Cocopeat Cucumber with Rising Drain EC and Hard Source Water
A 500 m² cocopeat grow-bag cucumber crop is irrigated by drip at 2 L/h per emitter. The feed solution is at EC 2.5 dS/m and pH 6.0. The drain reads EC 4.2 dS/m and pH 5.2, and the leaching fraction is only 10%. Source water has EC 1.2 dS/m and bicarbonate about 6 meq/L. Plants are weak, with marginal leaf necrosis, short internodes and small fruit. The grower has increased the feed EC to 3.0 to push growth, but the crop has worsened. There are 6 plants per square metre, and the timer runs five 10-minute pulses a day. The grower suspects a disease and wants to spray a fungicide. Before any spray, review the fertigation and substrate management.
1. Interpret the feed and drain EC and pH values. What is the most likely cause of the leaf-margin necrosis and small fruit?
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Drain EC 4.2 against feed EC 2.5 means salts are accumulating: the plant is taking up more water than nutrients, and the 10% leaching fraction is too low to push unused salts out. In cocopeat, a drain EC about 0.5–1.0 dS/m above feed is acceptable; +1.7 is high. The pH drop from 6.0 to 5.2 is not the main problem, but it suggests active root acidification and possibly high ammonium or urea in the feed; check the feed recipe and acid injection. Source water at EC 1.2 with bicarbonate 6 meq/L is hard water: it contributes salts and alkalinity, so the fertilizer recipe must be adjusted and the water acidified. Leaf-margin necrosis and small fruit fit salt and osmotic stress and poor calcium/magnesium uptake. Low leaching, hard water and possibly low irrigation volume form the likely chain. The grower's increase of feed EC to 3.0 made the salt load worse, not better.
2. Give a corrective leaching and fertigation plan, including a target leaching fraction and a check on irrigation volume.
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First leach, then rebalance. Run feed at EC 1.8–2.0 with the same nutrient ratios for 2–3 days, applying enough volume to get 30–40% drain, until drain EC falls below about 3.2. Do not leach with plain water; that strips nutrients and causes sudden deficiency. Then return to a feed EC of about 2.2–2.5 with a routine leaching fraction of 25–30%, which means 25–30 litres of drain for every 100 litres applied.
Check the application rate: a 2 L/h emitter run for 10 minutes gives only 0.33 L. Five such pulses give about 1.65 L per plant per day, which is likely below full-crop cucumber demand in this climate. Increase to 8–10 pulses or lengthen pulses, and use a timer with a solar-radiation sum or a substrate moisture sensor rather than a fixed schedule. Acidify the source water to neutralize bicarbonate, targeting about 0.5–1.0 meq/L residual bicarbonate. Nitric acid adds nitrogen, phosphoric acid adds phosphorus, and sulfuric acid adds sulfate, so choose according to the recipe. Recheck drain EC and pH daily for a week.
3. What substrate and record-keeping checks would you add before deciding that the crop has a disease problem?
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Inspect the bags first: compacted cocopeat, blocked drain holes and waterlogging will keep EC high no matter what is applied. Lift bags on slabs or rails, cut extra drain slits, and avoid reusing old bags without treatment. If the bags are more than two crops old, plan replacement or solarization/steam treatment because root disease and physical breakdown accumulate. Check roots for browning or root-knot galls; if roots are healthy and white, the problem is nutritional and water-related, not a fungal disease. Do not spray a fungicide as a substitute for fixing EC and leaching.
Keep a weekly record of feed EC, drain EC, drain percentage and pH; that record is the early warning. Send a water sample for sodium, chloride and bicarbonate, because high sodium in source water limits how far EC can be lowered without blending with low-salinity water or reverse osmosis. Also record the volume applied per plant and the drain volume from a marked bag.