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V. Responses to Molybdenum on Crops
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
MOLYBDENUM IN SOILS, PLANTS, A N D ANIMALS
unfilled heads. In southern New South Wales, Freney and Lipsett (1965) and
Lipsett and Simpson (1971, 1973) found that high levels of available N would
render wheat seedlings responsive to Mo. The Mo had a protective function in
alleviating damage (see Section 111) caused by accumulation of NO3- in the small
plant. Of course, the protection breaks down if NO,- reaches high levels.
It has long been known that fertilizer N commonly reduces grain yield in
Australian wheat crops (Stonier, 1965; Dann, 1969), an effect most often ascribed to early exhaustion of moisture reserves in the profile, with consequent
restriction of grain filling and hence of yield. There has been little experimental
backing for this explanation, and the results with Mo (which attribute the reduction in yield to damage in the early seedling stage) may better account for the
damage in many cases. Whatever the explanation, fertilizer N was avoided by
farmers and bare fallowing was relied on to mineralize N for the crop. It appears
that this intention may succeed too well at times, particularly in clover-based
rotations, by mineralizing undue amounts of N. Aspects of the matter still to be
investigated include the following: seasonal effects on mineralization and distribution of NO3- within the root profile; the role of seed reserves of Mo; the full
geographical and pedological extent of the deficiency or imbalance; managerial
practices in respect of N mineralization and N and Mo fertilizers.
Plainly the temperate cereals are not immune to Mo deficiency. Since they are
normally grown under cultivation, the appropriate techniques of applying Mo are
not necessarily those that were worked out for undisturbed pastures. There is a
relatively large gap between the plant requirement and the usual Mo fertilizer
dressings, which provides an opportunity for economies by such techniques as
seeddress or placement. These would appear mainly suited to crops that are
It is not clear whether the tropical cereals suffer from Mo deficiency to any
extent. Low available N and neutral to alkaline soils with adequate available Mo
probably combine to safeguard the dryland crops in general. The nutrition of
paddy rice (Oryza sativa L.) with respect to Mo is not clear, but should be
considered because of the status of rice as a staple foodstuff. Rice in aerobic
culture presumably exhibits at least the conventional requirement for Mo for
nitrate reductase. Shukla et al. (1976) reported a field response to Mo in India by
a new high-yielding variety. It might be expected that supplying N predominantly as NH4+ would lessen the demand for Mo in the paddy, whereas the
likelihood of the presence of Fe'+ and Sz- ions would reduce the availability of
Mo in the flooded soil.
Although grasses in general have been thought to have low requirements for
Mo, Lipsett (1975) found that Phularis tuberosa (now P . aquatica L.) was like
wheat in showing a marked response to Mo on a soil on which perennial ryegrass
grew strongly. It was not established whether the ryegrass was sustained by seed
reserves, recovery of more soil Mo, or some other means. Phalaris aquatica
UMESH C. GUPTA AND JOHN LIPSETT
(tuberma) proved to be extremely sensitive to Mo deficiency in the seedling
stage, particularly when supplied with NO3-. Johansen (1978a) examined three
tropical grasses but found no marked sensitivity. However, there were indications of the interaction between Mo and Nog- on which this response to Mo is
Since the yield response to Mo actually begins at an early growth stage,
attempts have been made to describe and diagnose responsiveness by characterizing the biochemical processes that are affected most directly by the application of
Mo. The activity of nitrate reductase in producing NO2-, and hence protein, and
in lessening NO,- within the plant is the process primarily involved. Randall
(1969) developed a method for diagnosing Mo deficiency in wheat by a bioassay
of leaf material. Johnson et al. (1976) have suggested that nitrate reductase
activity might serve as a predictive test of crop yield. One might expect a
correlation where N is the main limiting factor.
Responses by plants to Mo are closely related to soil properties, and consequently there are established geographical patterns of deficiency and of excess.
Large areas of North America, Australia, New Zealand, and probably eastern
Europe are potentially deficient. In Canada, responses to Mo have been limited
to the eastern part of the country. The soils of this area are leached, are acidic,
and have given response to Mo in controlling whiptail of cauliflower (Robinson
and Campbell, 1956) and in increasing the yield of grass-legume hay and the
nodule weight of red clover (Robinson et a!., 1957). Gupta (1969) reported that
in greenhouse experiments, crops grown on coarse-textured soils gave response
to Mo even when the soil was limed to pH 6.5. Results of field experiments
conducted in Prince Edward Island showed that in the case of a suspected Mo
deficiency, addition of about 0.2 kg Molha as a foliar spray or 0. 4 kg Molha
applied to the soil should alleviate a Mo deficiency problem, and the residual
effects at these levels of Mo should last 2-3 years (Gupta, 1979).
There are many abstracts of reports dealing with Mo in the Soviet Union and
associated countries, frequently with positive responses in yield. Valdek ( 1 974)
and Agafonova et al. (1975) indicate that Mo is in regular use. Again, mainly
light acid soils appear to be involved.
