soluble Mo after lime treatment may take many weeks to months.
If the soil's Mo content is low and lime isn't needed for crop development, then the following methods of fertilization with more Mo may be beneficial:
Soil: To enrich the soil with molybdenum, you may either band it or scatter it. Typical application rates are between 0.5 and 2 lb/A. To ensure even distribution, it is often combined with other fertilizers or dissolved in water and sprayed on the land before planting. However, the limited solubility of molybdenum trioxide (MoO3) limits its use in soil.
Foliar: Applying soluble Mo sources, such as sodium or ammonium molybdate, is common practice to a plant's foliage. Applying diluted Mo solutions through foliar spray is most beneficial during the early phases of plant growth.
As opposed to soil treatments, which have a more significant residual effect, foliar sprays are helpful for quickly repairing Mo deficient symptoms.
Seed: Fertilizing seeds with carefully controlled amounts of Mo can help crops in areas where the mineral is deficient. This ensures that each seed receives an adequate dose for strong and healthy growth while also guarding against giving too much, which could negatively affect germination rates or end up harmful to grazing animals if it accumulates over time. For legume crops such as beans, rhizobia inoculants are sometimes amended with just enough extra Mo to boost nitrogen fixation without putting any livestock at risk.
Signs of Molybdenum Toxicity
Molybdenum toxicity is highly uncommon; some plant tissues can contain up to a few thousand parts per million of the element without showing any ill effects. However, in very unusual instances, the leaves turn golden-yellow.
Molybdenosis is a disease that can affect ruminant animals if they consume grasses or feeds that contain high amounts of molybdenum (5-10 ppm in the tissue). Still, it does not affect horticultural crops. Instead, this disorder causes copper deficiency because of the excessive molybdenum competing with copper.
Conclusion
With the unpredictable season ahead, giving your plants every advantage is essential. Supplementation of Mo fertilizer can help increase crop yield and growth by allowing better utilization of N - especially in acid soils where lime application increases its availability. Where extra Mo levels are needed, research has shown significant improvements with applications- so don't let this valuable resource be left out.
