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V. Responses to Molybdenum on Crops

V. Responses to Molybdenum on Crops

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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



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

reseeded frequently.

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



(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



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.







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



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

olivine, respectively.

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

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V. Responses to Molybdenum on Crops

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