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II. Molybdenum Fertilizers, Their Rates and Methods of Application, and Industrial Uses of Molybdenum
UMESH C. GUPTA AND JOHN LIPSETT
and Smilde (1966) found that frits are good sources of Mo and reported that their
residual effect is somewhat larger than that of sodium molybdate.
Molybdenum can be applied as a seed treatment, soil application, or as a foliar
spray. The data summarized by Murphy and Walsh (1972) indicate that 50-100 g
Mo/ha are generally needed for soil treatments on most agronomic crops and that
as much as 400 g Mo/ha may be needed on vegetable crops, such as cauliflower.
Applications of greater than 1 kg Mo/ha (Gupta and MacLeod, 1975) produced
forages that could prove toxic if fed to livestock. For acid soils, broadcast
applications of Mo are best mixed with limestone to prevent fixation of Mo
Molybdenum has also been applied to soils in combination with superphosphates and has been found to be readily available in a few New Zealand soils
(Widdowson, 1966). Approximately 40-60% of the Mo applied by this method
to beans (Phaseolus spp.) was recovered in the tops and seeds. Lipsett and David
(1977) in Australia used molysuper, which is supposed to contain 0.04% Mo in
each bag. However, the percentage distribution of Mo varied according to the
fraction size, which was not evenly distributed within the bag. The fine material,
which was mostly in the lower layer, contained an average of about 1900 ppm
Mo compared with 1500 ppm in the medium-sized particles and only 260 ppm in
the coarse ones. It was suggested that the addition of Mo be made during the
early stages of manufacture, such as with the acid that is poured on the rock
phosphate to produce even mixing.'
The residual effect of Mo added to the soil varies from one soil to another.
McLeod (1976) reported, based on his studies in New Zealand, that Mo applications of 140 g sodium molybdate would last 4-5 years. Gupta (1979) reported
that the residual effect of Mo added at 0.4 kg/ha on some podzol soils should last
2-3 years from the crop sufficiency point of view.
Foliar-applied Mo for rapid uptake and for overcoming Mo deficiency has
been a common practice for many crops (Murphy and Walsh, 1972). Inden
(1975) recommends use of wetting agents in the spray when applying Mo in
foliar sprays on cauliflower or onions. Results of Gupta (1979) showed that foliar
sprays may be more desirable than soil applications under dry conditions. It has
been suggested that foliar sprays should allow a reduction in the rates of Mo
needed to maintain adequate levels in certified hybrid maize (Zea mays L.) seeds
(Weir et al., 1976); this method of application would also avoid the problem of
Mo fixation in acid soils (Bergeaux, 1976). Weir etal. (1976) reported that both
soil and foliar treatments of Mo raised the Mo concentration in corn (Zea mays
L.) grain and leaves, but the foliar sprays were more effective. Spraying when
the maize plants were 80 cm tall increased the Mo concentration in the seeds
'The company manufacturing the material now uses an oil carrier to try to improve adhesion.
MOLYBDENUM IN SOILS, PLANTS, AND ANIMALS
more than earlier or later spraying. Likewise Boswell er al. (1967) found that the
concentration of Mo in the kernels of peanuts (Arachis hypogaea L.) increased as
the time of spraying was delayed for up to 6 weeks after bloom, after which there
was a decline in the effect.
Because of the extremely low Mo requirement of crops, the most common
method of correcting a Mo deficiency is to treat the seeds with a Mo preparation.
Seed treatment of Mo has been used to prevent Mo deficiency in Brussels sprouts
(Brassica oleracea, var. gemmifera Zenker) (Gupta and Cutcliffe, 1968) and to
increase the Mo concentration of soybeans (Golov and Kazakhkhov, 1973). The
Mo content of seeds is important; for example, Hagstrom and Berger (1965)
observed that large-seeded crops, such as peas (Pisum sativum L.), responded to
soil applications of Mo when the seeds contained less than 0.2 pprn Mo, but not
when they contained enough Mo (0.5-0.7 ppm) to supply the Mo needs of the crop.
Gurley and Giddens (1969) also reported that high Mo content in large seeds may
supply enough Mo to plants grown on Mo-deficient soils. In maize, severe Mo
deficiency can be expected when seeds containing less than 0.02 ppm Mo are
sown on Mo-deficient soils, but not when the seeds contain more than 0.08 ppm
(Weir et al., 1966).
Application of Mo in the lime of lime-pelleted legume seeds is a practical way
of applying Mo in close proximity to the seeds and without detriment to the
rhizobia in the inoculum (Date and Hillier, 1968).
Results of Martinez et a f . (1977) indicated that when molybdic acid was
seed-applied (0.04 ppm on soil basis), soybean growth was reduced. Although
seed-applied Mo was at a low level when calculated on a soil basis, the concentration near the seeds would be higher than in the soil application and would
account for the greater decrease in growth. Gupta and Kunelius (1980) found that
use of moist seeds treated with a commercial Mo preparation at the rate of 14 g
Mo/ha resulted in large quantities of Mo in forage crops, which when fed to the
livestock could produce molybdenosis (Mo-induced Cu deficiency). In order to
avoid excessively high concentrations of Mo in the crops, it would therefore be
advisable not to treat germinating seeds with a Mo preparation.
