Chlorosis in a calcareous wheat field: iron or manganese?
A 1.5 ha wheat crop in a canal command area shows pale interveinal striping on the youngest fully expanded leaves, with irregular brown necrotic speckles and streaks; older leaves remain green. Soil (0-15 cm): pH 8.4 (1:2.5 water), EC 0.28 dS/m, free CaCO3 22%, organic carbon 0.31%, DTPA-Fe 3.6 mg/kg, DTPA-Mn 1.9 mg/kg, DTPA-Zn 0.9 mg/kg. Flag-leaf tissue at tillering: Fe 62 mg/kg, Mn 11 mg/kg, Zn 28 mg/kg, P 0.32%. The basal dose was 150 kg N as urea, 60 kg P2O5 as DAP drilled with the seed, and 40 kg K2O, plus 10 t/ha of old FYM. The farmer concluded it was iron chlorosis and sprayed FeSO4 at 0.5% twice, 8 days apart, in borewell water of pH 8.6. There was no improvement; if anything the striping spread. He now proposes to repeat the FeSO4 spray at 1.0% and to apply 2 t/ha of lime because the soil 'looks sick'.
1. Using the soil and tissue data, decide whether this is iron or manganese deficiency. Justify the diagnosis.
Read the worked answer
The data do not support iron.
- Tissue comparison with critical values at tillering: Fe about 50 mg/kg, Mn about 25 mg/kg, Zn about 20 mg/kg. Tissue Fe of 62 mg/kg is adequate; tissue Mn of 11 mg/kg is less than half the critical value. The plant is not short of iron; it is short of manganese.
- Soil agrees. At pH 8.4 with 22% free CaCO3, Mn2+ is oxidised to MnO2 and DTPA-Mn of 1.9 mg/kg falls below the 2.0 mg/kg critical level. DTPA-Fe of 3.6 mg/kg is borderline, yet the plant still accumulated adequate Fe, so Fe is not limiting.
- Symptom pattern fits. Fe and Mn are both phloem-immobile, so both appear on young leaves. But Mn chlorosis develops irregular brown necrotic speckles and streaks, exactly as described; Fe chlorosis usually stays a clean yellow.
- No other candidate. Zn is adequate (0.9 mg/kg soil, 28 mg/kg tissue), and there is no P-induced Zn problem.
Diagnosis: manganese deficiency induced by high pH and free lime, with iron only borderline. Treating it as iron explains the nil response.
2. Why did two FeSO4 sprays do nothing, and what is the correct spray chemistry for the actual deficiency?
Read the worked answer
Why the sprays failed:
- FeSO4 is ferrous sulphate dissolved in water. At pH 8.6 the Fe2+ oxidises within minutes to Fe3+, which precipitates as colloidal Fe(OH)3. What reaches the leaf is largely iron hydroxide, which is not absorbed - the spray was effectively water.
- The target was wrong. Tissue Fe was already adequate, so even a perfect iron spray had nothing to correct.
- Iron and manganese are antagonistic in uptake. Pushing Fe onto the leaf can aggravate the real Mn shortage.
Correct approach:
- Use MnSO4.H2O at 0.5% (5 g/L) with 0.1% citric acid, or acidify the spray water to about pH 6 before adding the salt; MnSO4 also oxidises in alkaline water, just less severely.
- Give 2-3 sprays at 10-day intervals, directed at young leaves, in the early morning.
- Add a soil application of 10-15 kg/ha MnSO4 banded with an acidifying fertiliser such as ammonium sulphate, since broadcast Mn is rapidly oxidised.
- Do not apply lime. The pH is already 8.4; lime would make the Mn problem permanent.
3. Sketch a two-season management plan to keep manganese available, and state what you would monitor.
Read the worked answer
While pH stays above 8 with 22% free lime, soil Mn will remain largely unavailable. The plan therefore has to change the root-zone chemistry, not just add Mn.
Season 1 (current wheat): two or three foliar MnSO4 sprays as a rescue; band 10-15 kg/ha MnSO4.H2O with ammonium sulphate at first irrigation; keep the crop well watered but not flooded.
Between seasons: substitute part of the urea with ammonium sulphate (21-0-0-24). Nitrification of NH4+ releases H+ and acidifies the band, raising DTPA-Mn around the root zone; the sulphate also supplies S, which is often low in this region. Continue FYM, which supplies chelating organic ligands and improves moisture retention.
Next season: band P instead of broadcasting it, because concentrated bands reduce contact with free CaCO3; do not apply lime; consider a wheat variety known to tolerate high pH, and use Mn-treated seed where available.
Monitoring: DTPA-Mn, DTPA-Fe, DTPA-Zn and pH on a 0-15 cm sample before each crop, plus flag-leaf tissue at tillering. The target is tissue Mn above 25 mg/kg and DTPA-Mn above 2 mg/kg; a yield response of 0.4-0.8 t/ha is realistic if tissue Mn is corrected.