I have referred to molybdenum (Mo) many times with respect to its key role in crop plant nitrogen metabolism. This micronutrient is needed by crop plants in such small quantities that we may take for granted it is unlikely to ever be deficient.
It may well be that on the Canadian Prairies it would be unusual to find any molybdenum-deficient cropland. That certainly would have been true, let’s say, 70 or so years ago, when cropland was tilled and nitrogen fertilizer levels were relatively low. Some 40-50 years ago, Prairie wheat yields were around 25 bushels per acre and canola around 20. Now, N fertilizer levels are more than double and wheat and canola yields are around 50 and 40 bushels annually respectively.
Molybdenum is needed to convert the nitrate taken up by all crop plants into protein via the Mo-enzyme (nitrite reductase). In addition, in legumes, another Mo-enzyme (nitrogenase) is needed by the root nodule bacteria for nitrogen (N) fixation. Only tiny quantities of molybdenum are needed — but there are cropland factors that can limit or prevent molybdenum uptake by crop plants. The result may be inhibited crop growth, reduced yields or delayed maturity.
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Let’s have a look at factors which may limit molybdenum crop plant availability and depress crop yields.
Nature
Molybdenum may be naturally deficient in a given area. In the northern U.S. state of Idaho, it was found that large areas — up to a quarter of the land in that state — could be naturally deficient in molybdenum. In a few areas in the central part of the state, the molybdenum level was so high in forage and hay in pastureland that it interfered with copper levels in livestock and caused a disease called molybdenosis, resulting in severe copper deficiency in pasturing animals.
Acidity
In very acidic soils — that is, those below pH 5 or 5.5 — there will be areas where the pH may be as low as 4 or 4.5, levels at which molybdenum may be unavailable and seeded canola in particular will remain in a tight rosette stage and grow very slowly if at all. The pH on such land must be adjusted either with lime (limestone) or wood ash.
Non-disturbance
In minimum or zero till, the surface soil of your cropland may be undisturbed for 10-15 years or more. If the cropland soil is perhaps between a pH of 5-6, then over this time the surface soils — say, down to two to three inches — can become acidified. Acidification of this surface soil is due to the use of nitrogen fertilizers, which acidify soil, and to crop residue, which can become acidic. The result is that this surface soil, at one to three inches, could have a pH of 3.5-4.5. This acidity immobilizes the soil molybdenum. The seedlings may pick up the soil nitrates, but without the molybdenum, the nitrates cannot be converted to proteins, in the absence of the molybdenum-based nitrate reductase enzymes. When the seedling roots move deeper into the soil with the higher pH, they likely will pick up sufficient molybdenum. This seedling delay could make the growing season longer — and the seedlings have a longer exposure to crop-damaging flea beetles.
Higher-demand crops
Legumes have greater needs for molybdenum than non-legume crops, since Mo is essential for N fixation in the legume root nodules. It is possible that legumes such as dry beans grown under irrigation in neutral or even in high-pH soil may have a need for Mo fertilization, due to the fact that the Mo which is normally present at low levels in soil may be leached below the root zones. We did run across irrigated bean growers who felt they got improved yields when they added a few ounces of Mo to the planted bean crop in southern Alberta.
Sulphate a suspect
In a review of several textbooks on the role of molybdenum, and one text in particular — Soil Nutrient Bioavailability, second edition, by Stanley Barber — I came across some new information. It appears sulphate in the soil can depress molybdenum availability. They compete for the same absorption sites on the root. For example, adding gypsum (calcium sulphate) to alkaline soil significantly decreases the uptake of molybdenum from 2.33 parts per million (p.p.m.) to 1.26 p.p.m. in tomatoes.
This same sulphate depression of molybdenum uptake was echoed in Marschner’s text and single superphosphate was identified as the cause. Triple superphosphate did not contain sulphate and was not a problem. Since our Prairie canola crops are high users of sulphur, this information should be explored. We may be depressing potential Prairie crop yields due to high or frequent applications of sulphate nutrients — a good reason to perhaps add an ounce or two of molybdenum to each acre of cropland.
Marschner also states that molybdenum deficiency is widespread in legumes growing in acidic soils. Additionally in this text, it states that molybdenum deficiency in cropland has a stalling effect on pollen formation in corn (maize). It causes delayed tasseling and results in a large proportion of flowers that fail to open. Both the above problems are preventable with a couple of ounces of molybdenum an acre.
When I was on the faculty at the University of Guelph, little attention was paid to crop-essential micronutrients. The rutabaga growers in Ontario, growing around 15,000 acres, worth about $12 million annually, invariably added a few ounces of sodium molybdate fertilizer to the intended rutabaga cropland. They said it enhanced rutabaga growth and quality. I would also point out that cauliflower growers in Ontario, on more than a few occasions, would come across a problem called whiptail. Whiptail is a disorder in the cabbage family (canola included) whereby the leaf rib develops but the leaf does not fill out. Search up “whiptail of cauliflower” on the internet. It’s a sign of molybdenum deficiency.
Another temporary way to alleviate Mo deficiency on acidic no-till or minimum-till soil is to deep harrow or till thin surface soil layer, to bring up the higher-pH soil that exists below the one- to three-inch level.
Sulphate, in the form of gypsum or single superphosphate, can depress plant uptake of Mo. Soil tests are not the most reliable methods for detecting available soil levels of Mo. So, if your cropland is on the acidic side — below pH 5.5 — and you follow a zero-till or minimum-till strategy, you may consider appropriate levels of Mo seed treatment, Mo foliar applications or Mo added to your fertilizer inputs at only a couple of ounces per acre. It’s a very small cost indeed if you suspect a Mo deficiency problem on your cropland.
