Soybean Nutrient Deficiency: Nutrient Requirement, Uptake, Symptoms, Look-alikes and How to Diagnose Them

Soybean takes much of its nitrogen from the air through root nodules, so the nutrients to watch are phosphorus, potassium and sulphur, with zinc in many Indian soils. Older leaves go first for nitrogen, phosphorus, potassium, magnesium and molybdenum. Young leaves go first for sulphur, iron and manganese. Before you spray, dig up a few plants and look at nodules, roots and whitefly, because yellow mosaic virus, herbicide injury, waterlogging and compaction look alike. Confirm with a soil test (yellow patch against a healthy patch), use a tissue test only as a second opinion, and send the dose question to your state recommendation or our schedule post.

There is no single national number. Indian university packages sit between 20:40:20 and 30:75:30 kg of N, P2O5 and K2O per hectare, with sulphur and zinc added where soils are short. Use these as reference ranges to understand the crop's needs, not as a schedule. Doses and timing are covered in our stage-wise schedule post.

Why do the numbers differ? Soil, rainfall, irrigation and yield target differ. The PDKV items are for Akola-region black soils. The TNAU numbers are for Tamil Nadu. Madhya Pradesh farmers should look for the JNKVV or ICAR-IISR Indore package for their district. The soil health card gives the starting point for your own field.

A reference dose is not the same as what the crop takes up. The next section shows the gap, and the gap is why potassium and sulphur are the ones that quietly run down.

A high-yielding crop takes up roughly 60 kg of nitrogen, 15 kg of P2O5, 38 kg of K2O and 3.5 kg of sulphur for every tonne of grain, but the grain carries away only part of it. The rest stays in straw and leaves. Figures differ by country, variety and how much nitrogen the nodules fix.

No Indian per-tonne uptake table turned up in the sources we could open, so the figures below are from a Wisconsin and Minnesota study, an Iowa State page and a Brazilian trial. The Wisconsin study (2014 and 2015, 6,672 tissue samples, yields from 40 to 100 bushels per acre) is the most complete. We converted its pounds per bushel to kg per tonne by multiplying by 16.67.

Source: Wisconsin/Minnesota bulletin, lb per bushel at 13 percent grain moisture, converted by us. The last column is simple subtraction. Micronutrients are tiny by weight. In grams per tonne of grain the same bulletin gives total uptake of about 100 g iron, 67 g manganese, 50 g zinc, 33 g boron and 17 g copper, with roughly 17 to 33 g of each leaving in the grain.

To compare P and K across rows, convert elemental P to P2O5 by multiplying by 2.29, and elemental K to K2O by multiplying by 1.205. Iowa then gives about 15.3 kg P2O5 and 25.1 kg K2O per tonne, and Brazil about 11.3 and 24.8. The spread is real. Nitrogen differs most because soybean fixes part of its own, and the share varies from field to field.

Worked example, using the Wisconsin figures only: a 2 tonne per ha crop takes up about 125 kg N, 30 kg P2O5, 77 kg K2O and 7 kg S, and removes about 110, 25, 39 and 5 kg in grain. A 3 tonne crop takes up about 188, 45, 115 and 11 kg. These illustrate the arithmetic and are not doses. Soil and nodules supply part of the total.

On nitrogen specifically: Iowa State notes the soybean plant needs more nitrogen than corn and can fix up to half of its own. In the Wisconsin study, high yields relied more on continued uptake after R5 (61.1 percent) than on moving nitrogen out of leaves (38.9 percent), so working nodules matter through pod fill. What to apply, if anything, is a schedule question for our stage-wise fertilizer post.

Most uptake happens after flowering. In the Wisconsin and Minnesota work, under 15 percent of dry matter was made before R1 (first flower) and half by R4 (full pod). Nitrogen uptake peaked between R4 and R5, phosphorus between R3 and R4, so a crop starved at flowering stays short at pod fill.

The two sources do not agree to the day on phosphorus timing. The US trial puts the peak rate at R3 to R4, and the Indian paper says the high-demand window runs from just before pod formation to near seed maturity. The practical point is the same: phosphorus has to be available in the root zone by flowering.

These curves come from US fields with yields above most Indian farms, so the stage timing is the useful part and the absolute rates are not. Local trials may shift the peaks by a week or so.

