Potassium and magnesium deficiency in a calcareous Limassol Mavro vineyard
A 2.5 ha Mavro vineyard at 350 m near Limassol, planted in 1994 on a deep, calcareous soil (pH 8.3, 35% total CaCO3, 1.8% organic matter). Vines are bilateral cordon, spaced 2.2 x 1.2 m, drip-irrigated with two 2.2 L/h emitters per vine. Borehole water has EC 1.9 dS/m and bicarbonate 340 mg/L. In spring the grower broadcast 200 kg/ha of 20-10-10 plus 15 t/ha of cattle manure, and irrigated 4 hours twice a week. At bloom, petiole analysis gave N 1.95% (high), P 0.14%, K 0.55% (low), Mg 0.22% (low), Ca 1.9%. In July, lower leaves showed interveinal chlorosis and some marginal scorch; berries were small and ripening uneven, and yield is estimated 25% below the 12 t/ha average. Emitters show white carbonate deposits and some are partly blocked. The grower proposes adding more 20-10-10 and increasing irrigation.
1. Interpret the petiole data and the leaf symptoms. Explain why K and Mg are low despite the fertiliser and manure applied.
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Interpretation. At bloom, K 0.55% and Mg 0.22% are below the usual sufficiency ranges (roughly 1.2-2.0% K and 0.3-0.6% Mg, depending on the laboratory), while N 1.95% is high. The symptoms match: interveinal chlorosis on basal leaves is Mg deficiency; marginal scorch on older leaves is K deficiency and/or salt stress. Small berries and uneven ripening follow from poor K supply during cell division and sugar accumulation.
Why, despite 200 kg/ha of 20-10-10 and manure?
- Placement. Potassium moves mainly by diffusion and is only slightly mobile in soil. Broadcasting on the surface under drip feeds only the wetted bulb; K held on the calcareous clay may never reach the roots.
- Antagonism. High Ca (pH 8.3, 35% CaCO3) and high Mg compete with K uptake, and excess N with vigorous growth dilutes K and Mg in the tissue.
- Bicarbonate. 340 mg/L HCO3 raises rhizosphere pH, inactivates iron and worsens chlorosis; EC 1.9 dS/m forces the vine to spend energy on osmotic adjustment.
- Salts and precipitation. Manure and 20-10-10 add salt; the white deposits and partly blocked emitters show carbonate precipitation. Sodium and chloride accumulation can produce marginal scorch that mimics K deficiency.
Adding more of the same fertiliser would increase salinity and vigour without correcting the imbalance.
2. Design a corrected fertigation and irrigation programme for the next season, including products, timing, water treatment and monitoring.
Read the worked answer
Stop the broadcast 20-10-10 and split nitrogen: 40-60 kg N/ha is enough for this vigour level.
Fertigation, bloom to veraison: potassium sulfate (about 50% K2O, no chloride) at 150-200 kg K2O/ha in 4-6 weekly doses, or potassium nitrate if some N is wanted. Magnesium sulfate 25-40 kg/ha through the drip plus two or three foliar sprays of 2% Epsom salt at 10-day intervals. Avoid KCl: the water already carries chloride, and the soil is saline enough.
Water treatment: acidify with sulfuric acid to neutralise bicarbonate and target pH 6.0-6.5 in the irrigation water. This protects the emitters from carbonate scaling and improves micronutrient availability. Add chelated iron (EDDHA) only if chlorosis persists after acidification. Apply a 15-20% leaching fraction to keep root-zone EC in check, and monitor with a soil paste extract rather than guessing.
Irrigation strategy: replace the two 4-hour weekly runs with a scheduled deficit: about 60-70% of ETc from fruit set to veraison, then moderate water during ripening. Mild stress concentrates fruit; severe stress on a saline soil causes scorch and berry shrivel.
Monitoring: petiole analysis at bloom and veraison, water analysis each season, and a soil sample from the wetted zone. Reassess rates next year instead of repeating the manure dose blindly.