Excessive amounts are of main concern in the western United States and
Europe. Information is lacking for most of Africa, South America, and Asia,
with the exception of India, where Mo appears to be generally in moderate
supply. There is a dearth of reports for tropical areas in particular, yet there are
large areas of leached, acid, ferruginous soils that would seem to be highly prone
to Mo deficiency. Prasad and Page1 (1976) examined a range of tropical soils for
ammonium acetate-extractable Mo, and reported a high incidence of deficiency
in ferrallitic soils. Molybdenum deficiencies have been readily found in Queensland, Australia, in investigations for pasture establishment and growth on infertile soils (Jones and Crack, 1970; Bishop, 1974). The preference often shown
MOLYBDENUM IN SOILS, PLANTS, AND ANIMALS
agriculturally for recent volcanic soils (as between Java and Sumatra) may reflect
better Mo supply. It seems likely that Mo deficiency may be widespread on
yellow earths and similar soils in resettlement areas in Indonesian Borneo (personal communication with L. F. Myers, CSIRO, Australia). The expectation is
that any plant species sown on such soils would be potentially at risk of Mo
deficiency, particularly if the crops involved are dependent on fixation of N, as
their source of N.
It is to be expected that the response in yield to Mo will be accompanied by an
increase in Mo content, since applied Mo (or soil Mo on liming) is readily taken
up by plants. Molybdenum contents can, in fact, reach levels at which the
material is toxic to animals, notably ruminants. A figure of 10 ppm Mo in forage
is widely assumed to be dangerous (see Section IX). This problem of
molybdenosis may reflect soil properties, but the use of lime, the rates and
frequency of application of Mo fertilizers, the composition of irrigation water,
and the possibility of contamination from mining or from burning coal are all
aspects to be considered. There were firm reports (Allaway, 1968), which seem
not to have been followed up, that a relatively high Mo content in plant material
in the diet favors dental health in humans.
VI. FACTORS AFFECTING THE MOLYBDENUM
A . PARENT
1 . Parent Rock
Molybdenum is a transition element in the fifth row of group VIB of the
periodic table. It is metallic and closely resembles tungsten (W) in chemical
properties. General principles of the occurrence of Mo in the igneous rocks of the
Earth’s crust are now fairly well established, since molten magmas represent
comparable starting points of relatively well-blended materials. However, the
concentration and form of Mo in other rocks and soils tend to vary according to
particular origins and conditions of formation. Molybdenum is a versatile element insofar as valence is concerned, and it may precipitate under either oxidizing (Mofi+predominant) or reducing (Mo4+) conditions (Manheim and Landergren, 1978). Consequently, there may be local enrichments or depletions, and
recent work is largely concerned with elucidating sequences of occurrence,
mobilization, and deposition in particular situations.
a . Occurrence in Igneous and Metamorphic Rocks. Igneous rocks make up
some 95% of the crust of the Earth (Mitchell, 1964), and Mo occurs in both acid
UMESH C. GUPTA AND JOHN LIPSETT
and basic igneous rocks. Manheim and Landergren (1978) suggest an overall
figure of nearly 2.0 ppm Mo for granitic rocks and somewhat lower for basalts.
The Mo is found in feldspar and ferromagnesian minerals such as biotite and
Although the occurrence of Mo in metamorphic rocks has not been widely
studied, metamorphism would be expected to alter the form and site of occurrence rather than the amount of Mo present. New minerals may be formed that
must undergo weathering-again, in the case of sedimentary parent materialbefore the Mo becomes available to plants.
b. Occurrence in Sedimentary Rocks. The sedimentary rocks that are
formed following weathering and transport usually retain some of the Mo of the
parent material. Concentrations of Mo may be high if the rocks have formed
under conditions favoring accumulation and precipitation of Mo, viz., at depth in
oceans or in the presence of carbon (coals, oil shales, some limestones). The
weighting given such sediments determines the average value actually quoted for
Mo content. Manheim and Landergren (1978) suggest < I ppm Mo overall, but
Norrish (1975) suggests 2 ppm. The lowest values are found in sandstones that
contain stable minerals and have undergone high drainage losses. The carbonaceous materials are of interest mainly in relation to either contamination of
the environment by spoil from mining and industrial uses or, in the case of some
limestones, their deliberate addition to the soil for agricultural purposes.
c . Weathering and Occurrence in Water and Soil; the Sedimentary Cycle.
The Mo is released from rocks by weathering (Mitchell, 1964), which involves one or more cycles of solution, oxidation, and precipitation before the
Mo from a given rock either appears in the soil formed from that rock or is
transported to ocean sediments as part of the sedimentary cycle. Molybdenum is
fairly readily released from primary minerals by weathering and, compared with
other metals, it remains relatively mobile as potentially soluble molybdates
(Mo")). Consequently, movement by leaching is likely, unless iron, aluminum,
or manganese oxides interfere under conditions appropriate for occlusion on
these minerals (Davies, 1956). Entry of Mo into surface or groundwaters is
normal (Table III), and may be marked near ore bodies (Jackson et al., 1975),
where the concentration may reach several ppm.
Manheim and Landergren (1978) quote high natural values for rivers in arid
regions, up to 10 p g Mo/liter, but suggest that pollution from industry and
agriculture regularly leads to much higher values, and has caused a probable
doubling in recent time of dissolved runoff of Mo. The amounts of Mo in coal
(Table 111) indicate it to be one of the sources of the extra Mo. It appears in the
ash and possibly in smoke and fumes.
This dissolved Mo may be intercepted by anaerobic layers in lakes, and incorporated as the sulphide in bottom deposits, but the ocean is the ultimate sink for