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IV. Determination of Molybdenum in Soils and Plants
UMESH C. GUPTA AND JOHN LIPSETT
I N SOILS
The most common method for extracting Mo from soils is by perchioric acid
digestion (Reisenauer, 1965). Dry ashing of soil and the extraction of ash using
concentrated acids was employed for determining total Mo in soils by Perrin
(1946) and Grigg (1953a). Total Mo has also been extracted by Na&O, fusion of
soil (Purvis and Peterson, 1956). Unpublished data of the first author of this
article showed that such extracts contained large quantities of interfering materials and required purification, which is time consuming. Little and Kerridge
(1978) used HF-HC104 digestion for determining total Mo in soils.
As for other plant nutrients, total Mo content of soils, except for very low
levels, is generally not a good indicator of plant Mo availability (Little and
Kemdge, 1978; Williams, 1971). Available Mo content has not been found to
be closely related to the total Mo content of soils (Stone and Jencks, 1963).
However, soil with a total Mo content of more than 20 ppm may be regarded as
potentially “teart” (producing Cu deficiency in animals) in Scotland (Williams,
1971). Soils with low total Mo and neutral to alkaline pH may be depleted by
many years of intensive cropping (Davies, 1956). Liming can correct Mo deficiency; therefore an estimate of total Mo content may provide some indication of
the Mo supplying power of acid soils. Details of the effect of liming on Mo
availability will be dealt with in Section VI,B.
Little information exists on the levels of Mo in various soils but, in general,
contents of 0.5-5 ppm are normal (Robinson and Alexander, 1953; Williams,
1971) and in agreement with the relative abundance of Mo in the lithosphere (2.3
ppm), whereas figures of 0.5 pprn or less would be considered particularly low
(Williams, 1971). The Mo content of a few soils selected from areas of Canada
close to industrial plants ranged from 1 .O to 11.3 ppm (Warren, 1973). MacLean
and Langille (1973) reported that the Mo content of Nova Scotia (Canada) podzol
soils ranged from 0.05 to 12.1 ppm.
The presence of extremely small quantities of Mo in the soil, the influence of
chemical characteristics of soils (Karimian and Cox, 1979), the importance of
seed reserves (Gurley and Giddens, 1969), and the possibility that seed reserves
may mask a deficiency in the soil make the problem of determining Mo
availability more difficult than for the other micronutrients. The first report on
the available Mo in soils, which related extracted Mo to plant uptake, was by
Grigg (1953b) in New Zealand. This involved an acid oxalate extractant buffered
at pH 3.3. The responses and lack of responses as related to Mo extracted by
Grigg’s reagent for a number of crops have been summarized by Reisenauer
MOLYBDENUM IN SOILS, PLANTS, AND ANIMALS
(1967). However, oxalate-extracted Mo has not been found to correlate with Mo
uptake by plants on various soils (Karimian and Cox, 1979; Little and Kerridge,
1978). The extracted-Mo values on some iron-rich soils may, however, be misleading (Little and Kerridge, 1978).
Water has been used as an extractant for determining Mo in soils (Gammon et
al., 1954; Lavy and Barber, 1964; Gupta and MacKay, 1965b). Difficulties are
encountered in the analyses because quantities extracted are very low. Lowe and
Massey (1965) used 10-hour leaching of soil with hot water and found the
method to be more reliable for extracting available Mo than the Grigg method.
Pathak et al. (1969) found hot-water-soluble Mo to be related to plant uptake of
the element in a number of soils from India. The hot-water method, however, is
time consuming and has not been tested extensively.
Recently, anion-exchange resin (Dowex 1-X4) has been used to extract Mo
from soils in Oregon (United States) by Bhella and Dawson (1972). The method
involved equilibration of an aqueous soil-resin suspension and measurement of
the sorbed molybdate using 2 M NaCl to displace it. The method was considered
to be satisfactory, theoretically reasonable, and practically acceptable in assaying
available soil Mo within the pH range 4.95-7.10. Later Jarrell and Dawson
(1978) found that Mo uptake by subterranean clover ( T . subterruneum L., cv.
Mt. Barker) grown in field experiments was significantly correlated with Mo
extracted by anion-exchange resin.
Although the Grigg and resin-soil test procedures may be used to separate
Mo-responsive from non-Mo-responsive soils, neither was considered by Karimian and Cox (1 979) to be a good predictor of the size of response to Mo
fertilization. They suggested that a measure of the degree of crystallinity of soil
iron oxides, the active Fe ratio (amorphous Fe/free Fe), along with the soil pH
could be used to reasonably predict response magnitude to Mo fertilization until a
more reliable procedure is developed.
