Analytical Techniques for the Determination of Molybdenum in Plants


Historical Information
  Determination of Essentiality
  Function in Plants
    - Nitrogenase
    - Nitrate Reductase
    - Xanthine Dehydrogenase
    - Aldehyde Oxidase
    - Sulfite Oxidase
Diagnosis of Molybdenum Status of Plants
  Molybdenum Concentration and Distribution in Plants
  Analytical Techniques for the Determination of Molybdenum in Plants
Assessment of Molybdenum Status of Soils
  Soil Molybdenum Content
  Forms of Molybdenum in Soils
  Interactions with Phosphorus and Sulfur
  Soil Analysis
    - Determination of Total Molybdenum in Soil
    - Determination of Available Molybdenum in Soil
Molybdenum Fertilizers
  Methods of Application
    - Soil Applications
    - Foliar Fertilization
    - Seed Treatment
  Crop Response to Applied Molybdenum

The molybdenum status of crops is often overlooked by the farming community, probably because of the relatively low crop requirement for molybdenum and because of a lack of education on the necessity of molybdenum in fertility programs. In addition, many commercial soil and plant analysis laboratories fail to report this nutrient in routine tissue and soil analyses. This omission may be partially due to the difficulties in accurately determining the small quantities of molybdenum that are normally present in plant tissues. It is possible that many molybdenum deficiencies in crop plants are misdiagnosed as nitrogen deficiency because of the similarity in their deficiency symptoms.

The two most common methods of molybdenum extraction from plant tissues are dry ashing (71) and wet digestion (72), both of which give similar results (12). Dry ashing is often the preferred method of extraction due to the potential hazards involved with the use of perchloric acid (HClO4) for wet digestion (72). Several analytical techniques have been proposed for the determination of molybdenum in the resulting extracts including the dithiol and thiocyanate colorimetric methods, determination by atomic absorption spectrometry (AAS), graphite furnace atomic absorption spectrometry (GF-AAS), and by inductively coupled plasma atomic emission spectrometry (ICP-AES). As the detection of molybdenum by ICP-AES is less sensitive than for other elements, this method should be used only for plant tissues suspected of having molybdenum concentrations >1.0mg Mo kg-1 (dry mass) (73,74). The dithiol colorimetric method and the AAS method are probably the most commonly used techniques for determining molybdenum in soil and plant materials (12).

The dithiol method developed by Piper and Beckworth (75) and modified by Gupta and MacKay (76) is more sensitive and precise than other colorimetric methods used for the determination of molybdenum in plant tissues. This method is based on precipitation and extraction of a green-colored molybdenum dithiol complex after removal of interfering ions from the test solution (77). The molybdenum concentration is determined by comparing the absorbance of the sample with known standards on a light spectrophotometer. The detection limit of the dithiol method is about 20 ng Mo mL-1, and the recovery of molybdenum added to the plant material has been greater than 90% (12). Although this method is relatively inexpensive, the procedure may be too tedious and time-consuming for use in many commercial analytical laboratories. For procedures of the dithiol method, readers are referred to Gupta (73).

Trace quantities of molybdenum in plant material have been determined by flame (78) or flameless AAS (79). These procedures provide adequate sensitivity for molybdenum and are relatively rapid, but are subject to matrix interferences (77). The GF-AAS method (80) improves the accuracy and precision of determining low concentrations of molybdenum, and the procedure is applicable to a range of different plant matrices (73). The detection limits for the determination of molybdenum by AAS using flame and graphite furnace are reported to be 10 and 2 ng mL-1, respectively (78), and the recovery of molybdenum by these two methods is similar to that of the dithiol colorimetric method, ranging from 92 to 95% (12). For details of the flame and graphite furnace AAS methods, the reader is referred to Khan et al. (78) and Gupta (73).