Micronutrient Soil Test & Apply or Skip
Reads Zn vs critical
Don't spray micronutrients blindly. Enter your DTPA Zn/Fe/Mn/Cu and hot-water B soil-test values and your crop, and each nutrient is plotted against its critical level — deficient, marginal or sufficient — so you know which one to fix first and which to skip.
Soil test (mg/kg = ppm)
Next: correct Zinc first — at 0.5 mg/kg it sits below the DTPA critical level of 1. Apply about 11 kg Zn/ha (≈ 25–35 kg/ha zinc sulphate (33% Zn) or 11 kg Zn/ha) or rescue with 0.5% ZnSO₄·7H₂O (5 g/L) + lime, 1–2 sprays. Re-test after the season to confirm the value clears the critical level.
Verdict = soil-test value vs the extractant's critical level (deficient < critical ≤ marginal < sufficient), with the critical level raised 25% for highly sensitive crops and lowered 20% for tolerant ones. Sources: Lindsay & Norvell (1978) DTPA; Sims & Johnson (1991); Havlin et al., Soil Fertility & Fertilizers; extension soil-test interpretation guides. Calibrate to a local correlation; B has a narrow deficiency-to-toxicity margin — never over-apply.
Micronutrient critical levels — key facts
- DTPA Zn critical
- ≈ 0.8 mg/kg (raised for corn/rice)
- DTPA Fe critical
- ≈ 4.5 mg/kg (foliar/chelate to fix)
- DTPA Mn critical
- ≈ 1.0 mg/kg (band, don't broadcast)
- DTPA Cu critical
- ≈ 0.2 mg/kg (long residual)
- Hot-water B critical
- ≈ 0.5 mg/kg (narrow toxicity margin)
- Crop sensitivity
- shifts the critical level ±20–25%
- Verdict
- deficient · marginal · sufficient
- Source
- Lindsay & Norvell 1978; Sims & Johnson 1991; Havlin et al.
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Micronutrient critical levels & sufficiency ranges
| Micronutrient | Extractant | Critical (mg/kg) | Sufficient ≥ (mg/kg) | Corrective soil rate | Foliar rescue |
|---|---|---|---|---|---|
| Zinc (Zn) | DTPA | 0.8 | 1.5 | 11 kg Zn/ha | 0.5% ZnSO₄·7H₂O (5 g/L) + lime, 1–2 sprays |
| Iron (Fe) | DTPA | 4.5 | 10 | soil ineffective — chelate/foliar | 0.5–1% FeSO₄ (5–10 g/L) or Fe-EDDHA chelate, repeat sprays |
| Manganese (Mn) | DTPA | 1 | 3 | 7 kg Mn/ha | 0.5% MnSO₄ (5 g/L), 1–2 sprays |
| Copper (Cu) | DTPA | 0.2 | 0.5 | 6 kg Cu/ha | 0.2–0.5% CuSO₄ (2–5 g/L) + lime to avoid burn |
| Boron (B) | Hot-water | 0.5 | 1 | 1.5 kg B/ha | 0.1–0.2% borax/Solubor (1–2 g/L), avoid at flowering on sensitive crops |
Representative critical levels and sufficiency floors for general field crops; the tool raises the critical level ~25% for highly sensitive crops and lowers it ~20% for tolerant ones. Source: Lindsay & Norvell (1978) DTPA soil test; Sims & Johnson (1991); Havlin et al., "Soil Fertility and Fertilizers"; university soil-test interpretation guides.
Reading the soil test, not the leaf
A micronutrient soil test only earns its keep when you interpret the number against the right critical level. A DTPA-zinc of 0.6 mg/kg is not just "low" — it is below the ~0.8 mg/kg critical level, so a corn crop will probably respond to applied zinc. The same value on a zinc-tolerant crop might fall in the marginal band where a response is uncertain. Treating every below-average value as a deficiency wastes money; treating every value as fine misses real yield-limiting shortfalls. The decision hinges on the extractant, the critical level, and crop sensitivity.
This tool plots each nutrient on a critical-level strip — a red deficient zone, an amber marginal band and a green sufficient zone — drops your soil value on it, sorts the nutrients so the most-deficient one comes first, and returns the corrective soil or foliar rate. Fix the limiting nutrient first, re-test the following season, and remember boron's narrow margin: under-apply and you lose yield, over-apply and you cause toxicity. Pair it with the Micronutrient Foliar Spray and Secondary Nutrient Deficiency tools.
How to use it in five steps
- 1
Pick your crop
The crop sets each nutrient's sensitivity, raising or lowering its effective critical level.
- 2
Enter the soil-test values
Type DTPA Zn/Fe/Mn/Cu and hot-water B in mg/kg (ppm) straight from your lab report.
- 3
Read each strip gauge
Each nutrient is plotted in its red, amber or green zone with the critical and sufficiency marks.
- 4
Find the limiting nutrient
The list sorts by severity, so the most-deficient micronutrient is named first.
- 5
Apply and re-test
Use the corrective soil or foliar rate for the deficient nutrients, then re-test next season.
Frequently Asked Questions
What is a micronutrient soil-test critical level?+
The critical level is the soil-test value below which a crop is likely to respond profitably to applied micronutrient. Below it the soil is deficient; just above it is a marginal band where a sensitive crop may still respond; above the sufficiency floor a response is unlikely, so you skip. For DTPA-extracted micronutrients the common critical levels are about 0.8 mg/kg Zn, 4.5 mg/kg Fe, 1.0 mg/kg Mn and 0.2 mg/kg Cu, and about 0.5 mg/kg for hot-water boron. The tool plots your value against these on a coloured strip for each nutrient.
