Crop Response to Applied Molybdenum

The effect of molybdenum fertilization on increasing plant yield is often related to an increased ability of the plant to utilize nitrogen. The activities of nitrogenase and nitrate reductase are affected by the molybdenum status of plants, and their activities are often suppressed in plants suffering from molybdenum deficiency (22,117). Foliar application of molybdenum at 40 g ha^-1 at 25 days after plant emergence greatly enhanced nitrogenase and nitrate reductase activities of common bean (Phaseolus vulgaris L.), resulting in an increase in total nitrogen accumulation in shoots (117). In addition, foliar fertilization of common bean with 40 g Mo ha^-1 increased nodule size, but not the quantity of root nodules (118). Therefore, the main effect of molybdenum on nodulation was suggested to be the avoidance of nodule senescence, thus maintaining a longer period of effective N₂ fixation.

The application of molybdenum to soils with low amounts of available molybdenum can improve crop yield dramatically, particularly for legumes, which have a high molybdenum requirement (12). Large-seeded legumes often do not require molybdenum fertilization if their seeds contain enough molybdenum to meet the requirements of the plant (119). But for plants suffering from molybdenum deficiency, the response to molybdenum fertility often varies. The lack of response to molybdenum can be related to other nutritional problems, such as the toxic effects of aluminum and manganese in acid soils, which mask the effects of molybdenum nutrition (116). In addition, molybdenum can be rendered unavailable to plants in acid soils if molybdenum is fixed by iron and manganese oxides (120). Crop plants also vary in their requirement for molybdenum (Table 13.1) and thus require different levels of molybdenum fertilization to achieve maximum growth.

Soybean yields in southeastern United States have been shown to increase by 30 to 80% following molybdenum fertilization on acid soils (33,121). Similar results have been obtained for peanut (Arachis hypogaea L.) grown on acid soils in western Africa (122). However, Rhoades and Nangju (123) found that at soil pH 4.5, soybeans did not respond to molybdenum. Differences in the response of legumes to molybdenum may be related to the timing of fertilizer applications. During the lag phase between infection and active N₂ fixation (between 10 and 21 days) (9), the addition of molybdenum fertilizers may be ineffective because the growth response to added molybdenum is related primarily to the molybdenum requirements of the N₂-fixing bacteria (18). In other studies where molybdenum was seed-applied, cowpea (Vigna sinensis Endl.) yields increased by 25% (123), and oat (Avena sativa L.) yields increased by 48% (124). Molybdenum fertilization has also been shown to increase the production of melons (Cucumis melo L.), with treated test plots yielding 254 melons compared to 19 in the untreated plots (39).

The efficiency of molybdenum fertilizers can be affected by soil pH. In acid soils, the availability of applied molybdenum can be limited due to the fixation of MoO₄^2- by iron and aluminum oxides, but the quantity of molybdenum in the soil solution increases with increasing soil pH (120). Liming materials can be used in conjunction with molybdenum fertilization to increase molybdenum uptake by plants, but the effect on plant growth is limited to soil pH levels<7.0 (48). Liming alone may liberate enough soil-bound molybdenum to sustain plant growth (89). However the effect of lime depends on the total molybdenum content of soils. On acid soils where aluminum toxicity can limit plant growth, adding both lime and molybdenum is often more beneficial than adding only one of them (125). Combined applications of lime and molybdenum to forage crops can lead to problems for grazing animals because the accumulation of molybdenum in plant tissues can be high enough to cause molybdenosis (126).

Other soil amendments such as phosphorus- or sulfur-containing fertilizers, may also influence the efficiency of molybdenum fertilizers by affecting the fixation of molybdenum in soils or its uptake by plant roots. The use of phosphate (H2PO₄^-), which has a high affinity for iron oxides, can lead to the release of adsorbed molybdenum and to an increase in the water-soluble MoO₄^2- concentration of the soil (8). As a result, phosphorus fertilization often increases the molybdenum absorption by roots and its accumulation in plant tissues (12,87). In contrast, sulfate and MoO₄^2- are strongly competitive during root absorption, and sulfur fertilization has been shown to decrease the uptake of molybdenum by plants (127). Studies with peanut have shown that providing phosphorus in the form of triple superphosphate is superior to single superphosphate for plants grown in molybdenum- deficient soils (128). This difference was attributed to the sulfur component of single superphosphate and its effect on inhibiting molybdenum uptake and suppressing plant growth.