The average abundance of total Mo in the earth’s crust is around 1.5 mg/kg: soils average around 2.0 mg Mo/kg (Ure and Berrow 1982), while total Mo concentrations in surface soils from Queensland canelands average 1.46 ± 2.14 mg/kg (Rayment et al. 1997). Typical total concentrations in common minerals include 0.2 mg Mo/kg in sandstones, 1–1.5 mg Mo/kg in basalts, 1.4–2.0 mg Mo/kg in granites, to around 3.0 mg Mo/kg in shales, noting that black bituminous shales can have total Mo concentrations of ≈70 mg/kg (Ure and Berrow 1982). Soil Mo concentrations typically increase with soil depth (Rayment et al. 1997).
Chemically, Mo occurs in soils at oxidation states Mo3+ to Mo6+, with Mo6+ species favoured under oxidising conditions and Mo3+ in reducing conditions. Naturally occurring sulfides of Mo (these can co-occur with sulfides of Cu, Fe, Pb and Zn) have low water solubility and are unavailable to plants.
Molybdenite oxidises fairly easily to MoO2SO4 (this is water soluble) and eventually to H2MoO4, which can migrate in soil solution until lost by leaching or ‘fixed’ by chemical sorption as a complex anion or (less common) in exchangeable cationic forms as a consequence of reduction with OM. Total Mo in soils commonly correlates with clay-size fractions (Ure and Berrow 1982).
Most arable soils in Australia and New Zealand that were naturally low in P also have limited reserves of plant-available Mo. Moreover, those with a medium to high propensity to ‘fix’ P typically have low plant-available Mo, irrespective of total Mo status. Moreover, Mo deficiencies in higher plants (particularly legumes and some vegetables) are likely to occur in soils with a pH (1:5 soil/water) <5.5 to 6.0. (This is common across coastal Australian soils supplied with fertiliser phosphate.) Molybdenum deficiency rarely occurs in plants, including legumes, grown on alkaline soils. The unavailability of Mo in acidic soils is associated with Al/Mo and Mn/Mo antagonisms. There is excellent evidence demonstrating that liming of acidic soils can overcome at least moderate Mo deficiency in pasture legumes, while soil ‘wetness’ is reported to increase the uptake of Mo by higher plants.
Typical recommendations to overcome Mo deficiency in legume-based sub-tropical and tropical pastures are to apply from 50 to 100 g Mo/ha every 3 to 4 years, when soil pH (1:5 soil/water) is <7.0. For Mo sensitive species such as the desmodiums, when grown on strongly Mo fixing soils such as the Ferrosols, recommended application rates can be up to 200 g Mo/ha every 2 to 3 years. Vegetable growers often (unwisely) apply more than these quantities. Molybdenum trioxide (MoO3; 66% Mo) and sodium molybdate (Na2MoO4.2H2O; 39% Mo) are the most commonly used Mo fertilisers.
There are well known Cu-Mo and Cu-Mo-S interrelationships in farm animals (e.g. the nervous disorder referred to as swayback in sheep due to Cu deficiency; teartness in cattle). In New Zealand, 10 mg Mo/kg in pasture dry matter was found to be ‘toxic’ when pasture Cu content was normal. Furthermore, concentrations in the range 3–10 mg Mo/kg were harmful to farm animals when Cu intake was low. That said, Mo mostly gives little cause for concern from a medical viewpoint, although there may be some association between ingestion of elevated levels of Mo (in water/food) and the incidence of dental caries in high Mo areas. There is some interest in levels of xanthine oxidase and uric acid levels in workers occupationally exposed to Mo.
Fifty g/ha/y of Mo was a loading rate suggested for sludge by Tjell (1985) but McBride and Hale (2004) see the need for downwards movement of these upper soil-loading limits.
The literature indicates little success in demonstrating a reliable soil chemical test for phyto-available soil Mo, although several have been ‘trialled’ with limited local success (e.g. Gupta 1993). McBride and Hale (2004) had reasonable success in relating hot (90°C) 0.01 M CaCl2-extractable Mo with concentrations of Mo in alfalfa (lucerne) following long-term additions of sewage sludge. This ‘experimental’ empirical test is similar to that of McBride and Hale (2004), but has a slightly narrower soil/solution ratio of 1:2, a shorter extraction time of 10 min, and a slightly higher reflux temperature of 100°C, which equates to those for soil B by Method 12C. Corresponding specifications used by McBride and Hale (2004) are 1:2.5, 30 min and reflux at 90°C. Final analysis by ICP-MS (preferred), by electrothermal atomisation AAS, or by a sensitive ICPAES is required.
Expect ‘normal’ background levels for this test of around 0.01 mg Mo/kg, whereas values ≥0.03 mg Mo/kg probably indicate the presence of more-than sufficient soil Mo reserves. Other factors also operate, such as soil pH.
0.01 M Calcium Chloride Extracting Solution
As for Method 12C1.
Molybdenum Primary Standard
1 L contains 100 mg of Mo.
Dilute accurately a certified Mo reference standard of higher concentration with 0.01 M CaCl2 Extracting Solution. Store in a pre-cleaned polyethylene or teflon bottle.
Molybdenum Secondary Standard
1 L contains 1.0 mg of Mo.
Pipette 5.0 mL Mo Primary Standard into a volumetric flask and make accurately to 500 mL with 0.01 M CaCl2 Extracting Solution. Store in a polyethylene bottle; shelf life is about 2 weeks.
Pipette 1.0, 2.5, 5.0, 7.5, 10.0, 20.0, 40.0 and 60.0 mL Mo Secondary Standard into separate 1.0 L volumetric flasks. Dilute accurately to volume with 0.01 M CaCl2 Extracting Solution. These solutions contain 0.001–0.06 mg Mo/L. For a 1:2 soil/solution ratio, these standards contain concentrations of Mo equivalent to 0.002, 0.005, 0.01, 0.015, 0.02, 0.04, 0.08 and 0.12 mg Mo/kg of soil. The extracting solution serves as a blank. Shelf life is about one week.
All glass and plasticware must be washed with dilute (1+4) HCl followed by deionised water before use. Filter papers should be checked for freedom from Mo contamination (and B contamination if it is also a required element). If ‘contaminated’, pretreat by washing with hot (≈80°C) CaCl2 Extracting Solution.
Add 10.0 g air-dry soil (<2 mm) and 20 mL of 0.01 M CaCl2 extracting solution into a flat-bottomed flask or bottle (150 mL) free of relevant micronutrient contamination. Record weight of each flask plus fresh sample and extracting solution.
Next insert a small, clean plastic funnel in the neck of the flask and quickly bring to the boil then gently reflux for 10 min. Remove the funnel and immediately bring the flask + sample back to its original weight with hot (>80°C), high quality deionised water. Quickly filter the extract through a Whatman No. 40 filter paper into a polyethylene container after discarding the first few mL.
Cool, then determine Mo concentrations from appropriate working standards using ICP-MS, electrothermal atomisation AAS or a sensitive ICPAES. Follow manufacturer’s instructions for instrument calibrations and wavelengths. The wavelength for Mo by electrothermal atomisation AAS is 313.3 nm. The preferred ICPAES wavelength is 202.03 nm with an alternate of 203.84 nm, noting that analysts attempting this option should demonstrate quantitation over the range 1–60 μg/kg. Irrespective of the instrumental option, make allowance for any significant calibration blank.
Report CaCl2-extractable Mo (mg Mo/kg) on an air-dry basis.
1. Sims (1996) notes that all glassware must be thoroughly cleaned, rinsed with HCl and rinsed again with distilled and/or double deionised water throughout all procedures.