Look at which leaves show it first. Nitrogen, phosphorus, potassium, magnesium and molybdenum show on older lower leaves. Sulphur, iron and manganese show on the youngest leaves. That one rule narrows the list, and the leaf pattern narrows it again. The table below uses only university sources.

Boron and calcium are in the immobile group in the K-State article, so any shortage would show in the youngest growth. We found no detailed symptom description for either on a university or ICAR page we could open, so none is given. Ask your KVK if you suspect them.

Mix-ups are common. Sulphur and nitrogen both give pale plants; sulphur is worse on young leaves. Iron and manganese give nearly identical interveinal chlorosis and K-State calls them hard to tell apart. Molybdenum and nitrogen deficiency are the same on the leaf and differ only underground, in the nodules. Do not guess from a photo; use the sections below.

All three show on older leaves, so use colour and edge. Nitrogen gives an even pale green. Phosphorus gives dark green stunted plants, sometimes purpling. Potassium gives yellow then brown leaf edges with green veins. If the lower leaves are plain pale and nodules are missing, think nitrogen or molybdenum first.

Phosphorus is the one that misleads people, because the plant stays dark green instead of yellow. Cold and wet soil can show phosphorus-type symptoms even when soil phosphorus is adequate, because uptake drops (K-State). If symptoms fade as the soil warms, no fertiliser was needed.

For potassium, remember the previous crop. Soybean removes a lot of K, and UNL lists heavy removal by the earlier crop among the causes. NAAS lists potassium as low only in Akola (Maharashtra) and Bundi (Rajasthan) among the sites it tabulated, so across most Indian soybean soils K deficiency is local rather than universal. Check your report before assuming it.

For NPK grades by name, see the NPK fertilizer collection. For a pale, slow canopy at vegetative stage, the fertilizer for soybean slow growth collection groups four products and is a starting point, not a diagnosis.

Sulphur deficiency makes the whole plant small, slender and pale green, with the young leaves worse. It is common in Indian soybean belts because soybean-after-soybean cropping and sulphur-free fertilisers mine the soil. NAAS says continuous monocropping in Malwa has led to severe sulphur deficiency and low yields.

NAAS lists sulphur as low or deficient in soils of Adilabad (Telangana), Bhopal, Rewa, Jabalpur, Sehore, Indore, Dewas and Vidisha (Madhya Pradesh), Akola (Maharashtra) and Kota and Bundi (Rajasthan). It also notes that farmers in most soybean areas do not apply sulphur. K-State adds that sulphur shortage is becoming more common with cleaner air and high-yielding varieties, and that it is more likely in cool, wet weather or on sandy, low-organic-matter soils.

How to tell it from nitrogen: nitrogen starts on older lower leaves, sulphur on upper young leaves (UNL, K-State). If lower leaves are fine and the top looks pale on a field that has had soybean year after year, test for sulphur.

On our catalogue, ammonium sulphate fertilizer is 21 percent N and 24 percent S. Its page lists oilseeds such as groundnut, mustard and sunflower and pulses, but not soybean, so no soybean dose is given here. For how much sulphur to apply and when, use the state recommendation or our stage-wise schedule post.

Zinc gives interveinal mottling that turns bronze, iron and manganese give yellowing between veins on new leaves, and molybdenum looks like nitrogen deficiency because nodules stop fixing. Each is tied to soil pH, organic matter or waterlogging, so the soil explains the symptom as much as the leaf does.

For India, NAAS lists zinc as low in Adilabad, the same Madhya Pradesh districts as sulphur, Parbhani and Akola in Maharashtra and Kota and Bundi in Rajasthan, and boron as low in Adilabad, Dewas, Parbhani and Akola. It recommends that potassium, sulphur and zinc be included in farmer recommendations. For Indore soils it reports a positive effect from 25 kg zinc sulphate per ha with the regular dose, and/or two foliar sprays of molybdenum at 0.1 percent, or 0.5 kg per ha of ammonium molybdate to soil.

Where our catalogue has a matching input, the pages are: zinc sulphate (lists pulses and oilseeds, soybean not named), boron 20 (names soybean), ammonium molybdate (names soybean), manganese sulphate (lists pulses and oilseeds, soybean not named), ferrous sulphate (all crops, soybean not named) and a mix micronutrient fertilizer (six nutrients; soybean not named). Check the pack label for rates. A pack-label rate is not a substitute for a soil test.