Should I apply zinc or skip it?+
Compare your DTPA-Zn soil test to the critical level. On a general crop the critical level is about 0.8 mg/kg; below that, apply. The catch is crop sensitivity: corn, rice, beans and citrus are highly Zn-responsive, so the tool raises their effective critical level by about 25% (to ~1.0 mg/kg for corn), meaning they read 'apply' at a soil test a tolerant crop would treat as sufficient. Enter your value and crop and the strip gauge gives the apply/skip verdict directly.
Why does boron need a different extractant?+
Boron is the only one of these micronutrients tested with hot water rather than DTPA, because boron is held as borate and is best extracted by hot-water solubilisation. The hot-water B critical level is about 0.5 mg/kg. Boron also has the narrowest deficiency-to-toxicity range of any nutrient — the gap between 'too little' and 'too much' is small — so corrective rates are low (1–2 kg B/ha) and over-application is genuinely toxic. Never broadcast boron heavily.
How much corrective micronutrient should I apply if deficient?+
Typical soil broadcast rates are about 11 kg Zn/ha (≈25–35 kg/ha zinc sulphate), 7 kg Mn/ha banded, 6 kg Cu/ha (long residual), and only 1–2 kg B/ha. Iron is the exception: soil iron is largely ineffective on calcareous soils, so a chelate (Fe-EDDHA) or foliar spray is preferred. The tool returns the soil rate and a foliar rescue concentration for the limiting nutrient. Banding Zn and Mn is more efficient than broadcasting on high-pH soils that fix them.
Which crops are most sensitive to micronutrient deficiency?+
Corn, rice, beans and citrus are highly zinc-sensitive; wheat and other cereals are copper- and manganese-sensitive on peat and sandy soils; soybean is prone to iron chlorosis and manganese deficiency on high-pH soils; and brassicas, sugar beet, cotton and many pulses are boron-sensitive (heart rot, hollow stem, poor seed/boll set). The tool adjusts each critical level for the crop you pick, so the same soil test can read 'apply' on a sensitive crop and 'skip' on a tolerant one.
What does the DTPA test measure?+
DTPA (diethylenetriaminepentaacetic acid) is a chelating extractant introduced by Lindsay and Norvell in 1978 to estimate plant-available zinc, iron, manganese and copper from near-neutral and calcareous soils. It chelates the small pool of metal ions a crop can actually access, which correlates with crop response far better than total soil metal. Your lab report should state whether micronutrients were run by DTPA or Mehlich-3, because the critical levels differ.
My zinc is 0.5 and the tool says deficient for corn but marginal for a tolerant crop — why?+
Because the critical level is crop-specific. The base Zn critical level is 0.8 mg/kg, but corn is highly responsive, so its effective critical level is raised about 25% to roughly 1.0 mg/kg — a soil test of 0.5 is well below that, hence deficient. A zinc-tolerant crop (e.g. some brassicas) has its critical level lowered, so 0.5 might fall in the marginal band instead. The deciding factor is the crop, which is exactly what the calculator captures.
What is the difference between deficient, marginal and sufficient?+
Deficient means the soil test is below the critical level and a yield response to application is likely — apply. Marginal (the amber band) means the value sits between the critical level and the sufficiency floor: a response is possible on sensitive crops but not guaranteed, so the tool only recommends application there if the crop is highly sensitive. Sufficient means the value is above the sufficiency floor — a response is unlikely and you should skip and re-test in a few years.
Can soil-test micronutrients be too high?+
Yes. Boron is toxic above its narrow sufficiency range — a few mg/kg of hot-water B can damage sensitive crops, which is why the tool caps corrective boron at 1–2 kg/ha. Copper, zinc and manganese accumulate from repeated application (and from some manures and fungicides) and can reach phytotoxic or environmentally regulated levels over time. The tool flags sufficiency so you stop applying once the value clears the floor rather than building up an excess.
How accurate are these critical levels?+
They are sound, widely cited interpretive values from DTPA and hot-water-B calibration research and university soil-test guides, with crop-sensitivity adjustments. Real critical levels vary with soil pH, texture, organic matter, the specific lab method and local calibration, so treat the verdict as strong planning guidance and confirm against your lab's own interpretation where it differs. The deficient/marginal/sufficient split and the limiting-nutrient ranking are directionally reliable.
Does soil pH affect micronutrient availability?+
Strongly. Zinc, iron, manganese and copper all become less available as pH rises, so high-pH and calcareous soils are the classic deficiency settings — lime-induced iron chlorosis is a textbook example. Boron and molybdenum behave differently. The critical levels here are extractant-based and already reflect the available pool, but if your soil is high-pH expect Zn, Fe and Mn deficiencies to be more common and harder to correct with soil application, favouring foliar or chelated sources.
Why correct only the limiting micronutrient first?+
By the law of the minimum, yield is held back by the most-deficient nutrient, so correcting it gives the biggest response per dollar. The tool sorts the micronutrients by severity and names the limiting one, so you fix that first rather than applying a shotgun blend. Once it clears the critical level, the next nutrient becomes limiting — re-test and re-assess rather than over-applying everything at once, especially boron, where excess is harmful.