Wednesday, 01 October 2014

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Molybdenum In Plant Nutrition

MOLYBDENUM IN THE SOIL
 
The content of molybdenum in most agricultural soils is usually between 0.6 and 3.5 ppm with an average molybdenum content of 2.0 ppm and an average available molybdenum content of 0.2 ppm.

Molybdenum largely occurs in the soil as an oxycomplex (MoO42-). Because of this, molybdenum more resembles phosphate or sulfate in its behavior in the soil. In a similar way to these two anions, molybdate is adsorbed by soil minerals and colloids.

This adsorption is closely dependent on soil pH. At neutrality it is very low but increases as the pH falls. Molybdenum availability to plants is thus poorest on acid soils and is improved by liming, provided the soil is not inherently deficient in molybdenum.
 


Molybdenum Deficiency
  MOLYBDENUM UTILIZATION BY PLANTS
 
Plants absorb molybdenum as molybdate. The form in which molybdenum is translocated is unknown. Molybdenum is located primarily in the phloem and vascular parenchyma and is only moderately mobile in the plant. The requirement of molybdenum in terms of dry matter is usually in the range from 0.1–1.0 ppm. Most plants are very tolerant of excessive amounts of molybdenum in the tissue with levels above 1000 ppm existing without any harmful effects. A unique feature of molybdenum nutrition is the wide variation between the critical deficiency and toxicity levels. These levels may differ by a factor of up to 104 (e.g., 0.1 to 1000 ppm dry weight).
 
Molybdenum is an essential component of two major enzymes in plants, nitrogenase and nitrate reductase. Nitrate reductase is the most well-studied molybdenum-containing enzyme. It catalyzes the reduction of NO3- to NO2-.

MOLYBDENUM IN CROP NUTRITION

Most soils contain enough molybdenum in available form to adequately meet the needs of crop plants. In some areas, however, particularly on acid soils (pH<5.5), molybdenum deficiency can arise because of high-molybdenum fixation in the soil. The geographic pattern of molybdenum deficiency mainly follows the regions of acid, sandy soils, although the effect may be masked by the common use of lime.1

The requirement for molybdenum by plants is varied. The Cruciferae, particularly cauliflower and cabbage, have a high-molybdenum demand. The same also applies to legumes because of the requirement of the root nodule bacteria. In a survey of 21 states in the USA, alfalfa was found to be the most common crop species showing molybdenum deficiency, followed by cauliflower, broccoli, soy beans, clover, and citrus. In general, the monocots are not very sensitive to molybdenum deficiency.

  MOLYBDENUM DEFICIENCY SYMPTOMS

Since the most important function of molybdenum in plant metabolism is in the NO3- (nitrate) reduction, molybdenum deficiency resembles nitrogen deficiency. Plants suffering from molybdenum deficiency are restricted in growth; their leaves become pale and eventually wither. Flower formation may be restricted. In dicotyledons a drastic reduction in size and irregularities in leaf blade formation (whiptail) are the most typical visual symptoms. These are caused by local necrosis in the tissue and insufficient differentiation of vascular bundles at early stages of leaf development.

CORRECTION WITH METALOSATE® MULTIMINERAL™ AND METALOSATE® TROPICAL™

As molybdenum is highly phloem-mobile, foliar application is an appropriate and effective procedure for correcting molybdenum deficiency. Since plants require such a low level of molybdenum, it does not take much to increase levels in plant tissue to the sufficient range. Given the fact that such small amounts are required, Albion Plant Nutrition supplies molybdenum as a nutrient available in Metalosate Multimineral and also in Metalosate Tropical.
 
In each of these products the level of molybdenum is 0.1% which has proven to be effective in increasing plant tissue levels to ranges that are acceptable. For more information on the Metalosate products and how they can add value to your programs please contact your local Albion Plant Nutrition representative.
REFERENCES
  1. Mengel, K., & Kirkby, E.A. (2001) Principles of Plant Nutrition (5th ed.) (p. 480). Dordrecht: Kluwer Academic Publishers.