Ammonium acetate and EDTA have also been used to extract Mo from soils;
the extracted Mo together with soil pH and organic matter provided useful
information on the potential uptake of Mo by pasture (Williams and Thornton,
The most recent studies suggest extraction of soil after wetting it to field
capacity and leaving it at this moisture content overnight (Little and Kemdge,
1978). A concentration of less than 4 p g Mo/liter thus obtained appeared to
indicate soils that would respond to Mo application.
A 12-hour extraction with 1 M (NH,),CO, at pH 9.0 was proposed by Vlek
and Lindsay (1977) to reflect the more labile fraction that could come into
solution. The amount of Mo obtained using this extractant on soils, with
pH a 7 . 0 , amended with 0-2 ppm Mo as N+MoO, showed a correlation of
r = 0.977 with Mo uptake by alfalfa. However, when soils with pH <7 were included, the correlation dropped considerably due to the extraction of more soil
UMESH C. GUFTA AND JOHN LIPSETT
Mo by (NH4)&03 than was available to plants. The method was considered to be
good for predicting Mo toxicity in alkaline soils.
With so many factors affecting the availability of Mo in soils and the uptake of
Mo by plants, it is rather obvious that soil tests are often unsuccessful in predicting Mo deficiencies and toxicities.
Microbiological assay of available soil Mo using A . niger has been used by
Mulder (1948). It was later pointed out that, using this method, the discrepancy
between the growth of cauliflower and the microbiological test could be ascribed
to the presence of a high concentration of available Mn in the acid soils, which
inhibited uptake of Mo by cauliflower but not by A . niger (Mulder, 1954). Soil
Mo values obtained with A . niger have been reported to be only moderately
correlated with field response (Donald et al., 1952) and not well related to soil
solution Mo (Lavy and Barber, 1964).
More recently Franco et al. (1978) used Azotobacter paspali nitrogenase
activity (measured by acetylene reduction) to detect Mo deficiency in soils. The
test is based on the percentage increase of nitrogenase activity of Azotobacter
paspali growing in a defined medium with a small amount of soil as the Mo
source. Using this test, 36 out of 41 soils tested showed a significant response to
added Mo with a tropical forage legume (Centrosema pubescens Beth) and gave
better correlation than the oxalate extractable Mo.
C . MOLYBDENUM
I N PLANTS
Due to the presence of Mo in microquantities in plant materials, its analysis is
somewhat more difficult. However, due to advances made in the methodology
for Mo as discussed earlier in this section, its analysis is carried out more
precisely than before in most laboratories. Because of the unreliability of various
methods of extracting Mo from soils from one region to another and from one
group of soils to another, the analysis of plant materials for Mo continues to be
one of the better indicators of Mo availability in soils as related to plants.
The two most common methods of extracting Mo are dry ashing and wet
digestion. Dry ashing has been used by Gupta and MacKay (1965a) and Fuge
(1970). Recently Khan et al. (1979) used the wet digestion method and found
that the results obtained were reasonably similar to those obtained by the dry
ashing technique used by Gupta and MacKay (1965a).
Wilson (1979) used matrix-compensated Mo standards to determine Mo in
plant tissue by flameless AAS. The need for preparing Mo standards in an
appropriate matrix solution was emphasized by the fact that an aqueous Mo
standard gave an apparent Mo concentration 41% greater than a Mo standard
prepared with the matrix solution of 0.55 M HC104. This acid concentration was
used because the concentration of HClO., in the diluted plant digests ranged from
MOLYBDENUM IN SOILS, PLANTS, AND ANIMALS
0.2 to 0.7 M. Concentrations of HCIOl ranging from 0.25 to 1.5 M had little
effect on the observed Mo values. The recovery of Mo added to the plant digests
of orchard leaves and alfalfa tops ranged from 95 to 105% (Wilson, 1979). Using
this procedure, tissue Mo concentration of 0.1 ppm could be determined with
good precision. However, tissue Mo values of 0.05 ppm and less were substantially less precise using this procedure.
V. RESPONSES TO MOLYBDENUM ON CROPS
The criterion for response to an added nutrient in agricultural terms is usually
an increase in growth of the plant, either in rate or in final yield. Sometimes a
visible symptom of deficiency may be present, and its disappearance or prevention form part of the response.
The first reported responses by higher plants to application of Mo were by
tomato in solution culture, following scrupulous removal of all traces of Mo from
the other nutrients (Amon and Stout, 1939). The purification was necessary
because only extremely small amounts of Mo are required for normal plant
growth. Some lower organisms-Azotobacter
(Bortels, 1930), Clostridium
(Bortels, 1936), and Aspergillus (Steinberg, 1936)-had already been shown to
require Mo for the fixation of N, and for utilization of NO3-. Mulder (1948)
showed that symbiotic Rhizobium in peas also required Mo for nitrogen fixation.
