Animals and plants require trace amounts of molybdenum. Its importance is vastly disproportionate with regard to the amount required for normal growth.
In past years, most farmers and soil scientists were just getting to grips with sulphur and phosphate requirements of crop plants, let alone nitrogen and potash. It seemed as long as you had nitrogen fertilizer everything else would fall into line. Micronutrients such as zinc, copper, boron and manganese would never be in short supply, let alone something as “insignificant” as molybdenum. Well, molybdenum (Mo) is the key to the nitrogen gate and is also involved in sulphur and phosphate metabolism.
Some time ago a knowledgeable canola expert in Alberta and I were asked by agronomists at the University of Idaho to look into a canola crop failure in Idaho. These agronomists supplied us with the field history, nutrients added, soil analysis, herbicide history, pH and weather conditions. Their canola would emerge satisfactorily but grew slowly into the initial rosette stage and just stalled. The crop would not develop. The canola expert and I came to the same conclusion: an absence of molybdenum. The answer came back from Idaho: they could not detect any molybdenum in the soil.
Molybdenum levels were checked across Idaho, and it was found that molybdenum deficiencies occur in the northern half of the state, the central part has very high levels and the lower part of the state is seldom deficient.
Agronomists in Idaho have found that sandy soils with pH values of less than six or soils very high in organic matter can be deficient in Mo. Molybdenum is a key nutrient in nitrogen fixation in legume nodules. No Mo means no fixed nitrogen. Such soils respond phenomenally when treated with a few ounces of sodium molybdate per acre. Crop yields can double or more. Seed treatment with as little as an ounce of Mo can be highly effective on deficient soils.
Without molybdenum, plants are unable to metabolize any form of nitrogen (i.e. nitrate to ammonium) and to a lesser extent sulphur and phosphate. The whole plant process comes to a halt.
Molybdenum in Canada
I suspect we may have Mo-deficient areas on the Canadian prairies. Several Saskatchewan farmers east of North Battleford report that peas produce well-below expected yields on their cropland. I encouraged them to use molybdenum to no avail.
In the Peace Region of Alberta and B.C., significant acres of cropland have a pH of around 5.5 or less. At this pH, Mo becomes restricted or unavailable to field crops. In-furrow applications of crushed limestone on these low-pH soils show significant yield results, often wrongly attributed to the addition of boron. Crushed limestone raises the soil pH and releases Mo. Molybdenum becomes progressively more soluble as the pH level increases. Liming or wood ash application to acidic crop soils has an important role in making Mo available for plant growth and nitrogen fixation in legumes.
I grew up on a farm in Wales where the soil pH was around four to five. We never seeded a legume crop without a prior lime application.
Some 15 years back a colleague and I were visiting dry bean growers near Lethbridge. They remarked that in recent years, beans grown under irrigation needed more nitrogen to reach their target yield. Dry beans fix a percentage of their nitrogen via rhizobia. It occurred to us that in this high pH soil, around 7.5 to 8.2 pH, perhaps over many years of irrigation, available Mo had leached to the subsoil. My colleague suggested they try a few ounces of Mo per acre. The following year both growers reached their target yields without additional nitrogen. There is a strong possibility that Mo can be leached out of or into the subsoil on high-pH soils under irrigation.
If your legumes or other crops are not performing up to expectations, try a few ounces of molybdenum on a few acres or invest in some very inexpensive seed treatment. It will be the most inexpensive fertilizer that you will ever buy.
In more than a few parts of the Canadian Prairies we have areas of Mo toxicity. This occurs in alkaline, poorly drained cropland areas where soluble Mo can accumulate in the soil and consequently reaches high levels, particularly in hay crops. Cattle feeding on high Mo hay can develop severe copper deficiency, resulting in death or poor performance of milkers. This is referred to as molybdenum-induced copper deficiency. Cases have been identified in Manitoba, around Vita, south of Winnipeg, and the Swan River area, and in the northern Peace Region. I suspect there are more of these areas on the Prairies.
In a visit to a field at Fort Vermillion in the northern Peace Region, I diagnosed a failed canola field as herbicide damage. Soil analysis showed that samples taken from the field had a pH of 3.5. Now, you figure out the crop failure. I lost a $10 bet on this field to an Alberta soil scientist.