Molybdenum deficiency
The micronutrient of acid soils — it halts nitrogen fixation in legumes and throws the strap-leaf "whiptail" of cauliflower and broccoli.
- Nutrient type
- Micronutrient
- Mobility in plant
- Mobile
- Symptoms start on
- Old leaves; cole growing points
- Most at risk
- Acidic soils; legumes & cole crops
Overview
Molybdenum is needed in the tiniest amount of any plant nutrient, but it does two jobs nothing else can: it lets legumes fix nitrogen from the air (the enzyme needs molybdenum), and it lets any plant use the nitrate it takes up. So a shortage looks a lot like nitrogen deficiency — pale older leaves, poor growth — because the plant cannot process nitrogen properly.
It is the odd one out among micronutrients: it becomes LESS available as soil turns acidic, the opposite of iron, zinc and manganese. That makes it a problem of acid soils, correctable simply by liming. In cole crops it produces a distinctive symptom — "whiptail", where the leaf blade fails to form and the leaf is reduced to a narrow, distorted strap.
How to spot it
- "Whiptail" in cauliflower, broccoli and cabbage — the leaf blade fails to develop and the leaf becomes a narrow, twisted strap, with distorted, cupped growing points.
- In legumes (pea, gram, soybean, groundnut) the crop looks nitrogen-starved — pale, stunted, few root nodules — because it cannot fix nitrogen.
- General pale yellow-green older leaves with upward-cupped margins, since the plant cannot process nitrate.
- It appears on acid soils, exactly where you would expect molybdenum to be least available.
Why it happens
- Acidic soils (low pH) — molybdenum becomes progressively unavailable as pH falls, so acid soils are the classic setting.
- Sandy, weathered and leached soils that are naturally low in molybdenum.
- Legume and cole crops, which have the highest molybdenum demand, growing on such soils.
- It is one of the rarer deficiencies overall — but predictable where acid soil and a high-demand crop meet.
Confirm before you spend
- Whiptail in a cole crop is close to diagnostic of molybdenum. In legumes, molybdenum deficiency mimics nitrogen deficiency — check the soil pH: if it is acidic and nodulation is poor, suspect molybdenum.
- Because the requirement is so tiny, a soil pH check is often more useful than chasing a molybdenum soil-test number.
How to correct it
- Liming an acid soil to a healthy pH often corrects molybdenum deficiency on its own, by releasing what is already there — the cheapest fix.
- Where a direct dose is needed, apply a very small amount of sodium or ammonium molybdate — grams per acre, not kilograms — as a seed treatment or foliar spray; the requirement is minute.
- Treating legume seed with a molybdenum + rhizobium inoculant before sowing is a common, effective step on acid soils.
- Never over-apply molybdenum — excess makes forage toxic to grazing livestock (molybdenosis), so measure carefully.
Prevent it coming back
- Keep acid soils limed to a healthy pH — this is the single most reliable prevention.
- Use a molybdenum seed treatment for legumes and cole crops on known acid soils.
- Build organic matter, which helps hold and cycle the tiny amount of molybdenum the crop needs.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My cauliflower leaves are becoming thin straps. What is that?
That is "whiptail", the classic symptom of molybdenum deficiency in cole crops — the leaf blade fails to form and the leaf is reduced to a narrow, twisted strap. It occurs on acid soils. Liming to correct pH, plus a tiny molybdenum seed treatment or spray, prevents and treats it.
Why is molybdenum deficiency worse on acid soil, when zinc and iron are worse on alkaline soil?
Molybdenum is the exception among micronutrients: its availability falls as pH drops, so acid soils starve the crop of it while iron, zinc and manganese are locked up by high pH instead. That is why liming an acid soil often fixes molybdenum deficiency at the same time as raising pH.
How much molybdenum should I apply?
Very little — molybdenum is needed in the smallest amount of any nutrient, grams per acre as sodium or ammonium molybdate, usually as a seed treatment or foliar spray. Never overdo it: excess molybdenum in forage can poison grazing livestock. Often, simply liming the acid soil is enough without adding molybdenum at all.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.