Diagnosing Stunted Corn with Interveinal Chlorosis

The symptoms-interveinal chlorosis on older leaves with marginal necrosis-are classic for magnesium (Mg) deficiency, but the tissue and soil data point to a more complex situation. Let's analyze each nutrient. Sulfur (S): Soil S is 8 ppm (low), tissue S is 0.12% (below the sufficiency range of 0.15-0.50% for corn at V6). Sulfur deficiency causes uniform chlorosis of younger leaves, not interveinal chlorosis on older leaves. However, the high rainfall (15 cm) could have leached sulfate, and the no-till history with low organic matter mineralization may contribute. But the symptom pattern doesn't match S alone. Zinc (Zn): Soil Zn is 0.6 ppm (low; critical level ~1.0 ppm), tissue Zn is 15 ppm (below sufficiency 20-50 ppm). Zn deficiency causes interveinal chlorosis on younger leaves, sometimes with a whitish band, not typically on older leaves. The high pH (7.4) reduces Zn availability, and the high P application (60 kg P2O5/ha) can induce Zn deficiency by forming insoluble Zn-phosphate complexes. This is a strong candidate. Iron (Fe): Tissue Fe is 45 ppm, just below the sufficient range. High pH and no-till (cooler, wetter soils) can reduce Fe availability, but Fe deficiency typically shows interveinal chlorosis on younger leaves, not older. Magnesium (Mg): Not tested in soil or tissue, but the symptoms (interveinal chlorosis on older leaves with necrosis) are classic for Mg. The high K application (40 kg K2O/ha) and high CEC could create K-Mg antagonism, especially if Mg is low. The soil pH 7.4 is favorable for Mg availability, but high K can suppress Mg uptake. Potassium (K): Soil K is 110 ppm (medium), tissue K is 1.8% (below the sufficiency range of 2.0-3.0% for V6 corn). K deficiency causes marginal necrosis on older leaves, which matches the necrotic spots. The high rainfall could have leached K, and the no-till surface application may have left K in the top few cm where it is less accessible in dry conditions, but the soil was wet. The K/Mg ratio in tissue is not given, but K deficiency is plausible. Phosphorus (P): Soil P is 12 ppm (low), tissue P is 0.25% (below sufficiency 0.30-0.50%). P deficiency causes stunting and purpling, not interveinal chlorosis. The high pH and no-till (cool soils) reduce P availability, and the fall-applied anhydrous ammonia may have created acidic bands that immobilized P. However, the symptoms are not typical. Nitrogen (N): Tissue N is 2.8% (sufficient for V6 is 2.5-3.5%). Fall-applied anhydrous ammonia with high rainfall could have leached nitrate, but N deficiency causes uniform chlorosis on older leaves, not interveinal. Boron (B): Soil B is 0.3 ppm (low), tissue B is 8 ppm (below sufficiency 10-20 ppm). B deficiency causes stunted growth and chlorosis of young leaves, not typical here. Conclusion: The most likely primary deficiencies are potassium (marginal necrosis on older leaves, low tissue K) and zinc (interveinal chlorosis, low soil and tissue Zn, high pH and high P). Magnesium may also be involved due to K-Mg antagonism, but we lack tissue Mg data. The high pH and no-till history contribute to Zn and P deficiency, while high rainfall and high K application contribute to K and Mg issues. The farmer's suspicion of a micronutrient deficiency is partially correct (Zn), but K is a macronutrient also deficient. The priority is to address K and Zn first, then consider Mg and S.

For the current crop, immediate corrective measures are needed. Potassium: Apply 60-80 kg K2O/ha as potassium chloride (MOP) or potassium sulfate (SOP) broadcast over the rows, followed by irrigation if possible. Alternatively, foliar spray with 2-3% KCl solution (about 10-15 kg K2O/ha in 200 L water) can provide rapid but temporary relief. Since the corn is at V6, sidedress application of K is effective. Zinc: Apply 5-10 kg Zn/ha as zinc sulfate (ZnSO4·7H2O) broadcast or banded, or foliar spray with 0.5% ZnSO4 solution (2-3 kg Zn/ha) plus a surfactant. Foliar Zn is quickly absorbed and can correct deficiency within a week. Magnesium: If tissue tests confirm Mg deficiency, apply 20-30 kg Mg/ha as magnesium sulfate (Epsom salts) foliar or 100-200 kg/ha broadcast. Sulfur: Apply 20-30 kg S/ha as ammonium sulfate or gypsum. Nitrogen: Ensure adequate N; the fall application may have leached, so sidedress 50-70 kg N/ha as urea ammonium nitrate (UAN) if tissue N is low. For long-term management: Soil pH: Maintain pH between 6.0 and 6.8 for corn. The current pH 7.4 is high; avoid over-liming and consider elemental sulfur to lower pH gradually if Zn deficiency persists. Organic matter: Continue no-till but add cover crops (e.g., cereal rye) to increase organic matter and improve nutrient cycling. Fertilizer placement: Band P and K instead of broadcast to increase efficiency, especially in no-till. Use a starter fertilizer with Zn (e.g., 10-34-0 + Zn) at planting. Nutrient balance: Avoid excessive K applications that antagonize Mg; base rates on soil tests. Monitoring: Take tissue samples from both affected and healthy plants 10-14 days after treatment to confirm nutrient levels. Use drone imagery or NDVI to assess recovery. Soil test every 2-3 years, and consider in-season petiole or tissue testing for Zn and K. Track yields and keep records to adjust the plan.