The rate of 28 g commercial source of Mo per 27 kg of seed (1 oz/bushel) is
sufficient to meet the Mo requirement for soybeans (Bergeaux, 1976). According
to the recommendation of Lancaster (personal communication from J. D. Lancaster of Mississippi State University in 1968) about 7-35 g Mo/ha (0.1-0.5 oz
Mo/acre) annually is sufficient for seed treatment.
Reisenauer (1963) also showed that seed application of sodium molybdate was
much more effective than soil application for peas. The Mo supply was considered to be adequate when applied at the rate of 18-36 g/ha.
Besides its use in fertilizers, Mo is used industrially as a component of hard,
corrosion-resistant alloys with steel, a lubricant, and a pigment or other reagent,
UMESH C. GUPTA AND JOHN LIPSE'IT
Production of Molybdenum in 1976"
( lo3 tons)
"From Manheim and Landergren (1978)
notably catalytic. These uses account for the production shown in Table 11.
Reserves amounting to a supply of 6- 10 years at this rate of usage appear to have
Molybdenum is not abundant in the Earth's mantle, but it is widespread, as
one would expect from its essential role in plants. Large amounts of Mo occur in
sedimentary formations, especially marine manganiferous concretions. Concentrations may exceed 0.04%, but these amounts are not yet recoverable since the
Mo tends to be dispersed in the sediments (Manheim and Landergren, 1978).
The concentrations that are attributed to various materials are given in Table
111, based on Norrish (1975) and Manheim and Landergren (1978).
111. PHYSIOLOGICAL ROLE OF MOLYBDENUM
Molybdenum is a component of at least five distinct enzymes that catalyze
diverse and unrelated reactions, namely nitrogenase, nitrate reductase, xanthine
oxidase, aldehyde oxidase, and sulfite oxidase (Nicholas, 1975). Three of these
enzymes, nitrate reductase, nitrogenase, and sulfite oxidase, are found in plants.
The principal functions of Mo in plants are implicated in the electron-transfer
system; for instance, nitrate reductase and nitrogenase require Mo in the reducTable 111
Concentration of Molybdenum in Rocks, Soils, Natural Waters, and Coal"
"Values given in parts per million.
11.0 x 10-3
MOLYBDENUM IN SOILS, PLANTS, A N D ANIMALS
tion of NO,- and in the fixation of N,, respectively. This section will include a
discussion of these two enzymes (molybdoproteins) as they function in the plant
The first molybdoprotein, nitrate reductase, is known to require Mo and flavin
for its activity and in the reduction of NO,- to No,- as follows:
Reduced N A D
+ NO,- + H,O
where NAD is nicotinamide adenine dinucleotide. The reduction mechanism
from NO,- to NOz- has been proposed by Nicholas (1975) as
+ 2H' + 2 e - + NO,- + H,O
+ H+ + 2e- -+ NO,- + OH-
Reaction (2) is based on the acidic half reaction, whereas reaction (3) allows for
OH- participation at physiological pH.
Nitrate reductase is found in most plant species as well as fungi and bacteria
(Price et al., 1972). The increased Mo requirement of most plants grown on
No,--N compared with NH,+-N can be almost completely accounted for by the
Mo in nitrate reductase (Evans, 1956).
The other major known molybdoprotein of plants, nitrogenase, fixes elemental
nitrogen in the form of NH,, which is then assimilated by the plant (Koch ef al.,
1967). The role of Mo in the fixation of N, has been reviewed in detail by Chatt
(1974). The unique role of Mo in biological systems is exemplified by nitrogenase, the enzyme that converts N, into NH, at room temperature and normal
pressure (Schrauzer, 1976). The nitrogenase is an enzyme complex composed of
two distinct components that combine to reduce N, to NH, [reaction (4)] or
acetylene to ethylene [reaction (5)] (Nicholas, 1975):
+ 6H+ + 6 e C,H, + 2H+ + 2eN,
Nitrogenases have been isolated from a variety of different sources, for example, from Azotobacter vinelandii, Rhizobium japonicum, Azotobacter chroococcum, and Klebsiella pneumonianum (Schrauzer, 1976).
Recent studies by Agarwala et al. (1978) have shown that in addition to
reduced nitrate reductase activity, Mo deficiency in corn resulted in significantly
lower activities of catalase, aldolase, and alanine aminotransferase and higher
activities of peroxidase, P-glycerophosphatase, and ribonuclease.
In addition to the involvement of Mo in the fixation of N, and nitrate reduction, Mo is associated with other processes in plants. However, many of these
processes are interrelated with the two main functions. For example, experiments
of Malonosova (1 968) showed that addition of Mo to the soil resulted in better
development of lupine (Lupinus spp.) and increased weight of its roots and
nodules although the N, was not fixed. In this review of Russian work it was