In plant nutrition, molybdenum is classified as a micro nutrient as the amount required is very, very small, as little as 50g/ha will satisfy the needs of most crops.
Molybdenum (Mo) is a transition metal that sits on the second larow in the middle of the Periodic Table. It can exist in many transition states, from zero to VI, ideal properties for a catalyst. Mo is an essential nutrient for prokaryotes, plants and animals.
Role of Molybdenum in Crop Production
In plant nutrition, molybdenum is classified as a micro or minor nutrient as the amount required is very, very small, as little as 50g/ha will satisfy the needs of most crops. The word minor nutrient is misleading. Molybdenum is an essential nutrient and is used as a catalyst in many plant enzymes that are crucial to plant growth. Small amounts of molybdenum (100g/ha) can have major yield impacts and make or break a crop.
Molybdenum deficiencies are generally rare because it is found in manure, compost, soils, waters, fertilizers, seeds and parent plant material. However, every now and then circumstances come together and there is a molybdenum deficiency in a crop that can be quite cataclysmic. Either, there is no molybdenum, or conditions in the root zone make it impossible for the plants to take it up.
Mo is not biologically active by itself but is an integral part of an organic complex called the molybdenum co-factor (Moco for short). Moco binds to molybdenum-requiring-enzymes that are required in most biological systems essential in plants, animals and prokaryotes.
Molybdenum is essential inside and outside the plant for the metabolism of nitrogen. Nitrogen is a major nutrition and the biggest building block for plant tissue. The root nodule bacteria (rhizobia) that fix nitrogen for leguminous plants need molybdenum to do so. If there is no molybdenum in the soil, there is no nitrogen fixation. In leguminous plants, a seed dressing of Mo is generally applied because of this. Plants suffer a nitrogen deficiency – leaves start yellowing and may have scorched edges, plant are very stunted, and eventually die. Production is severely compromised. In fact, strangely enough, the amount of Mo required by the rhizobia to fix nitrogen is MORE than the amount of Mo required by the plants to utilize nitrates.
Moco enzymes are required for the nitrogen cycle in plants and the soil. Enzymes include nitrate reductase and nitrogen fixing nitrogenase. No Mo, no N fixation, no nitrate utilization within the plant, and no protein production in some plants. Mo deficiencies can therefore be expressed as N deficiencies.
Mo is a cofactor in sulphite oxidase, which catalyzes the conversion of sulphites to sulphates, a reaction necessary for the metabolism of sulphur containing amino acids (methionine and cysteine). Important for plants and animals. Mo deficiencies can therefore look similar to sulphur deficiencies.
Plant growth and development is regulated by auxins. These auxins regulate cell division, cell expansion, cell differentiation, lateral root formation, flowering, seed production and tropic responses in plants. Indole-3-acetic acid (IAA) is the major auxin. Auxin production and distribution within the plant is very complicated and not totally understood.
However, molybdenum cofactors (Moco), notably molybdenum cofactor sulphurase, play a major role in regulation and distribution of IAA around the plant. Mo deficiencies can therefore come out as abnormal plant growth, distortion of tissue and tissue death (some very virus like symptoms!).
Plants use molybdenum to convert inorganic phosphorous into organic forms in the plants – a deficiency can also look like a phosphorous deficiency.
So, in a nutshell, Mo deficiencies can look like nitrogen, sulphur and phosphorous deficiencies or a viral infection in a plant, can severely affect yield and quality, and can cause plant death. The uptake of Mo into the plant is dependent on many soil properties, and the definitive test to determine a deficiency is a leaf nutrient analysis. Once identified Mo deficiencies are easily and cheaply treated.
Uptake of Molybdenum
Molybdenum uptake is strongly dependent on soil pH. The most available form of Mo for plants is the anion MoO4(-). In alkaline soils this anion is very soluble and available, in fact high levels of Mo in plant tissue generally indicate alkaline soil / media. In acidic soils (pH <5.5) or soils with high concentrations of adsorbing oxides (eg Al or iron oxides), Mo uptake is severely restricted.
Water application and drainage also play a big part in Mo uptake. As an anion, Mo can quickly leach out of alkaline soils if they are over-irrigated. Mo uptake is restricted in waterlogged organic soils (eg old compacted cocopeat).
Once an molybdenum deficiency has been established through leaf analysis – a grower should do further testing / exploration to identify the root cause of the problem. Liming acidic soils will increase availability and uptake. Reduce watering in waterlogged organic soils – or apply a foliar feed (50g/ha!). Remember its not just the plants that need the Mo – the soil microbes need it too.
More and more, Mo deficiencies are popping up. They may be there because Mo is not applied – in cocopeat or crops irrigated with rain water or reverse osmosis water – a tiny amount of Mo needs to be added to the fertigation water (0.2-0.5 ppm).
Short term crops planted from seed may have enough Mo in the seed to last their crop cycle (so seed levels of Mo are important, and these are determined by the Mo in the seed producing plants), or the seeds can be dressed with Mo seed dressings. Mo can also be applied into the seed bed of seedlings before transplanting.
In long term crops like roses and some summer flowers, there may be enough Mo in the young plants to take the plants through a few months growing, but as the plants continue to grow and produce the Mo runs out.
Excess Molybdenum In Plants
Excess Molybdenum in plants is generally thought not to cause too many problems except in two cases.
- Excess application of Mo may cause problems with copper uptake and cause copper deficiencies in plants – especially in high Mo alkaline conditions.
- Excessive rates of Mo application, irrigation fields with alkaline water, liming of high Mo acidic soils, may cause high levels of Mo in fodder crops, that can be harmful to ruminants. Molybdenum combines with copper in the rumen, which causes a copper deficiency in stock called molybdenosis, a condition that can be fatal. A condition that I know about to my detriment, after losing 5 cows from this in the last drought. Molybdenosis can be prevented by 1) identification – a leaf analysis of the fodder will give the Mo/Copper ratio, and 2) augmenting copper in the livestock diet.
In conclusion the effects of the mighty molybdenum nutrient are greatly out of proportion to the amount that is required.
To know more about soil and plant health or have your plants tested for a molybdenum deficiency please contact us at firstname.lastname@example.org.
Till next time
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods