All About Moly

By: Dr. Ieuan R. Evans, Senior Agri-Coach

Moly (molybdenum) is the wallflower of the plant essential micronutrients; no one wants to dance or find out if this lonesome element is present, absent or even around in overabundance. Moly is the only significant micronutrient that becomes more available as the soil pH increases from pH to pH 8.5. In our intensive crop management strategies we take "moly" for granted - no questions, no answers. 

In my native Wales, molybdenum was used as a seed treatment on cole crops (kale and turnips) or the usually acidic soil at around pH 4,5 to 6 was limed. This addi­tion of lime increased soil pH and thus the availability of moly. In Southern Ontario moly is routinely added at a few ounces actual per acre to cole crops such as cauliflower and rutabaga on fertile soils at around pH 7. This prevented the disease known as whiptail, a moly deficiency in cruciferous crops that could result in crop failure. In Western Canada I have only seen moly deficiency occasionally in cauliflower and canola, but remember obvious symptom usually means a severe deficiency. In my own garden, pH 6.2, I can induce moly deficiency in cabbage seedlings by applying very high levels of N fertilizer.

WHAT DOES MOLY DO? 

While the actual requirement for moly is low, it's the key element in nitrogen (N) metabolism. The enzymes nitrate reductase, nitrogenase and possibly sulphite reductase all must have molybdenum to function effectively. 

In legumes the moly requirement is very large in the root nodules in the role of N fixation. Its function is to convert nitrate to the ammonium form, so it’s not surprising that molybdenum is less important to- crops fed ammonium rather than nitrate fertilizer. 

Research has shown (attention: bean and pea growers) that>n soils with low molybdenum availability it is possible to replace the application of nitrogen fertilizer to legumes by the application of molybdenum fertilizer combined with proper rhizobial infection. On field beans, is a seed treatment with moly or a foliar application of a few ounces comparable in price to 100 Ibs. or more of actual N that might be applied to the crop? 

I'm beginning to suspect that moly deficiency can occur on high pH soils 7 - 8.5 subject to frequent irrigation; i.e. the moly's been leached out. Could this be happening to dry bean and peppermint crops? On the other hand there could be moly unavailability on soils with a pH of between 5 and 6. Maybe that's why alfalfa doesn't fix N below pH 6 and canola can't utilize all the N of a high yield expectation on acidic soils. 

In addition to the well-documented problems with cole crops, moly deficiency causes pollen failure in corn leading to grain set failure and premature sprouting in wheat and corn. Premature sprouting in wheat maybe strongly depressed by foliar sprays of molybdenum particularly in soils high in nitrogen although critical levels of moly in leaves varies widely between 0.1 and 1 ppm.  Seed pelleting of sensitive crops with molybdenum at the rate of 100 gm per hectare or 1 ounce per acre of actual molybdenum trioxide is highly effective  Anyone tried this yet? Foliar applications are also highly effective at an early growth stage, particularly on legumes. Just because we never test for mo!y or never use moly doesn't mean that we shouldn't be trying it out on our crops. 

On the negative side, high levels of moly cause problems in livestock by inducing a severe imbalance of copper. This situation occurs in many areas of the prairies such as Southern Manitoba and the Swan River region. Molybdenum excess occurs in soils with poor drainage and high in organic matter.

All in all let's tango with Moly ...let's do a series of strip trials or seed treatments with a select number of clients this year - let's plan for it now.