<div>From Plant Analysis Handbook III pages 151+152</div>

Chapter 15

Molybdenum is one of the most common micronutrient deficiencies found in agriculture. It is routinely misdiagnosed for several factors 1) molybdenum is not routinely tested in the soil or plant tissues, 2) deficiency symptoms appear as nitrogen deficiency, 3) the erroneous assumption that if the soil pH is neutral or above, then plenty of molybdenum is available from the soil. There is also a fear than high molybdenum levels are toxic to plants and animals. However, molybdenum toxicities in plants are virtually non-existent in normal crop production.<div>

Molybdenum in the soil solution occurs predominantly as MoO4², HM0O4 and H₂MoO4. The main reservoir of Mo in the soil is molybdenite sulfide (Mo₂S). The sulfide molybdenite (MoS₂) is common in certain granitoids. Most excess Mo is associated with Fe and Mn hydroxides. Molybdenum is found as a relatively insoluble salts in association with Pb, Ca or Fe, and in association with organic matter. The weathering of these materials and the decomposition of organic matter release MoO4 into the soil solution.</div><div>

</div><div>Molybdenum exists largely as impurities in soil minerals, especially in the clays and oxides of secondary minerals, and in freshly added organic materials, particularly in leaves. The secondary minerals usually surround the soluble molybdenum and occlude it, thus reducing its availability. Decomposition of organic material releases the molybdenum in a soluble form.</div><div>

Molybdenum can be fixed in the soil at low pH levels, but is readily displaced and released by hydroxyl (OH) ions. Therefore, it becomes more available at higher soil pH. Mo reacts differently to soil conditions than do the micronutrients B, Cu, Fe, Mn and Zn, all of which tend to decrease in availability as the pH level rises. Mo behaves more like phosphorus, with availability increasing with rising soil pH. Concentration of MoO and HMOO, increases dramatically with increasing soil pH. The molybdate anion’s (Mo0,2) availability is increased 10-fold for each unit increase in soil pH. At low pH, Mo is strongly absorbed by iron and aluminum oxides in the soil. Mo can be a limiting factor in growth for plants with relatively high Mo requirements grown on acid/clay soils (e.g., soybeans grown in Georgia). Liming acid soils will make Mo in the soils more available to plants. However, available Mo is commonly deficient in soils regardless of pH, and simply raising the pH of the soil does not guarantee adequate Mo is available.

Mo levels in the soil solution can vary widely typically from <0.01 to 20-30 ppm. Some soils in Hawaii with Mo content as high as 73.8 ppm. The main factor that controls the quantity of available Mot is the quantity of Mo contained in the native soil minerals. Also, much of the Mo in the soil is tied up in organic matter, including plant residues. The mineralization of this organic component in the soil often provides much of the Mo provided to crops. In many intensively cultivated soils, Mo deficiency has become a more common problem. However, because Mo is not routinely included in soil and tissue tests, growers and agronomists do not know that it is a problem. Low available Mo in the soil occurs because of low Mo content in the native minerals, low pH, heavy rainfall and/or excessive irrigation, limited organic matter, low Mo content in organic matter, and poor mineralization of the organic matter in the soil. Some soils naturally have low levels of Mo and others have high levels of Mo. Traditionally, Mo levels in the soil solution are considered to be very low, in the range from 0.2 to 5 ppm. However, with high yield cropping programs, Mo is also removed with the harvested crop. Thus molybdenum demand often exceeds the capability of soils to provide, and the application of Mo fertilizers is becoming more prevalent, especially foliar application of chelated or other readily available Mo.

Uptake and Assimilation by Higher Plants

Molybdenum is taken up as the molybdate anion (MoO4) and translocated preferentially to the leaves. Molybdenum can be absorbed through leaves using foliar application of chelated Mo. However, active nitrogen fixing structures, such as nodules in soybeans, are sinks for molybdenum, and Mo tends to accumulate in the roots and nodules during active nitrogen fixation. Molybdenum is required in the enzyme system for nitrogen fixation.</div>