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  • Tom Zogas

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    September 1, 2021 at 7:05 am
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    Pp. 157, 158, 159

    promoting absorption of this element by plant roots. On the other hand, molybdenum deficiency caused a decrease in the translocation of iron from veinal to interveinal tissue, the result of a decrease in the reduction of iron into more soluble forms. This also accounted for decreased absorption by roots. High levels of molybdenum similarly reduced iron uptake, although by a chemical mechanism external to the plant. The excess molybdenum became coated on iron oxide compounds, resulting in a decreased solubility (and hence availability) of iron of the plant.

    Molybdenum is essential for the operation of plant enzymes and for the functions of the enzymes nitrogenase and nitrate reductase. Also, the fixation of atmospheric N by both free living and symbiotic microorganisms and the utilization of nitrate-N by plants depend upon Mo. Lack of Mo can visually appear as N deficiency in plants dependent on such N. Petioles of cauliflower, mustard, and tomato suffering from Mo deficiency had much higher levels of nitrate-N than did N sufficient plants. Plants on ammonium-N nutrition tend to exhibit less Mo deficiency symptoms than do those receiving nitrate-N, but some studies indicate that Mo may be involved in more than nitrate-N reduction reactions.

    Phosphorus has been shown to enhance Mo absorption by plants, probably due to exchange of adsorbed MoO.. A positive correlation between soil-adsorbed P and Mo at a given pH was found in the pH range of 4.0 to around 5.2. Bingham and Garber (1960) and Bingham (1963) conducted greenhouse experiments on the effect of application of P on the micronutrient content of sour orange seedlings. The experiments showed that under the acid soil conditions, P application enhanced the uptake of Mo. On the other hand, in alkaline soils, the uptake of Mo was reduced with excessive P. According to these workers, more Mo is released from the exchange complex under acid soil condition with H₂PO4 replacement, thus more Mo is available for plant uptake. Molybdenum uptake is enhanced by phosphates. The application of P increases the Mo concentration in alfalfa, lading clover, and Brussels sprouts. Adding 0.25 mg L (ppm) Mo to nutrient solution cultures increases the P content in white clover shoots, while concentrations of 1.25 and 5.0 mg L¹ (ppm) Mo decreases the P content. A positive correlation between soil-adsorbed P and Mo at a given pH was found in the pH range of 4.0 to about 5.2. Practically no Mo was adsorbed at a pH level of 5.6 and higher. Phosphates replace molybdate from the anion exchange sites at the surface of soil colloids. This increases Mo concentration in the soil solution and Mo availability to plants,

    Potassium applications reduce the Mo content of corn leaves, with the highest concentration of Mo being present in K-deficient leaves.

    A deficiency of Mo increases Fe in white clover and wheat shoots. The greatest increase of Fe occurred in a wheat genotype susceptible to Mo deficiency.

    Molybdenum uptake is inhibited by sulfates. On soils with marginal Mo deficiencies, the application of heavy rates of sulfate containing fertilizers may induce a Mo deficiency in plants. During initial nutrient absorption, sulfates compete directly with molybdate for absorption sites on the roots.

    High Mn increased the uptake of Mo in tomato. Adding Mo to a nutrient solution increased the Mn content of shoots of white clover and several tropical legumes.

    Using Molybdenum Fertilizers

    A few ounces per acre of molybdenum will usually correct a deficiency. This small amount may be mixed into and applied with a fertilizer, or foliar applied (also see pp. 187, 194, 234-235). When mixed with a fertilizer, phosphorus in the blend will promote Mo uptake. The rate and frequency of Mo application is dependent on the concentration in the soil and/or tissue, the specific crop, stage of crop development, soil type, amount of rainfall and/or irrigation, and soil pH. An advantage of foliar application using chelated Mo is that soil pH and other characteristics do not influence availability and Mo uptake.

    Rates of Mo application are very low – 0.5 to 5 oz./acre. The solution may be applied to soil, sprayed on foliage, or put on seed prior to planting (latter two methods use lowest application rates). Mo can be combined with N-P-K fertilizers. Foliar applications of Mo may be made with ammonium or sodium molybdate, however, chelated Mo is more easily absorbed by the plant tissues and is more effective in correcting deficiencies. In acids soils when adequate Mo is present, liming can used to increase Mo availability. A soil test for Mo is necessary to know if adequate Mo is in the soil. With high yielding crops, Mo deficiency is becoming more prevalent. Additionally, Mo application is always preferable to liming when an increase in soil pH is not desired.

    Sodium molybdate, which ranges from around 40 to 46% molybdenum, is probably the most common Mo fertilizer source. Ammonium molybdate is often used in more soluble clear liquid fertilizer solution. Chelated molybdenum is becoming more popular because of easy of use, incl ng mixing properties with other fertilizers and chemicals, and efficiency of Mo uptake by plant tissues.

    Brassicas such as cauliflower need high rates of Mo. This may be applied in the seedbeds before planting, mixed with dry and liquid fertilizers, or foliar applied. With adequate Mo in the seedbed, the plant may be able to accumulate enough Mo to last through the plants’ life. Mo and insecticidal root drenches have been developed to prevent cabbage root fly.

    Other sources of Mo include molybdic oxide (58 to 62% Mo), and molybdic acid (approximately 58% molybdenum). The oxide and acid forms contain no sodium or ammonium ions, thus reducing salting out. However, these forms must be dissolved in acids before being added to a mixed fertilizer solution.