It was further reported that “the growth-rate curve and the increasing sporulation
of Aspergillus niger with increasing amounts of Mo were used in estimating very
small amounts of this element in various materials. ”
The first reports of responses by agricultural plants in the field soon appeared,
notwithstanding reservations on the part of the first workers that a nutrient
needed in such small amounts would be lacking in the field (Stout, 1972). Anderson (1942) obtained responses to Mo (applied as ammonium molybdate at a rate
of 1 kg/ha) by the clover component of subterranean clover-perennial ryegrass
(Lolium perenne L.)-Phalaris tuberosa (now P . aquatica L . ) pastures in South
Australia. Much work followed in Australia and other countries, and several
conditions (“diseases”) in plants that had hitherto been unexplained were shown
to respond to application of Mo. Probably the best known and most widespread
of these was in cauliflower. “Whiptail” of cauliflower was marked by distortion
of the leaf lamina. Quite early, cereals and other grasses were shown to respond
to Mo (Fricke, 1947; Lobb, 1953; and Mulder, 1954, in Tasmania, New Zealand, and Holland, respectively). Several compilations (which will not be reviewed here) report the accumulated knowledge of Mo deficiency in agricultural
plants, including field, horticulture, and pasture species (Wallace, 1951; Hewitt,
1956; Johnson, 1966; Lucas and Knezek, 1972).
UMESH C . GUPTA AND JOHN LIPSE'M
The early reports of responses by cereals to Mo application seem not to have
affected commercial practice in cereal growing to any marked extent. Particularly in Australia (where deficient soils proved to be widespread), Mo deficiency
was regarded as a complaint of Rhizobium, particularly that associated with
subterranean clover. Because of this, subterranean clover with fertilizer N omitted became the standard experimental indicator plant. Molybdenum was applied
in the field primarily in order to secure symbiotic fixation of N, for permanent
pastures or leys. It was not applied unless the clover showed a response; associated grasses, or cereals in the rotation, were not assessed.
However, Mo deficiency was reported in maize in a coastal area of New South
Wales (Noonan, 1953). Further work of Weir and Hudson (1966) clarified the
association of deficiency symptoms with seed reserves: symptoms were unlikely,
even on low-Mo soils, for Mo contents in seed >0.08 ppm and likely for <0.02
ppm. Accordingly, application of Mo to crops producing hybrid seed of unsatisfactory content was stipulated as a requirement for certification (Weir et al.,
1966). However, it was found that high rates of sidedress were often required.
These were expensive and potentially hazardous to grazing ruminants in the
rotation, so foliar sprays are now preferred (Weir et al., 1976).
Maize appears to be relatively susceptible to Mo deficiency, particularly on
acid soils. Tanner (1976) readily produced symptoms in the greenhouse and
suggests that the problem is widespread in Rhodesia. Premature sprouting of the
grain on the cob in Rhodesia has been linked with Mo-N balance (Tanner, 1978).
Most of the corn in the United States seems to be adequately supplied with Mo
from the soil. However, Brown and Clark (1974) reported Mo deficiency in a pot
study of two inbred lines, grown on acid (pH 4.3) soil. One line developed
symptoms (twisted leaves, chlorosis, and necrosis), whereas the other did not.
Application of either Mo or lime cured the symptoms. This suggests different
genetic abilities in taking up Mo from soil of low Mo availability.
A relationship between deficiency symptoms and seed reserves has been found
in some cases (Mulder, 1954; Hewitt, 1956), and has been advanced (Anderson,
1956) as a probable reason for the relative absence in early experience of deficiency in large-seeded legumes. Nevertheless, these species are by no means
immune (de Mooy, 1970; Sedbeny et a f . , 1973; Parker and Hams, 1977), and
the results suggest that deficiency may occur in the period of dependence on seed
reserves of Mo before the new seedling of whatever species taps soil sources by
means of an expanding root system. On the other hand, it is possible for a seed to
contain more Mo than the whole new plant will require (Meagher et al., 1952).
Meanwhile, application of Mo specifically to wheat (Triticum aesiivum L.)
came to be practiced in Australia on soils that grew subterranean clover satisfactorily without it. In Western Australia, Gartrell (1966) found responses in grain
yield by wheat and oats (Avenu sativa L.) on light sandy soils, particularly if
ammonium sulfate was added. The untreated plants had a pale color and many