Soil Analysis

The use of soil testing to predict the soil’s capacity to supply molybdenum for plant growth can be difficult because of the relatively small amounts of molybdenum in soil, the differences in plant requirement for molybdenum, and because of the importance of seed molybdenum reserves in supplying crop needs (74). In addition, the total molybdenum content of soils can differ considerably from the plant-available molybdenum fraction (77). The total molybdenum content in soils usually ranges between 0.013 and 17.0 mg Mo kg^-1 (44) and is dependent on the molybdenum content of the parent material (101). However, the quantity of molybdenum available for plant uptake can be substantially less and is dependent on soil pH and other chemical and biological factors. For pollution monitoring, a method for determining the total molybdenum in soils is necessary. If the objective is to quantify the available molybdenum for plant uptake, then a method for determination of the mobile or readily extractable molybdenum is required (77). Several excellent reviews on the determination of molybdenum in soils are provided by Sims (84), Eivazi and Sims (77), and Sims and Eivazi (74). The reader is referred to these references for detailed explanations of methods and procedures described here.

Determination of Total Molybdenum in Soil

Several extraction methods have been developed for the determination of molybdenum in soils. The most common method of soil extraction is by perchloric acid digestion (102). Dry ashing followed by acid extraction of the ash has also been used (103). Purvis and Peterson (104) proposed the sodium carbonate fusion method for extraction of total molybdenum.

The thiocyanate-stannous chloride spectrophotometric procedure revised by Johnson and Arkley (105) and modified by Sims (84), is used extensively for the determination of total molybdenum in soils. Details of the procedure are provided by Sims (84). Molybdenum in the soil extract reacts with thiocyanate and excess iron in the presence of stannous chloride to form the colored complex Fe(MoO(SCN)₅). The complex is extracted from the aqueous phase with isoamyl alcohol that has been dissolved in carbon tetrachloride (CCl4). The amount of molybdenum present is determined on a light spectrophotometer by comparison of the absorbance of the sample with appropriate standards. Difficulties associated with the thiocyanate method include interference from iron and the use of stannous chloride, which can vary in purity and consistency (77).

Graphite furnace atomic absorption spectrometry has also been used for the analysis of extract having a low concentration of molybdenum (<1.0 mg kg^-1) (106,107). For extracts high in molybdenum, AAS or ICP-AES have been used, but Sims (84) indicates that owing to low detection limits, interferences from other elements, or the enhancement of molybdenum readings, the usefulness of these methods is limited.

Determination of Available Molybdenum in Soil

According to Gupta and Lipsett (12), the first report on the available molybdenum in soils was given by Grigg (103) wherein soils were extracted with acid oxalate buffered at pH 3. Other extractants have been used with varying degrees of success for the determination of available molybdenum in soils including ammonium oxalate, hot water, anion-exchange resin, and ammonium bicarbonatediethylenetriamine- pentaacetic acid (AB-DTPA) (84). The most common method for the determination of molybdenum in soil extracts is the thiocyanate method as described previously.

Although the ammonium oxalate procedure is the method most commonly used to determine available molybdenum in soils, the findings have not been consistent (77). Grigg (108) decided that the method was unreliable for diagnosis of molybdenum deficiencies, because oxalate extracts a portion of iron-bound molybdenum that is unavailable to plants. Water extraction has been shown to be well correlated with available molybdenum in some studies (109), but has failed to give positive results in others (110). Difficulties are encountered with water extraction because the quantities extracted are very low (12). Sims (84) indicates that anion-exchange resins have been used with success to extract molybdenum, but that the method has not been tested widely.

According to Sims and Eivazi (74), the AB-DTPA method was developed for the simultaneous soil extraction of macronutrients and micronutrients such as phosphorus, potassium, iron, manganese, copper, and zinc, and the method has been extended to include molybdenum. Molybdenum extracted with AB-DTPA increases with increasing soil pH (84), and the method has been used most often for soils or sediments high in molybdenum, such as calcareous or polluted soils (111,112). Because the extractant can be used in conjunction with ICP-AES, it offers the added potential for measuring molybdenum during routine analysis of multiple nutrients (74).