Both are usually fine in a normal soil. Magnesium deficiency shows as pale older leaves with yellow blotches between veins, mostly on very sandy or low-pH soils. NAAS lists both as low only around Parbhani. Most of both nutrients the crop takes up stays in the straw, so a normal crop returns most of it.

The Wisconsin bulletin gives total uptake of about 16 kg Ca per tonne of grain but only about 2 kg leaves in the grain. Magnesium follows the same pattern, 8.5 kg taken up and 2.7 kg removed. K-State puts magnesium risk on very sandy soils, and UNL adds low-pH soils.

Low pH is the common thread. Our post on soil pH and nutrient lock explains how pH changes what the crop can reach. For calcium and boron symptoms we found no university description to quote, so check with your KVK.

Yellow mosaic virus, poor nodulation, abnormal soil pH, restricted roots, and root injury from insects, disease or herbicide all cause yellow, stunted plants. If the pattern is patchy, spreads from one spot, or comes with spots and mottles you can feel on the leaf, treat it as a non-nutrient problem until a test says otherwise.

The yellow mosaic mix-up is the most Indian one. The Jabalpur paper sampled leaves from 10 villages in Kharif 2016, and 9 of the 10 tested positive for the virus. One sample with mosaic-like symptoms was virus-free. The authors attributed that to non-viruliferous whiteflies and floated the micronutrient idea. It is one observation, so use it only as a reason to check both: whitefly on the underside of the leaf and new leaves emerging with yellow patches point to virus.

Not sure whether the fault is below or above ground? Ten minutes with a spade settles most of it. See the step-by-step check later in this post. For the cereal and vegetable version of yellowing, see nitrogen deficiency in plants; soybean is the exception to much of it because nodules supply nitrogen.

Black cotton soils and shallow black soils of Madhya Pradesh and Maharashtra are low in organic carbon, and the NAAS paper marks sulphur and zinc as widespread gaps. Waterlogged seasons, repeated soybean on the same field and cheap NP-only fertilisers make it worse. Sandy, acid or very alkaline patches add magnesium, iron or manganese problems.

The NAAS strategy paper describes the main soils as deep clayey black soils and shallow black soils with swell-shrink behaviour, prone to erosion, poor drainage and slow infiltration, which hurt root distribution and uptake. It says soils in Madhya Pradesh, Maharashtra, Gujarat, Rajasthan and Telangana are low in organic carbon. Table 5 of that paper lists the deficient districts quoted here.

This table is 2007 survey data, quoted in 2017. It tells you where to look first, not what your own field holds. Soil types are explained in our post on types of soil in India, and the soil health card guide shows which lines to read.

Take a soil sample from the yellow patch and a second from a healthy patch of the same field, send both to a lab, and ask for organic carbon, pH, available N, P, K, S and zinc. Symptoms alone can mislead. The two-sample comparison shows whether the soil or something else is to blame.

UNL says tissue analysis may need a laboratory to confirm whether a nutrient is the problem, and refers to soil test values for fertiliser rates. K-State notes that causes include low soil level, poor inoculation, abnormal soil pH, restricted root access, root injury from insects, disease or herbicide, and plant genetics. A soil test only covers the first two.

• Sample the patch and the good area separately. Same depth, same field, same day.
• Ask for pH and organic carbon. High pH ties up zinc, iron and manganese; very low pH is where molybdenum runs short.
• Ask for sulphur and zinc by name. They are not always in a basic NPK report.
• Use the soil health card if the field has one. Our how to read the soil health card post lists the lines to check.
• Test before you buy. A bag of the wrong nutrient costs more than the test.

It works best for phosphorus and potassium, and poorly for sulphur and micronutrients. Virginia Tech gives sufficiency ranges for the top fully expanded trifoliate at early bloom, but Iowa State says its research supports tissue tests for only P and K in soybean and that published micronutrient ranges are too high. Use tissue results as a second opinion.

Source: Virginia Tech SPES-716. Virginia Tech says to sample the uppermost fully expanded trifoliate at or before initial bloom and during R1 to R5, collecting from at least 18 plants (the page also cites 18 to 25). The ranges are survey-based, and the potassium values are replicated-research critical levels.

