Sodium molybdate (Na₂MoO₄·2H₂O) is a water-soluble crystalline fertilizer containing 39% molybdenum by weight. Mo is an essential micronutrient that serves as the metal cofactor for two key enzymes: nitrate reductase, which converts nitrate to nitrite as the first step in protein synthesis in all plants, and nitrogenase, which converts atmospheric N₂ to plant-available ammonium inside legume root nodules. Without adequate Mo, plants accumulate nitrate without metabolizing it — producing symptoms that mimic nitrogen deficiency even when N is abundant. For more on micronutrient roles, see our guide to Essential Micronutrients for Healthier Plants.

Symptoms vary by crop. Brassicas (cauliflower, broccoli) show the classic "whiptail" pattern — narrow, strap-like, distorted leaves with ragged edges. Legumes show poor nodulation: healthy active nodules are pink or red inside (from leghemoglobin); Mo-deficient nodules turn green or white with little to no nitrogen fixation. Most other crops show pale green to yellow leaves resembling nitrogen deficiency, sometimes with upward leaf cupping or scorched margins on older leaves.

Mo deficiency is most common in acid soils (pH below 6.0), sandy soils with low organic matter, and after heavy rainfall events that leach molybdate ions. Because Mo deficiency often looks like nitrogen deficiency, see our guide to 8 reasons leaves turn yellow for help differentiating causes — soil or tissue testing is the most reliable confirmation.

It depends on the crop and the situation. Seed treatment (1–2 oz per 100 lb of seed) is the most efficient method overall — it places Mo exactly where Rhizobium bacteria need it for nitrogen fixation and supports the full season for legume annual crops. Foliar spray (1–4 oz per acre in 20–30 gallons of water) gives the fastest correction in standing crops, best applied at the 4–6 leaf stage before symptoms appear. Soil broadcast (4–8 oz per acre) is less efficient per application but can support correction for 3–5 years from a single treatment. As an alternative, liming acidic soils to pH 6.2–6.5 often releases enough native Mo to correct the deficiency without direct application.

Critical: never exceed recommended rates. Mo above 5–10 ppm in forage tissue can cause copper deficiency (molybdenosis) in grazing livestock.

Mo deficiency is a common but often overlooked cause of poor nitrogen fixation in legumes. The nitrogenase enzyme that splits atmospheric N₂ requires Mo as a cofactor — without adequate Mo, even well-inoculated legumes cannot fix nitrogen effectively. Check nodule color first: healthy active nodules are pink or red inside; Mo-deficient nodules turn green or white. Seed treatment with 1–2 oz of sodium molybdate per 100 lb of seed at planting usually addresses this for the entire season.

Also check soil pH — acidic conditions below pH 6.0 severely limit Mo availability, and the deficiency may resolve with liming alone. For broader nitrogen-source guidance, see our article on the best nitrogen fertilizer for your situation.

Unlike most micronutrients, Mo becomes less available as soil pH drops. Each one-unit decrease in pH reduces plant-available Mo by roughly 100-fold — so a soil at pH 5.5 can behave as severely deficient even if total Mo content is adequate. This is the opposite of iron, manganese, and zinc, which all become more available in acid conditions.

This gives growers two correction paths: apply Mo directly (seed, foliar, or soil), or raise pH through liming with Dolomite Lime to 6.2–6.5 so native Mo becomes plant-available. In moderately acid soils with adequate total Mo, liming alone often resolves the deficiency.

Several important interactions. Mo is essential for nitrogen metabolism — without adequate Mo, plants can't convert nitrate to amino acids even when nitrogen is abundant. Sulfate ions compete with molybdate for plant uptake, so high-sulfur fertility programs can induce Mo deficiency even in soils with adequate total Mo — if you're applying significant sulfur, monitor Mo more closely.

There's also an antagonistic relationship with copper: high tissue Mo can induce copper deficiency in grazing livestock (molybdenosis), which is why forage Mo above 5–10 ppm is unsafe for cattle and sheep. For background on nutrient interactions and how toxicity can mimic deficiency, see Fertilizer Toxicity vs. Nutrient Deficiency.

Sodium molybdate is generally permitted in organic systems as a remedial input when a documented deficiency exists, but always verify with your certifying agency before application. Typical requirements include: soil or tissue tests confirming Mo deficiency, documentation of deficiency and application rates, and applying only the amount necessary to correct the deficiency — not as a routine input. The tiny quantities needed (often less than 0.1 lb of actual Mo per acre) make Mo correction both environmentally low-impact and economical for organic production.

Duration depends on the method. Seed treatment supports the full season for annual legume crops — one application at planting is typically enough. Foliar correction addresses the current season but may need repeating in subsequent years if the underlying soil deficiency isn't resolved. Soil broadcast applications generally support correction for 3–5 years before reapplication is needed, making them the most efficient long-term investment for chronic Mo-deficient soils. In hydroponic systems, maintain continuous low levels of 0.02–0.05 ppm in the working solution and monitor monthly.

Yes — sodium molybdate is compatible with most fertilizers and can be tank-mixed for convenience with nitrogen sources, micronutrient blends, and foliar sprays. Avoid mixing with highly acidic solutions, which can reduce Mo availability. When banding with starter fertilizer, placement efficiency improves significantly compared to broadcast at the same rate.

As with any fertilizer combination, jar-test unfamiliar mixtures before tank-mixing at scale to check for precipitation or compatibility issues.

This is the single most important safety consideration for Mo. While plants need Mo to function, grazing cattle and sheep are sensitive to molybdenum in forage tissue. When forage Mo exceeds approximately 5–10 ppm, it can induce copper deficiency in the animal — a condition called molybdenosis that causes scouring, weight loss, and reduced productivity. The effect is worse when forage copper is also low.

For pasture and forage crops, never apply Mo without first establishing a baseline through forage tissue testing. If you must correct a confirmed deficiency in grazed acreage, use the lowest effective rate (seed treatment is preferable to soil broadcast) and retest forage tissue before resuming grazing.