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II. Molybdenum Fertilizers, Their Rates and Methods of Application, and Industrial Uses of Molybdenum

II. Molybdenum Fertilizers, Their Rates and Methods of Application, and Industrial Uses of Molybdenum

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76



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

(Berger, 1962).

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



77



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,



78



UMESH C. GUPTA AND JOHN LIPSE'IT



Table I1

Production of Molybdenum in 1976"

Production

( lo3 tons)



Source



50.8



United States

Canada

Chile

U.S.S.R.



13.4



9.8

8.8



Source



Production

(lo3 tons)



China

PeN

Bulgaria

Japan



I .8

0.8

0. I

0. I



"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

been identified.

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

IN PLANTS

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"

Earth's

CNSt



Igneous

rocks



Sedimentary

rocks



Soils



1



2



1-2



2 .O-2.5



"Values given in parts per million.



River, lake,

and groundwater



0.5 x



10-3



Ocean



Coal



11.0 x 10-3



3-5



MOLYBDENUM IN SOILS, PLANTS, A N D ANIMALS



79



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

system.

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,-



+ NAD



+ NO,- + H,O



(1)



where NAD is nicotinamide adenine dinucleotide. The reduction mechanism

from NO,- to NOz- has been proposed by Nicholas (1975) as

NO,NO,-



+ 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,



-+



2NH3



+



GH,



(4)



(5)



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



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