The Iowa State note is the counterweight. It says tissue testing for P and K does not substitute for soil testing, that drought, excess moisture, pests, disease or another nutrient can shift results, that its sulphur tissue test is for alfalfa and not soybean, and that micronutrient trials showed little yield response. Its mid-season P and K sampling takes the top three trifoliate leaves with petioles at R2 to R3, from at least ten plants. The two states sample differently, so stay with one lab's method. These ranges are from the US, and Indian labs may use other cut-offs; ask your lab which one it follows.

Correct by cause. A confirmed zinc or molybdenum shortage is corrected with a foliar spray of that nutrient at the university rate, and the fix must also deal with the root cause, such as pH or waterlogging. Spray timing and N, P, K schedules are in our stage-wise fertilizer post, not here.

A general water-soluble option for a pale canopy with healthy roots is NPK 19:19:19, whose page lists soybean. It is a nutrition spray, not a sulphur or zinc fix. Our post on water-soluble versus granular fertiliser helps decide when it is worth it.

Before You Spray: Dig up three plants first. If the roots are brown, nodules are missing, or whitefly and mosaic are present, a nutrient spray will not fix it. Wet leaves in hot sun scorch. Spray in the cool part of the day, and try a few rows before the whole field.

Check roots and weather first, then leaf age, then the pattern across the field, then test the soil. This order stops you buying fertiliser for a virus, a root injury or a drainage problem. Most wrong sprays come from skipping the first two steps, so take the spade to the field before the sprayer.

1. Dig three to four plants. Look at roots and nodules. Brown or damaged roots or missing nodules change the plan (K-State, UNL).
2. Look at the weather. Waterlogging or a cold spell in the last two weeks can cause temporary yellowing that fades without fertiliser (K-State, NAAS).
3. Check the spray history. K-State lists herbicide among causes of root injury that restrict uptake.
4. Look under the leaves. Whitefly plus yellow-green patches points to yellow mosaic virus, not nutrition (Biotech Asia).
5. Which leaves? Older and lower means N, P, K, Mg or Mo. Young and upper means S, Fe or Mn. Zinc: UNL puts it on older leaves.
6. Is it patchy? Even across the field points to a nutrient or soil-wide cause. Patchy, spreading from a point, points to disease, pests or a waterlogged low spot.
7. Sample soil from good and bad patches and ask for the tests listed above.
8. Record what you found so next season's basal dose is based on a diagnosis, not a guess.

Test the soil before sowing, ask for sulphur and zinc by name, rotate out of continuous soybean, drain low spots and keep a record of each diagnosis. Prevention here means knowing your field; the dose and timing that follow from the test are in our stage-wise schedule post.

If DAP is short or dear, see DAP alternative fertilizers and the DAP and urea shortage post for what can stand in. For how biofertilisers fit, see the integrated nutrient management post in this soybean series.

The common ones are guessing from leaf colour alone, treating a virus with a nutrient spray, skipping the root check, and sampling only the sick patch. Each costs money and gives no yield. Most come from skipping the order of checks above.

• Judging by colour only. Nitrogen, sulphur and molybdenum all give pale plants; leaf age and nodules separate them.
• Spraying zinc on a virus. Mosaic with whitefly needs a pest and virus plan, not micronutrients.
• Skipping the spade. Poor nodulation, root injury and waterlogging cannot be seen from the bund.
• Sampling only the bad patch. Without the healthy comparison, a soil test cannot show what is different.
• Trusting one tissue test. Iowa State warns that drought, pests or another nutrient can shift tissue results, and its tissue tests cover only P and K.
• Reading too much into cold-weather symptoms. Phosphorus-like symptoms in cold, wet soil can fade as soil warms (K-State).
• Treating the last-year diagnosis as permanent. Soil pH, rain and crop history change; retest every few seasons.

Conclusion

Soybean nutrient problems are easier to avoid than to fix, and they are easier to fix when the cause is right. Start below ground: roots, nodules, drainage and whitefly. Then use leaf age to separate nitrogen, phosphorus, potassium, magnesium and molybdenum from sulphur, iron and manganese, and use NAAS district data to know which gaps are likely in your belt. Confirm with a two-patch soil test, treat a tissue test as a second opinion, and take the dose and timing question to your state recommendation or our schedule post.