The role of molybdenum in soils and patterns of its distribution

Molybdenum is a crucial micronutrient, the availability of which to plants directly depends on the soil type, its acidity, and particle-size distribution. The total molybdenum content in arable soil varies within a wide range — from 0.2 to 36 mg/kg. It is important for an agronomist to know how this element behaves in the soil profile in order to correctly assess the need for fertilizer application containing molybdenum.

• Total content in soil — 0.2–36 mg/kg
• Maximum adsorption by clays — pH 4–5
• Rank by intensity of profile migration — 5
• Rank by coefficient of aqueous migration — 3 (after iodine and silver)
• Clarke value of molybdenum in soil — 3·10⁻⁴ %
• Clarke value of molybdenum in plants — 2·10⁻⁵ %

Mobility and reserves of molybdenum in soils

Molybdenum reserves in soil depend primarily on its content in soil-forming rocks. A certain increase in the concentration of the element is noted in acidic igneous rocks and sedimentary rocks rich in organic matter. Conversely, soils formed on serpentinites are initially poor in molybdenum.

The distribution of molybdenum by soil types is uneven. Chernozems have the highest average content, while chestnut soils and sierozems are the poorest in it. Red soils and podzolic soils occupy an intermediate position. The reserves of the element are also influenced by particle-size distribution: light sandy and loamy sand soils contain little molybdenum, whereas clay and loamy soils are significantly richer.

On the Earth's surface, the behavior of molybdenum is determined by climate and the reaction of the environment. In dry climate conditions on alkaline soils, tetravalent molybdenum is easily oxidized to soluble molybdates, migrates actively, and accumulates upon evaporation in solonchaks and salt lakes. In a humid climate on acidic soils, it, on the contrary, transitions to a low-mobility state. In terms of the intensity of migration along the soil profile, this element ranks fifth (in descending order), and in terms of the coefficient of aqueous migration, it ranks third, yielding only to iodine and silver.

During the process of pedogenesis, molybdenum accumulates in the humus-accumulative horizon due to fixation by organic matter. During podzolization, it partially leaches from the upper layers into the lower ones. Because of this, in strongly podzolized soils, the eluvial (middle) horizon is depleted of molybdenum, while increased concentrations are found in the humus-accumulative and illuvial horizons. Accumulation in the illuvial layer is explained by a high proportion of silt particles, as well as the ability of iron and aluminum oxides to firmly bind the molybdate-ion migrating from above.

Natural forms of molybdenum and its fixation mechanisms

In nature, molybdenum is found in the form of disulfides, molybdates, oxides, and other compounds within disseminated minerals. The main minerals of the element include molybdenite, molybdite, ilsemannite, wulfenite, powellite, ferrimolybdite, chillagite, lindgrenite, belonosite, and patterite. Molybdenum is also present as impurities in the following minerals:

• plagioclase;
• pyroxene;
• olivine;
• sphene;
• zircon;
• magnetite;
• ilmenite;
• hypersthene;
• hornblende.

Due to its radius, high charge density, and strong polarization properties, the Mo⁴⁺ ion is incorporated into accessory minerals (zircon, sphene, magnetite, ilmenite). In these, molybdenum is capable of isomorphously replacing titanium in TiO₂ and divalent or trivalent iron (Fe²⁺, Fe³⁺) in ilmenite and magnetite. In addition, isomorphic substitution of aluminum and sometimes silicon is possible according to the scheme MoO₄²⁻ – SiO₄⁴⁻.

In the hypergenic zone, hexavalent molybdenum prevails in the form of the molybdate-ion, although in an alkaline environment with a deficiency of oxygen, a pentavalent form is also possible. All elements with a valence higher than four form complex anions with oxygen, rather than free cations. These anions form sparingly soluble and low-migrating compounds with strongly polarizing ions. The composition of such molybdates is written by the formula Me(MoO₄)·nH₂O, where calcium (Ca), lead (Pb), and more rarely copper (Cu), bismuth (Bi), and iron (Fe) ions act as the metal (Me).

The structure of these minerals is based on deformed (MoO₄)²⁺ tetrahedra, which are crystallochemically close to WO₄²⁻ tetrahedra. For this reason, isomorphic mixtures of molybdates and tungstates are often found in soils. Molybdenum itself is usually associated with alkali and plagioclase feldspars.

Like phosphate-ions, molybdenum is adsorbed on clay minerals, as well as on iron and aluminum hydroxides. Exchange sorption by clays can occur in the form of the Mo₂O₇Н⁻ ion. Also, the uptake of molybdate-ions occurs with the alkalization of the medium according to the following scheme: Clay-(OH)₂ + 2MoO₄²⁻ → Clay-MoO₄ + 4OH⁻.

How soil acidity and organic matter control molybdenum availability

The behavior of molybdenum in the soil is largely determined by its interaction with iron and aluminum hydroxides. It is on these sesquioxides that the bulk of the element is adsorbed, while only about 2% of the trace element is fixed on clay minerals. Iron accounts for about 70% of the total soil molybdenum.

In acidic soils with a high content of active iron, molybdenum is firmly fixed in the form of iron molybdate Fe2(MoO4)4. Aluminum also precipitates molybdenum, forming compounds with the composition 2[Al(H2O)4OH]2·MoO4, with the lowest solubility of this complex observed at pH 2.7. In practice, this means that highly acidic soils firmly block molybdenum with iron, while slightly acidic soils bind it with aluminum hydroxides. Freshly precipitated hydroxides absorb the element most vigorously.

Soil organic matter also actively binds molybdenum, but the nature of this process depends on the acidity of the environment. The element is adsorbed by decomposition products of plant residues at pH 1 to 4 (maximum at pH 1.5), and by humic substances in a wider pH range from 1.5 to 6.5. Molybdenum is prone to complexation with organic oxy-compounds containing a hydroxyl group in the ortho-position (alcohols, phenols, oxy-acids), forming water-soluble complexes with them.

The mechanism of binding by humic acids has its own specifics: they cannot adsorb anions directly. First, the MoO42– anion must be reduced to a pentavalent molybdenum cation, which is then easily bound by humic acid. Fulvic acids form less stable complexes due to a less favorable spatial arrangement of carbonyl groups. Molybdenum fixation by organic matter occurs only in a strongly acidic environment at pH 1–2, whereas at pH 5–7, in the presence of humic substances, the element remains in solution and is available to plants.

Migration pathways of molybdenum and the influence of the water regime

In the soil profile, molybdenum moves in two ways: via diffusion or along with the flow of the soil solution. The prevalence of one or the other mechanism depends on the concentration of the mobile element.

• Concentration for diffusion — less than 0.004 µg/ml
• Concentration for migration with solution — more than 0.004 µg/ml
• Sorption on clay minerals — about 2 %
• Binding by iron — about 70 % of total Mo

The availability of molybdenum for plants directly depends on the redox regime of the soil. Oxidative processes promote the transition of the element into readily available forms, while reductive reactions convert it into a poorly available state. The balance is regulated by the transition in the "molybdenum trioxide MoO3 — salt form" system. In an acidic environment, the equilibrium shifts towards the formation of poorly soluble MoO3, and in an alkaline one, towards the accumulation of mobile salts.

In the soil, molybdenum is distributed in the following forms:

• In the crystal lattice of primary and secondary minerals — a poorly available form.
• As part of clay minerals and colloidal iron and aluminum oxides (in the form of the MoO42– anion) — a potentially available reserve.
• In organic matter — becomes available to plants only after the mineralization processes are complete.
• In the form of water-soluble compounds — the most available form, but rarely found in large quantities.

The final mobility of the trace element in a specific field is determined by four factors:

1. The content and form of molybdenum compounds in the parent soil-forming rock.
2. Soil composition: the amount of clay minerals, free iron and aluminum oxides, pH level, as well as water and redox regimes.
3. The equilibrium between organic and mineral compounds in the "solid phase — soil solution" system.
4. The intensity of the biological cycle in the "soil — plant" system.

Fe2+, Fe3+⇄Fe2O3·H2O; Al3+⇄Al2O3·H2O; Ca2 3+CO2.

Shifting the equilibrium towards the formation of element hydroxides promotes the coprecipitation of molybdenum. In the Ca2 3+CO2 system, which should be considered as a carbonate-bicarbonate buffer, the partial pressure of CO2 is of great importance. An increase in pressure in the soil solution promotes the formation of soluble bicarbonates, including those of molybdenum, whereas its decrease leads to the formation of poorly soluble carbonates, including those of molybdenum, which precipitate from the solution. The amount of mobile molybdenum closely correlates with its total reserves in the soil. Approximately 5–10% of its total content passes into the oxalate extract. Molybdenum deficiency is generally observed in sod-podzolic soils, drained acidic peatlands, grey forest soils, and leached chernozems. This is explained by the fact that in acidic soils, molybdenum ions are bound by aluminum and iron sesquioxides and become poorly available to plants. A study of the effect of pH on molybdenum forms in dilute solutions showed that at a pH of 5 and above, the MoO42– form prevails; at pH 4.5–2.5, alongside MoO42–, HMoO4 and H2MoO4 are present; at pH 2.5–1.0, H2MoO4 dominates; and finally, at a pH of 1, H2MoO4 disappears and cationic forms appear. Taking into account that most soils have a pH>5, it can be considered that in the pedosphere, the anionic form of the trace element MoO42– is predominant.

Factors that increase the mobility of molybdenum in the soil and its availability to plants include an increase in the concentration of OH–, PO43–, and COO– anions in the soil solution (observed upon the application of lime CaO, Ca(OH)2, CaCO3, and phosphate fertilizers) and the mineralization of soil organic matter. Factors that decrease the mobility of molybdenum in the soil and its availability to plants include an increase in the concentration of H+ ions; enrichment of the soil with Fe and Al oxides; the application of physiologically acidic mineral fertilizers without combining them with lime; and the immobilization of molybdenum by soil humic acids. Molybdenum deficiency in the soil can occur upon the application of sulfur fertilizers. The reason for the negative effect of sulfates may lie in their acidifying action on the soil. This effect may also be related to the competition for absorption sites on the roots between MoO42– and SO42– ions, which have the same size and carry the same charge.

Redox processes also have a significant influence on the mobility of molybdenum. A high degree of soil moisture leads to a decrease in the content of mobile molybdenum in it as a result of more intensive development of reduction processes.